23 Containers library [containers]

23.1 General [containers.general]

This Clause describes components that C++ programs may use to organize collections of information.

The following subclauses describe container requirements, and components for sequence containers and associative containers, as summarized in Table 69 .

Table 69 — Containers library summary [tab:containers.summary]

🔗	Subclause	Header
🔗 [container.requirements]	Requirements
🔗 [sequences]	Sequence containers	<array>, <deque>, <forward_list>, <inplace_vector>,
🔗		<list>, <vector>
🔗 [associative]	Associative containers	<map>, <set>
🔗 [unord]	Unordered associative containers	<unordered_map>, <unordered_set>
🔗 [container.adaptors]	Container adaptors	<queue>, <stack>, <flat_map>, <flat_set>
🔗 [views]	Views	<span>, <mdspan>

23.2 Requirements [container.requirements]

23.2.1 Preamble [container.requirements.pre]

Containers are objects that store other objects.

They control allocation and deallocation of these objects through constructors, destructors, insert and erase operations.

All of the complexity requirements in this Clause are stated solely in terms of the number of operations on the contained objects.

[Example 1:

The copy constructor of type vector<vector<int>> has linear complexity, even though the complexity of copying each contained vector<int> is itself linear.

— end example]

Allocator-aware containers ([container.alloc.reqmts]) other than basic_string construct elements using the function allocator_traits<allocator_type>::rebind_traits::construct and destroy elements using the function allocator_traits<allocator_type>::rebind_traits::destroy ([allocator.traits.members]), where U is either allocator_type::value_type or an internal type used by the container.

These functions are called only for the container's element type, not for internal types used by the container.

[Note 1:

This means, for example, that a node-based container would need to construct nodes containing aligned buffers and call construct to place the element into the buffer.

— end note]

23.2.2 General containers [container.requirements.general]

23.2.2.1 Introduction [container.intro.reqmts]

In [container.requirements.general],

(1.1)
X denotes a container class containing objects of type T,
(1.2)
a denotes a value of type X,
(1.3)
b and c denote values of type (possibly const) X,
(1.4)
i and j denote values of type (possibly const) X::iterator,
(1.5)
u denotes an identifier,
(1.6)
v denotes an lvalue of type (possibly const) X or an rvalue of type const X,
(1.7)
s and t denote non-const lvalues of type X, and
(1.8)
rv denotes a non-const rvalue of type X.

The following exposition-only concept is used in the definition of containers: template<class R, class T> concept container-compatible-range = // exposition only ranges::input_range<R> && convertible_to<ranges::range_reference_t<R>, T>;

23.2.2.2 Container requirements [container.reqmts]

A type X meets the container requirements if the following types, statements, and expressions are well-formed and have the specified semantics.

🔗

typename X::value_type

Result: T

Preconditions: T is Cpp17Erasable from X (see [container.alloc.reqmts], below).

🔗

typename X::reference

Result: T&

🔗

typename X::const_reference

Result: const T&

🔗

typename X::iterator

Result: A type that meets the forward iterator requirements ([forward.iterators]) with value type T.

The type X::iterator is convertible to X::const_iterator.

🔗

typename X::const_iterator

Result: A type that meets the requirements of a constant iterator and those of a forward iterator with value type T.

🔗

typename X::difference_type

Result: A signed integer type, identical to the difference type of X::iterator and X::const_iterator.

🔗

typename X::size_type

Result: An unsigned integer type that can represent any non-negative value of X::difference_type.

🔗

X u;
X u = X();

Postconditions: u.empty()

Complexity: Constant.

🔗

X u(v);
X u = v;

Preconditions: T is Cpp17CopyInsertable into X (see below).

Postconditions: u == v.

Complexity: Linear.

🔗

X u(rv);
X u = rv;

Postconditions: u is equal to the value that rv had before this construction.

Complexity: Linear for array and inplace_vector and constant for all other standard containers.

🔗

t = v;

Result: X&.

Postconditions: t == v.

Complexity: Linear.

🔗

t = rv

Result: X&.

Effects: All existing elements of t are either move assigned to or destroyed.

Postconditions: If t and rv do not refer to the same object, t is equal to the value that rv had before this assignment.

Complexity: Linear.

🔗

a.~X()

Result: void.

Effects: Destroys every element of a; any memory obtained is deallocated.

Complexity: Linear.

🔗

b.begin()

Result: iterator; const_iterator for constant b.

Returns: An iterator referring to the first element in the container.

Complexity: Constant.

🔗

b.end()

Result: iterator; const_iterator for constant b.

Returns: An iterator which is the past-the-end value for the container.

Complexity: Constant.

🔗

b.cbegin()

Result: const_iterator.

Returns: const_cast<X const&>(b).begin()

Complexity: Constant.

🔗

b.cend()

Result: const_iterator.

Returns: const_cast<X const&>(b).end()

Complexity: Constant.

🔗

i <=> j

Result: strong_ordering.

Constraints: X::iterator meets the random access iterator requirements.

Complexity: Constant.

🔗

c == b

Preconditions: T meets the Cpp17EqualityComparable requirements.

Result: bool.

Returns: equal(c.begin(), c.end(), b.begin(), b.end())

[Note 1:

The algorithm equal is defined in [alg.equal].

— end note]

Complexity: Constant if c.size() != b.size(), linear otherwise.

Remarks: == is an equivalence relation.

🔗

c != b

Effects: Equivalent to !(c == b).

🔗

t.swap(s)

Result: void.

Effects: Exchanges the contents of t and s.

Complexity: Linear for array and inplace_vector, and constant for all other standard containers.

🔗

swap(t, s)

Effects: Equivalent to t.swap(s).

🔗

c.size()

Result: size_type.

Returns: distance(c.begin(), c.end()), i.e., the number of elements in the container.

Complexity: Constant.

Remarks: The number of elements is defined by the rules of constructors, inserts, and erases.

🔗

c.max_size()

Result: size_type.

Returns: distance(begin(), end()) for the largest possible container.

Complexity: Constant.

🔗

c.empty()

Result: bool.

Returns: c.begin() == c.end()

Complexity: Constant.

Remarks: If the container is empty, then c.empty() is true.

In the expressions i == j i != j i < j i <= j i >= j i > j i <=> j i - j where i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

Unless otherwise specified, all containers defined in this Clause obtain memory using an allocator (see [allocator.requirements]).

[Note 2:

In particular, containers and iterators do not store references to allocated elements other than through the allocator's pointer type, i.e., as objects of type P or pointer_traits::template rebind<unspecified>, where P is allocator_traits<allocator_type>::pointer.

— end note]

Copy constructors for these container types obtain an allocator by calling allocator_traits<allocator_type>::select_on_container_copy_construction on the allocator belonging to the container being copied.

Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved.

Such move construction of the allocator shall not exit via an exception.

All other constructors for these container types take a const allocator_type& argument.

[Note 3:

If an invocation of a constructor uses the default value of an optional allocator argument, then the allocator type must support value-initialization.

— end note]

A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced.

The allocator may be replaced only via assignment or swap().

Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only if

(64.1)
allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value,
(64.2)
allocator_traits<allocator_type>::propagate_on_container_move_assignment::value, or
(64.3)
allocator_traits<allocator_type>::propagate_on_container_swap::value

is true within the implementation of the corresponding container operation.

In all container types defined in this Clause, the member get_allocator() returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.

The expression a.swap(b), for containers a and b of a standard container type other than array and inplace_vector, shall exchange the values of a and b without invoking any move, copy, or swap operations on the individual container elements.

Any Compare, Pred, or Hash types belonging to a and b shall meet the Cpp17Swappable requirements and shall be exchanged by calling swap as described in [swappable.requirements].

If allocator_traits<allocator_type>::propagate_on_container_swap::value is true, then allocator_type shall meet the Cpp17Swappable requirements and the allocators of a and b shall also be exchanged by calling swap as described in [swappable.requirements].

Otherwise, the allocators shall not be swapped, and the behavior is undefined unless a.get_allocator() == b.get_allocator().

Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap.

It is unspecified whether an iterator with value a.end() before the swap will have value b.end() after the swap.

Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], [inplace.vector.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

(66.1)
If an exception is thrown by an insert() or emplace() function while inserting a single element, that function has no effects.
(66.2)
If an exception is thrown by a push_back(), push_front(), emplace_back(), or emplace_front() function, that function has no effects.
(66.3)
No erase(), clear(), pop_back() or pop_front() function throws an exception.
(66.4)
No copy constructor or assignment operator of a returned iterator throws an exception.
(66.5)
No swap() function throws an exception.
(66.6)
No swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped.

[Note 4:
The end() iterator does not refer to any element, so it can be invalidated.
— end note]

Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.

A contiguous container is a container whose member types iterator and const_iterator meet the Cpp17RandomAccessIterator requirements ([random.access.iterators]) and model contiguous_iterator ([iterator.concept.contiguous]).

The behavior of certain container member functions and deduction guides depends on whether types qualify as input iterators or allocators.

The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.

Likewise, the extent to which an implementation determines that a type cannot be an allocator is unspecified, except that as a minimum a type A shall not qualify as an allocator unless it meets both of the following conditions:

(69.1)
The qualified-id A::value_type is valid and denotes a type ([temp.deduct]).
(69.2)
The expression declval<A&>().allocate(size_t{}) is well-formed when treated as an unevaluated operand.

23.2.2.3 Reversible container requirements [container.rev.reqmts]

A type X meets the reversible container requirements if X meets the container requirements, the iterator type of X belongs to the bidirectional or random access iterator categories ([iterator.requirements]), and the following types and expressions are well-formed and have the specified semantics.

🔗

typename X::reverse_iterator

Result: The type reverse_iterator<X::iterator>, an iterator type whose value type is T.

🔗

typename X::const_reverse_iterator

Result: The type reverse_iterator<X::const_iterator>, a constant iterator type whose value type is T.

🔗

a.rbegin()

Result: reverse_iterator; const_reverse_iterator for constant a.

Returns: reverse_iterator(end())

Complexity: Constant.

🔗

a.rend()

Result: reverse_iterator; const_reverse_iterator for constant a.

Returns: reverse_iterator(begin())

Complexity: Constant.

🔗

a.crbegin()

Result: const_reverse_iterator.

Returns: const_cast<X const&>(a).rbegin()

Complexity: Constant.

🔗

a.crend()

Result: const_reverse_iterator.

Returns: const_cast<X const&>(a).rend()

Complexity: Constant.

23.2.2.4 Optional container requirements [container.opt.reqmts]

The following operations are provided for some types of containers but not others.

Those containers for which the listed operations are provided shall implement the semantics as described unless otherwise stated.

If the iterators passed to lexicographical_compare_three_way meet the constexpr iterator requirements ([iterator.requirements.general]) then the operations described below are implemented by constexpr functions.

🔗

a <=> b

Result: synth-three-way-result<X::value_type>.

Preconditions: Either T models three_way_comparable, or < is defined for values of type (possibly const) T and < is a total ordering relationship.

Returns: lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(),
synth-three-way)

[Note 1:

The algorithm lexicographical_compare_three_way is defined in [algorithms].

— end note]

Complexity: Linear.

23.2.2.5 Allocator-aware containers [container.alloc.reqmts]

Except for array and inplace_vector, all of the containers defined in [containers], [stacktrace.basic], [basic.string], and [re.results] meet the additional requirements of an allocator-aware container, as described below.

Given an allocator type A and given a container type X having a value_type identical to T and an allocator_type identical to allocator_traits<A>::rebind_alloc<T> and given an lvalue m of type A, a pointer p of type T*, an expression v that denotes an lvalue of type T or const T or an rvalue of type const T, and an rvalue rv of type T, the following terms are defined.

If X is not allocator-aware or is a specialization of basic_string, the terms below are defined as if A were allocator<T> — no allocator object needs to be created and user specializations of allocator<T> are not instantiated:

(2.1)
T is Cpp17DefaultInsertable into X means that the following expression is well-formed: allocator_traits<A>::construct(m, p)
(2.2)
An element of X is default-inserted if it is initialized by evaluation of the expression allocator_traits<A>::construct(m, p) where p is the address of the uninitialized storage for the element allocated within X.
(2.3)
T is Cpp17MoveInsertable into X means that the following expression is well-formed: allocator_traits<A>::construct(m, p, rv) and its evaluation causes the following postcondition to hold: The value of *p is equivalent to the value of rv before the evaluation.
[Note 1:
rv remains a valid object.

Its state is unspecified.
— end note]
(2.4)
T is Cpp17CopyInsertable into X means that, in addition to T being Cpp17MoveInsertable into X, the following expression is well-formed: allocator_traits<A>::construct(m, p, v) and its evaluation causes the following postcondition to hold: The value of v is unchanged and is equivalent to *p.
(2.5)
T is Cpp17EmplaceConstructible into X from args, for zero or more arguments args, means that the following expression is well-formed: allocator_traits<A>::construct(m, p, args)
(2.6)
T is Cpp17Erasable from X means that the following expression is well-formed: allocator_traits<A>::destroy(m, p)

[Note 2:

A container calls allocator_traits<A>::construct(m, p, args) to construct an element at p using args, with m == get_allocator().

The default construct in allocator will call ::new((void*)p) T(args), but specialized allocators can choose a different definition.

— end note]

In this subclause,

(3.1)
X denotes an allocator-aware container class with a value_type of T using an allocator of type A,
(3.2)
u denotes a variable,
(3.3)
a and b denote non-const lvalues of type X,
(3.4)
c denotes an lvalue of type const X,
(3.5)
t denotes an lvalue or a const rvalue of type X,
(3.6)
rv denotes a non-const rvalue of type X, and
(3.7)
m is a value of type A.

A type X meets the allocator-aware container requirements if X meets the container requirements and the following types, statements, and expressions are well-formed and have the specified semantics.

🔗

typename X::allocator_type

Result: A

Mandates: allocator_type::value_type is the same as X::value_type.

🔗

c.get_allocator()

Result: A

Complexity: Constant.

🔗

X u;
X u = X();

Preconditions: A meets the Cpp17DefaultConstructible requirements.

Postconditions: u.empty() returns true, u.get_allocator() == A().

Complexity: Constant.

🔗

X u(m);

Postconditions: u.empty() returns true, u.get_allocator() == m.

Complexity: Constant.

🔗

X u(t, m);

Preconditions: T is Cpp17CopyInsertable into X.

Postconditions: u == t, u.get_allocator() == m.

Complexity: Linear.

🔗

X u(rv);

Postconditions: u has the same elements as rv had before this construction; the value of u.get_allocator() is the same as the value of rv.get_allocator() before this construction.

Complexity: Constant.

🔗

X u(rv, m);

Preconditions: T is Cpp17MoveInsertable into X.

Postconditions: u has the same elements, or copies of the elements, that rv had before this construction, u.get_allocator() == m.

Complexity: Constant if m == rv.get_allocator(), otherwise linear.

🔗

a = t

Result: X&.

Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Postconditions: a == t is true.

Complexity: Linear.

🔗

a = rv

Result: X&.

Preconditions: If allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is false, T is Cpp17MoveInsertable into X and Cpp17MoveAssignable.

Effects: All existing elements of a are either move assigned to or destroyed.

Postconditions: If a and rv do not refer to the same object, a is equal to the value that rv had before this assignment.

Complexity: Linear.

🔗

a.swap(b)

Result: void

Effects: Exchanges the contents of a and b.

Complexity: Constant.

23.2.3 Container data races [container.requirements.dataraces]

For purposes of avoiding data races ([res.on.data.races]), implementations shall consider the following functions to be const: begin, end, rbegin, rend, front, back, data, find, lower_bound, upper_bound, equal_range, at and, except in associative or unordered associative containers, operator[].

Notwithstanding [res.on.data.races], implementations are required to avoid data races when the contents of the contained object in different elements in the same container, excepting vector<bool>, are modified concurrently.

[Note 1:

For a vector<int> x with a size greater than one, x[1] = 5 and *x.begin() = 10 can be executed concurrently without a data race, but x[0] = 5 and *x.begin() = 10 executed concurrently can result in a data race.

As an exception to the general rule, for a vector<bool> y, y[0] = true can race with y[1] = true.

— end note]

23.2.4 Sequence containers [sequence.reqmts]

A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement.

The library provides the following basic kinds of sequence containers: vector, inplace_vector, forward_list, list, and deque.

In addition, array is provided as a sequence container which provides limited sequence operations because it has a fixed number of elements.

The library also provides container adaptors that make it easy to construct abstract data types, such as stacks, queues, flat_maps, flat_multimaps, flat_sets, or flat_multisets, out of the basic sequence container kinds (or out of other program-defined sequence containers).

In this subclause,

(2.1)
X denotes a sequence container class,
(2.2)
a denotes a value of type X containing elements of type T,
(2.3)
u denotes the name of a variable being declared,
(2.4)
A denotes X::allocator_type if the qualified-id X::allocator_type is valid and denotes a type ([temp.deduct]) and allocator<T> if it doesn't,
(2.5)
i and j denote iterators that meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to value_type,
(2.6)
[i, j) denotes a valid range,
(2.7)
rg denotes a value of a type R that models container-compatible-range<T>,
(2.8)
il designates an object of type initializer_list<value_type>,
(2.9)
n denotes a value of type X::size_type,
(2.10)
p denotes a valid constant iterator to a,
(2.11)
q denotes a valid dereferenceable constant iterator to a,
(2.12)
[q1, q2) denotes a valid range of constant iterators in a,
(2.13)
t denotes an lvalue or a const rvalue of X::value_type, and
(2.14)
rv denotes a non-const rvalue of X::value_type.
(2.15)
Args denotes a template parameter pack;
(2.16)
args denotes a function parameter pack with the pattern Args&&.

The complexities of the expressions are sequence dependent.

A type X meets the sequence container requirements if X meets the container requirements and the following statements and expressions are well-formed and have the specified semantics.

🔗

X u(n, t);

Preconditions: T is Cpp17CopyInsertable into X.

Effects: Constructs a sequence container with n copies of t.

Postconditions: distance(u.begin(), u.end()) == n is true.

🔗

X u(i, j);

Preconditions: T is Cpp17EmplaceConstructible into X from *i.

For vector, if the iterator does not meet the Cpp17ForwardIterator requirements ([forward.iterators]), T is also Cpp17MoveInsertable into X.

Effects: Constructs a sequence container equal to the range [i, j).

Each iterator in the range [i, j) is dereferenced exactly once.

Postconditions: distance(u.begin(), u.end()) == distance(i, j) is true.

🔗

X(from_range, rg)

Preconditions: T is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

For vector, if R models neither ranges::sized_range nor ranges::forward_range, T is also Cpp17MoveInsertable into X.

Effects: Constructs a sequence container equal to the range rg.

Each iterator in the range rg is dereferenced exactly once.

Postconditions: distance(begin(), end()) == ranges::distance(rg) is true.

🔗

X(il)

Effects: Equivalent to X(il.begin(), il.end()).

🔗

a = il

Result: X&.

Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Effects: Assigns the range [il.begin(), il.end()) into a.

All existing elements of a are either assigned to or destroyed.

Returns: *this.

🔗

a.emplace(p, args)

Result: iterator.

Preconditions: T is Cpp17EmplaceConstructible into X from args.

For vector, inplace_vector, and deque, T is also Cpp17MoveInsertable into X and Cpp17MoveAssignable.

Effects: Inserts an object of type T constructed with std::forward<Args>(args)... before p.

[Note 1:

args can directly or indirectly refer to a value in a.

— end note]

Returns: An iterator that points to the new element.

🔗

a.insert(p, t)

Result: iterator.

Preconditions: T is Cpp17CopyInsertable into X.

For vector, inplace_vector, and deque, T is also Cpp17CopyAssignable.

Effects: Inserts a copy of t before p.

Returns: An iterator that points to the copy of t inserted into a.

🔗

a.insert(p, rv)

Result: iterator.

Preconditions: T is Cpp17MoveInsertable into X.

For vector, inplace_vector, and deque, T is also Cpp17MoveAssignable.

Effects: Inserts a copy of rv before p.

Returns: An iterator that points to the copy of rv inserted into a.

🔗

a.insert(p, n, t)

Result: iterator.

Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Effects: Inserts n copies of t before p.

Returns: An iterator that points to the copy of the first element inserted into a, or p if n == 0.

🔗

a.insert(p, i, j)

Result: iterator.

Preconditions: T is Cpp17EmplaceConstructible into X from *i.

For vector, inplace_vector, and deque, T is also Cpp17MoveInsertable into X, and T meets the Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable ([swappable.requirements]) requirements.

Neither i nor j are iterators into a.

Effects: Inserts copies of elements in [i, j) before p.

Each iterator in the range [i, j) shall be dereferenced exactly once.

Returns: An iterator that points to the copy of the first element inserted into a, or p if i == j.

🔗

a.insert_range(p, rg)

Result: iterator.

Preconditions: T is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

rg and a do not overlap.

Effects: Inserts copies of elements in rg before p.

Each iterator in the range rg is dereferenced exactly once.

Returns: An iterator that points to the copy of the first element inserted into a, or p if rg is empty.

🔗

a.insert(p, il)

Effects: Equivalent to a.insert(p, il.begin(), il.end()).

🔗

a.erase(q)

Result: iterator.

Preconditions: For vector, inplace_vector, and deque, T is Cpp17MoveAssignable.

Effects: Erases the element pointed to by q.

Returns: An iterator that points to the element immediately following q prior to the element being erased.

If no such element exists, a.end() is returned.

🔗

a.erase(q1, q2)

Result: iterator.

Preconditions: For vector, inplace_vector, and deque, T is Cpp17MoveAssignable.

Effects: Erases the elements in the range [q1, q2).

Returns: An iterator that points to the element pointed to by q2 prior to any elements being erased.

If no such element exists, a.end() is returned.

🔗

a.clear()

Result: void

Effects: Destroys all elements in a.

Invalidates all references, pointers, and iterators referring to the elements of a and may invalidate the past-the-end iterator.

Postconditions: a.empty() is true.

Complexity: Linear.

🔗

a.assign(i, j)

Result: void

Preconditions: T is Cpp17EmplaceConstructible into X from *i and assignable from *i.

For vector, if the iterator does not meet the forward iterator requirements ([forward.iterators]), T is also Cpp17MoveInsertable into X.

Neither i nor j are iterators into a.

Effects: Replaces elements in a with a copy of [i, j).

Invalidates all references, pointers and iterators referring to the elements of a.

For vector and deque, also invalidates the past-the-end iterator.

Each iterator in the range [i, j) is dereferenced exactly once.

🔗

a.assign_range(rg)

Result: void

Mandates: assignable_from<T&, ranges::range_reference_t<R>> is modeled.

Preconditions: T is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

For vector, if R models neither ranges::sized_range nor ranges::forward_range, T is also Cpp17MoveInsertable into X.

rg and a do not overlap.

Effects: Replaces elements in a with a copy of each element in rg.

Invalidates all references, pointers, and iterators referring to the elements of a.

For vector and deque, also invalidates the past-the-end iterator.

Each iterator in the range rg is dereferenced exactly once.

🔗

a.assign(il)

Effects: Equivalent to a.assign(il.begin(), il.end()).

🔗

a.assign(n, t)

Result: void

Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

t is not a reference into a.

Effects: Replaces elements in a with n copies of t.

Invalidates all references, pointers and iterators referring to the elements of a.

For vector and deque, also invalidates the past-the-end iterator.

For every sequence container defined in this Clause and in [strings]:

(67.1)
If the constructor template<class InputIterator> X(InputIterator first, InputIterator last, const allocator_type& alloc = allocator_type()); is called with a type InputIterator that does not qualify as an input iterator, then the constructor shall not participate in overload resolution.
(67.2)
If the member functions of the forms: template<class InputIterator> return-type F(const_iterator p, InputIterator first, InputIterator last); // such as insert template<class InputIterator> return-type F(InputIterator first, InputIterator last); // such as append, assign template<class InputIterator> return-type F(const_iterator i1, const_iterator i2, InputIterator first, InputIterator last); // such as replace are called with a type InputIterator that does not qualify as an input iterator, then these functions shall not participate in overload resolution.
(67.3)
A deduction guide for a sequence container shall not participate in overload resolution if it has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter, or if it has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

The following operations are provided for some types of sequence containers but not others.

Operations other than prepend_range and append_range are implemented so as to take amortized constant time.

🔗

a.front()

Result: reference; const_reference for constant a.

Returns: *a.begin()

Remarks: Required for basic_string, array, deque, forward_list, inplace_vector, list, and vector.

🔗

a.back()

Result: reference; const_reference for constant a.

Effects: Equivalent to: auto tmp = a.end(); --tmp; return *tmp;

Remarks: Required for basic_string, array, deque, inplace_vector, list, and vector.

🔗

a.emplace_front(args)

Result: reference

Preconditions: T is Cpp17EmplaceConstructible into X from args.

Effects: Prepends an object of type T constructed with std::forward<Args>(args)....

Returns: a.front().

Remarks: Required for deque, forward_list, and list.

🔗

a.emplace_back(args)

Result: reference

Preconditions: T is Cpp17EmplaceConstructible into X from args.

For vector, T is also Cpp17MoveInsertable into X.

Effects: Appends an object of type T constructed with std::forward<Args>(args)....

Returns: a.back().

Remarks: Required for deque, inplace_vector, list, and vector.

🔗

a.push_front(t)

Result: void

Preconditions: T is Cpp17CopyInsertable into X.

Effects: Prepends a copy of t.

Remarks: Required for deque, forward_list, and list.

🔗

a.push_front(rv)

Result: void

Preconditions: T is Cpp17MoveInsertable into X.

Effects: Prepends a copy of rv.

Remarks: Required for deque, forward_list, and list.

🔗

a.prepend_range(rg)

Result: void

Preconditions: T is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

For deque, T is also Cpp17MoveInsertable into X, and T meets the Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable ([swappable.requirements]) requirements.

Effects: Inserts copies of elements in rg before begin().

Each iterator in the range rg is dereferenced exactly once.

[Note 2:

The order of elements in rg is not reversed.

— end note]

Remarks: Required for deque, forward_list, and list.

🔗

a.push_back(t)

Result: void

Preconditions: T is Cpp17CopyInsertable into X.

Effects: Appends a copy of t.

100

Remarks: Required for basic_string, deque, inplace_vector, list, and vector.

🔗

a.push_back(rv)

101

Result: void

102

Preconditions: T is Cpp17MoveInsertable into X.

103

Effects: Appends a copy of rv.

104

Remarks: Required for basic_string, deque, inplace_vector, list, and vector.

🔗

a.append_range(rg)

105

Result: void

106

Preconditions: T is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

For vector, T is also Cpp17MoveInsertable into X.

107

Effects: Inserts copies of elements in rg before end().

Each iterator in the range rg is dereferenced exactly once.

108

Remarks: Required for deque, inplace_vector, list, and vector.

🔗

a.pop_front()

109

Result: void

110

Preconditions: a.empty() is false.

111

Effects: Destroys the first element.

112

Remarks: Required for deque, forward_list, and list.

🔗

a.pop_back()

113

Result: void

114

Preconditions: a.empty() is false.

115

Effects: Destroys the last element.

116

Remarks: Required for basic_string, deque, inplace_vector, list, and vector.

🔗

a[n]

117

Result: reference; const_reference for constant a.

118

Effects: Equivalent to: return *(a.begin() + n);

119

Remarks: Required for basic_string, array, deque, inplace_vector, and vector.

🔗

a.at(n)

120

Result: reference; const_reference for constant a.

121

Returns: *(a.begin() + n)

122

Throws: out_of_range if n >= a.size().

123

Remarks: Required for basic_string, array, deque, inplace_vector, and vector.

23.2.5 Node handles [container.node]

23.2.5.1 Overview [container.node.overview]

A node handle is an object that accepts ownership of a single element from an associative container ([associative.reqmts]) or an unordered associative container ([unord.req]).

It may be used to transfer that ownership to another container with compatible nodes.

Containers with compatible nodes have the same node handle type.

Elements may be transferred in either direction between container types in the same row of Table 70 .

Table 70 — Container types with compatible nodes [tab:container.node.compat]

🔗 map<K, T, C1, A>	map<K, T, C2, A>
🔗 map<K, T, C1, A>	multimap<K, T, C2, A>
🔗 set<K, C1, A>	set<K, C2, A>
🔗 set<K, C1, A>	multiset<K, C2, A>
🔗 unordered_map<K, T, H1, E1, A>	unordered_map<K, T, H2, E2, A>
🔗 unordered_map<K, T, H1, E1, A>	unordered_multimap<K, T, H2, E2, A>
🔗 unordered_set<K, H1, E1, A>	unordered_set<K, H2, E2, A>
🔗 unordered_set<K, H1, E1, A>	unordered_multiset<K, H2, E2, A>

If a node handle is not empty, then it contains an allocator that is equal to the allocator of the container when the element was extracted.

If a node handle is empty, it contains no allocator.

Class node-handle is for exposition only.

If a user-defined specialization of pair exists for pair<const Key, T> or pair<Key, T>, where Key is the container's key_type and T is the container's mapped_type, the behavior of operations involving node handles is undefined.

template<unspecified> class node-handle { public: // These type declarations are described in [associative.reqmts] and [unord.req]. using value_type = see below; // not present for map containers using key_type = see below; // not present for set containers using mapped_type = see below; // not present for set containers using allocator_type = see below; private: using container_node_type = unspecified; // exposition only using ator_traits = allocator_traits<allocator_type>; // exposition only typename ator_traits::template rebind_traits<container_node_type>::pointer ptr_; // exposition only optional<allocator_type> alloc_; // exposition only public: // [container.node.cons], constructors, copy, and assignment constexpr node-handle() noexcept : ptr_(), alloc_() {} node-handle(node-handle&&) noexcept; node-handle& operator=(node-handle&&); // [container.node.dtor], destructor ~node-handle(); // [container.node.observers], observers value_type& value() const; // not present for map containers key_type& key() const; // not present for set containers mapped_type& mapped() const; // not present for set containers allocator_type get_allocator() const; explicit operator bool() const noexcept; bool empty() const noexcept; // [container.node.modifiers], modifiers void swap(node-handle&) noexcept(ator_traits::propagate_on_container_swap::value || ator_traits::is_always_equal::value); friend void swap(node-handle& x, node-handle& y) noexcept(noexcept(x.swap(y))) { x.swap(y); } };

23.2.5.2 Constructors, copy, and assignment [container.node.cons]

🔗

node-handle(node-handle&& nh) noexcept;

Effects: Constructs a node-handle object initializing ptr_ with nh.ptr_.

Move constructs alloc_ with nh.alloc_.

Assigns nullptr to nh.ptr_ and assigns nullopt to nh.alloc_.

🔗

node-handle& operator=(node-handle&& nh);

Preconditions: Either !alloc_, or ator_traits::propagate_on_container_move_assignment::value is true, or alloc_ == nh.alloc_.

Effects:

(3.1)
If ptr_ != nullptr, destroys the value_type subobject in the container_node_type object pointed to by ptr_ by calling ator_traits::destroy, then deallocates ptr_ by calling ator_traits::template rebind_traits<container_node_type>::deallocate.
(3.2)
Assigns nh.ptr_ to ptr_.
(3.3)
If !alloc_ or ator_traits::propagate_on_container_move_assignment::value is true, move assigns nh.alloc_ to alloc_.
(3.4)
Assigns nullptr to nh.ptr_ and assigns nullopt to nh.alloc_.

Returns: *this.

Throws: Nothing.

23.2.5.3 Destructor [container.node.dtor]

🔗

~node-handle();

Effects: If ptr_ != nullptr, destroys the value_type subobject in the container_node_type object pointed to by ptr_ by calling ator_traits::destroy, then deallocates ptr_ by calling ator_traits::template rebind_traits<container_node_type>::deallocate.

23.2.5.4 Observers [container.node.observers]

🔗

value_type& value() const;

Preconditions: empty() == false.

Returns: A reference to the value_type subobject in the container_node_type object pointed to by ptr_.

Throws: Nothing.

🔗

key_type& key() const;

Preconditions: empty() == false.

Returns: A non-const reference to the key_type member of the value_type subobject in the container_node_type object pointed to by ptr_.

Throws: Nothing.

Remarks: Modifying the key through the returned reference is permitted.

🔗

mapped_type& mapped() const;

Preconditions: empty() == false.

Returns: A reference to the mapped_type member of the value_type subobject in the container_node_type object pointed to by ptr_.

Throws: Nothing.

🔗

allocator_type get_allocator() const;

Preconditions: empty() == false.

Returns: *alloc_.

Throws: Nothing.

🔗

explicit operator bool() const noexcept;

Returns: ptr_ != nullptr.

🔗

bool empty() const noexcept;

Returns: ptr_ == nullptr.

23.2.5.5 Modifiers [container.node.modifiers]

🔗

void swap(node-handle& nh)
  noexcept(ator_traits::propagate_on_container_swap::value ||
           ator_traits::is_always_equal::value);

Preconditions: !alloc_, or !nh.alloc_, or ator_traits::propagate_on_container_swap::value is true, or alloc_ == nh.alloc_.

Effects: Calls swap(ptr_, nh.ptr_).

If !alloc_, or !nh.alloc_, or ator_traits::propagate_on_container_swap::value is true calls swap(alloc_, nh.alloc_).

23.2.6 Insert return type [container.insert.return]

The associative containers with unique keys and the unordered containers with unique keys have a member function insert that returns a nested type insert_return_type.

That return type is a specialization of the template specified in this subclause.

template<class Iterator, class NodeType> struct insert-return-type { Iterator position; bool inserted; NodeType node; };

The name insert-return-type is exposition only.

insert-return-type has the template parameters, data members, and special members specified above.

It has no base classes or members other than those specified.

23.2.7 Associative containers [associative.reqmts]

23.2.7.1 General [associative.reqmts.general]

Associative containers provide fast retrieval of data based on keys.

The library provides four basic kinds of associative containers: set, multiset, map and multimap.

The library also provides container adaptors that make it easy to construct abstract data types, such as flat_maps, flat_multimaps, flat_sets, or flat_multisets, out of the basic sequence container kinds (or out of other program-defined sequence containers).

Each associative container is parameterized on Key and an ordering relation Compare that induces a strict weak ordering ([alg.sorting]) on elements of Key.

In addition, map and multimap associate an arbitrary mapped type T with the Key.

The object of type Compare is called the comparison object of a container.

The phrase “equivalence of keys” means the equivalence relation imposed by the comparison object.

That is, two keys k1 and k2 are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.

[Note 1:

This is not necessarily the same as the result of k1 == k2.

— end note]

For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value.

An associative container supports unique keys if it may contain at most one element for each key.

Otherwise, it supports equivalent keys.

The set and map classes support unique keys; the multiset and multimap classes support equivalent keys.

For multiset and multimap, insert, emplace, and erase preserve the relative ordering of equivalent elements.

For set and multiset the value type is the same as the key type.

For map and multimap it is equal to pair<const Key, T>.

iterator of an associative container is of the bidirectional iterator category.

For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators.

It is unspecified whether or not iterator and const_iterator are the same type.

[Note 2:

iterator and const_iterator have identical semantics in this case, and iterator is convertible to const_iterator.

Users can avoid violating the one-definition rule by always using const_iterator in their function parameter lists.

— end note]

In this subclause,

(7.1)
X denotes an associative container class,
(7.2)
a denotes a value of type X,
(7.3)
a2 denotes a value of a type with nodes compatible with type X (Table 70),
(7.4)
b denotes a value of type X or const X,
(7.5)
u denotes the name of a variable being declared,
(7.6)
a_uniq denotes a value of type X when X supports unique keys,
(7.7)
a_eq denotes a value of type X when X supports multiple keys,
(7.8)
a_tran denotes a value of type X or const X when the qualified-id X::key_compare::is_transparent is valid and denotes a type ([temp.deduct]),
(7.9)
i and j meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to value_type,
(7.10)
[i, j) denotes a valid range,
(7.11)
rg denotes a value of a type R that models container-compatible-range<value_type>,
(7.12)
p denotes a valid constant iterator to a,
(7.13)
q denotes a valid dereferenceable constant iterator to a,
(7.14)
r denotes a valid dereferenceable iterator to a,
(7.15)
[q1, q2) denotes a valid range of constant iterators in a,
(7.16)
il designates an object of type initializer_list<value_type>,
(7.17)
t denotes a value of type X::value_type,
(7.18)
k denotes a value of type X::key_type, and
(7.19)
c denotes a value of type X::key_compare or const X::key_compare;
(7.20)
kl is a value such that a is partitioned ([alg.sorting]) with respect to c(x, kl), with x the key value of e and e in a;
(7.21)
ku is a value such that a is partitioned with respect to !c(ku, x), with x the key value of e and e in a;
(7.22)
ke is a value such that a is partitioned with respect to c(x, ke) and !c(ke, x), with c(x, ke) implying !c(ke, x) and with x the key value of e and e in a;
(7.23)
kx is a value such that
- (7.23.1)
  a is partitioned with respect to c(x, kx) and !c(kx, x), with c(x, kx) implying !c(kx, x) and with x the key value of e and e in a, and
- (7.23.2)
  kx is not convertible to either iterator or const_iterator; and
(7.24)
A denotes the storage allocator used by X, if any, or allocator<X::value_type> otherwise,
(7.25)
m denotes an allocator of a type convertible to A, and nh denotes a non-const rvalue of type X::node_type.

A type X meets the associative container requirements if X meets all the requirements of an allocator-aware container ([container.alloc.reqmts]) and the following types, statements, and expressions are well-formed and have the specified semantics, except that for map and multimap, the requirements placed on value_type in [container.reqmts] apply instead to key_type and mapped_type.

[Note 3:

For example, in some cases key_type and mapped_type need to be Cpp17CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not Cpp17CopyAssignable.

— end note]

🔗

typename X::key_type

Result: Key.

🔗

typename X::mapped_type

Result: T.

Remarks: For map and multimap only.

🔗

typename X::value_type

Result: Key for set and multiset only; pair<const Key, T> for map and multimap only.

Preconditions: X::value_type is Cpp17Erasable from X.

🔗

typename X::key_compare

Result: Compare.

Preconditions: key_compare is Cpp17CopyConstructible.

🔗

typename X::value_compare

Result: A binary predicate type.

It is the same as key_compare for set and multiset; is an ordering relation on pairs induced by the first component (i.e., Key) for map and multimap.

🔗

typename X::node_type

Result: A specialization of the node-handle class template ([container.node]), such that the public nested types are the same types as the corresponding types in X.

🔗

X(c)

Effects: Constructs an empty container.

Uses a copy of c as a comparison object.

Complexity: Constant.

🔗

X u = X();
X u;

Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.

Effects: Constructs an empty container.

Uses Compare() as a comparison object.

Complexity: Constant.

🔗

X(i, j, c)

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses c as a comparison object.

Complexity:

N log N

in general, where N has the value distance(i, j); linear if [i, j) is sorted with respect to value_comp().

🔗

X(i, j)

Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses Compare() as a comparison object.

Complexity:

N log N

in general, where N has the value distance(i, j); linear if [i, j) is sorted with respect to value_comp().

🔗

X(from_range, rg, c)

Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

Effects: Constructs an empty container and inserts each element from rg into it.

Uses c as the comparison object.

Complexity:

N log N

in general, where N has the value ranges::distance(rg); linear if rg is sorted with respect to value_comp().

🔗

X(from_range, rg)

Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

Effects: Constructs an empty container and inserts each element from rg into it.

Uses Compare() as the comparison object.

Complexity: Same as X(from_range, rg, c).

🔗

X(il, c)

Effects: Equivalent to X(il.begin(), il.end(), c).

🔗

X(il)

Effects: Equivalent to X(il.begin(), il.end()).

🔗

a = il

Result: X&

Preconditions: value_type is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Effects: Assigns the range [il.begin(), il.end()) into a.

All existing elements of a are either assigned to or destroyed.

Complexity:

N log N

in general, where N has the value il.size() + a.size(); linear if [il.begin(), il.end()) is sorted with respect to value_comp().

🔗

b.key_comp()

Result: X::key_compare

Returns: The comparison object out of which b was constructed.

Complexity: Constant.

🔗

b.value_comp()

Result: X::value_compare

Returns: An object of value_compare constructed out of the comparison object.

Complexity: Constant.

🔗

a_uniq.emplace(args)

Result: pair<iterator, bool>

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of t.

Returns: The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t.

Complexity: Logarithmic.

🔗

a_eq.emplace(args)

Result: iterator

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

Effects: Inserts a value_type object t constructed with std::forward<Args>(args)....

If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range.

Returns: An iterator pointing to the newly inserted element.

Complexity: Logarithmic.

🔗

a.emplace_hint(p, args)

Result: iterator

Effects: Equivalent to a.emplace(std::forward<Args>(args)...), except that the element is inserted as close as possible to the position just prior to p.

Returns: An iterator pointing to the element with the key equivalent to the newly inserted element.

Complexity: Logarithmic in general, but amortized constant if the element is inserted right before p.

🔗

a_uniq.insert(t)

Result: pair<iterator, bool>

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

Effects: Inserts t if and only if there is no element in the container with key equivalent to the key of t.

Returns: The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t.

Complexity: Logarithmic.

🔗

a_eq.insert(t)

Result: iterator

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

Effects: Inserts t and returns the iterator pointing to the newly inserted element.

If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range.

Complexity: Logarithmic.

🔗

a.insert(p, t)

Result: iterator

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

Effects: Inserts t if and only if there is no element with key equivalent to the key of t in containers with unique keys; always inserts t in containers with equivalent keys.

t is inserted as close as possible to the position just prior to p.

Returns: An iterator pointing to the element with key equivalent to the key of t.

Complexity: Logarithmic in general, but amortized constant if t is inserted right before p.

🔗

a.insert(i, j)

Result: void

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Neither i nor j are iterators into a.

Effects: Inserts each element from the range [i, j) if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys.

Complexity:

N log (a.size() + N)

, where N has the value distance(i, j).

🔗

a.insert_range(rg)

Result: void

Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

rg and a do not overlap.

Effects: Inserts each element from rg if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys.

Complexity:

N log (a.size() + N)

, where N has the value ranges::distance(rg).

🔗

a.insert(il)

Effects: Equivalent to a.insert(il.begin(), il.end()).

🔗

a_uniq.insert(nh)

Result: insert_return_type

Preconditions: nh is empty or a_uniq.get_allocator() == nh.get_allocator() is true.

Effects: If nh is empty, has no effect.

Otherwise, inserts the element owned by nh if and only if there is no element in the container with a key equivalent to nh.key().

Returns: If nh is empty, inserted is false, position is end(), and node is empty.

Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty; if the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key().

Complexity: Logarithmic.

🔗

a_eq.insert(nh)

Result: iterator

Preconditions: nh is empty or a_eq.get_allocator() == nh.get_allocator() is true.

Effects: If nh is empty, has no effect and returns a_eq.end().

Otherwise, inserts the element owned by nh and returns an iterator pointing to the newly inserted element.

If a range containing elements with keys equivalent to nh.key() exists in a_eq, the element is inserted at the end of that range.

Postconditions: nh is empty.

Complexity: Logarithmic.

🔗

a.insert(p, nh)

Result: iterator

Preconditions: nh is empty or a.get_allocator() == nh.get_allocator() is true.

Effects: If nh is empty, has no effect and returns a.end().

Otherwise, inserts the element owned by nh if and only if there is no element with key equivalent to nh.key() in containers with unique keys; always inserts the element owned by nh in containers with equivalent keys.

The element is inserted as close as possible to the position just prior to p.

Postconditions: nh is empty if insertion succeeds, unchanged if insertion fails.

Returns: An iterator pointing to the element with key equivalent to nh.key().

Complexity: Logarithmic in general, but amortized constant if the element is inserted right before p.

🔗

a.extract(k)

100

Result: node_type

101

Effects: Removes the first element in the container with key equivalent to k.

102

Returns: A node_type owning the element if found, otherwise an empty node_type.

103

Complexity:

log (a.size())

🔗

a_tran.extract(kx)

104

Result: node_type

105

Effects: Removes the first element in the container with key r such that !c(r, kx) && !c(kx, r) is true.

106

Returns: A node_type owning the element if found, otherwise an empty node_type.

107

Complexity:

log (a_tran.size())

🔗

a.extract(q)

108

Result: node_type

109

Effects: Removes the element pointed to by q.

110

Returns: A node_type owning that element.

111

Complexity: Amortized constant.

🔗

a.merge(a2)

112

Result: void

113

Preconditions: a.get_allocator() == a2.get_allocator().

114

Effects: Attempts to extract each element in a2 and insert it into a using the comparison object of a.

In containers with unique keys, if there is an element in a with key equivalent to the key of an element from a2, then that element is not extracted from a2.

115

Postconditions: Pointers and references to the transferred elements of a2 refer to those same elements but as members of a.

Iterators referring to the transferred elements will continue to refer to their elements, but they now behave as iterators into a, not into a2.

116

Throws: Nothing unless the comparison object throws.

117

Complexity:

N log (a.size()+ N)

, where N has the value a2.size().

🔗

a.erase(k)

118

Result: size_type

119

Effects: Erases all elements in the container with key equivalent to k.

120

Returns: The number of erased elements.

121

Complexity:

log (a.size()) + a.count(k)

🔗

a_tran.erase(kx)

122

Result: size_type

123

Effects: Erases all elements in the container with key r such that !c(r, kx) && !c(kx, r) is true.

124

Returns: The number of erased elements.

125

Complexity:

log (a_tran.size()) + a_tran.count(kx)

🔗

a.erase(q)

126

Result: iterator

127

Effects: Erases the element pointed to by q.

128

Returns: An iterator pointing to the element immediately following q prior to the element being erased.

If no such element exists, returns a.end().

129

Complexity: Amortized constant.

🔗

a.erase(r)

130

Result: iterator

131

Effects: Erases the element pointed to by r.

132

Returns: An iterator pointing to the element immediately following r prior to the element being erased.

If no such element exists, returns a.end().

133

Complexity: Amortized constant.

🔗

a.erase(q1, q2)

134

Result: iterator

135

Effects: Erases all the elements in the range [q1, q2).

136

Returns: An iterator pointing to the element pointed to by q2 prior to any elements being erased.

If no such element exists, a.end() is returned.

137

Complexity:

log (a.size()) + N

, where N has the value distance(q1, q2).

🔗

a.clear()

138

Effects: Equivalent to a.erase(a.begin(), a.end()).

139

Postconditions: a.empty() is true.

140

Complexity: Linear in a.size().

🔗

b.find(k)

141

Result: iterator; const_iterator for constant b.

142

Returns: An iterator pointing to an element with the key equivalent to k, or b.end() if such an element is not found.

143

Complexity: Logarithmic.

🔗

a_tran.find(ke)

144

Result: iterator; const_iterator for constant a_tran.

145

Returns: An iterator pointing to an element with key r such that !c(r, ke) && !c(ke, r) is true, or a_tran.end() if such an element is not found.

146

Complexity: Logarithmic.

🔗

b.count(k)

147

Result: size_type

148

Returns: The number of elements with key equivalent to k.

149

Complexity:

log (b.size()) + b.count(k)

🔗

a_tran.count(ke)

150

Result: size_type

151

Returns: The number of elements with key r such that !c(r, ke) && !c(ke, r).

152

Complexity:

log (a_tran.size()) + a_tran.count(ke)

🔗

b.contains(k)

153

Result: bool

154

Effects: Equivalent to: return b.find(k) != b.end();

🔗

a_tran.contains(ke)

155

Result: bool

156

Effects: Equivalent to: return a_tran.find(ke) != a_tran.end();

🔗

b.lower_bound(k)

157

Result: iterator; const_iterator for constant b.

158

Returns: An iterator pointing to the first element with key not less than k, or b.end() if such an element is not found.

159

Complexity: Logarithmic.

🔗

a_tran.lower_bound(kl)

160

Result: iterator; const_iterator for constant a_tran.

161

Returns: An iterator pointing to the first element with key r such that !c(r, kl), or a_tran.end() if such an element is not found.

162

Complexity: Logarithmic.

🔗

b.upper_bound(k)

163

Result: iterator; const_iterator for constant b.

164

Returns: An iterator pointing to the first element with key greater than k, or b.end() if such an element is not found.

165

Complexity: Logarithmic.

🔗

a_tran.upper_bound(ku)

166

Result: iterator; const_iterator for constant a_tran.

167

Returns: An iterator pointing to the first element with key r such that c(ku, r), or a_tran.end() if such an element is not found.

168

Complexity: Logarithmic.

🔗

b.equal_range(k)

169

Result: pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant b.

170

Effects: Equivalent to: return make_pair(b.lower_bound(k), b.upper_bound(k));

171

Complexity: Logarithmic.

🔗

a_tran.equal_range(ke)

172

Result: pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant a_tran.

173

Effects: Equivalent to: return make_pair(a_tran.lower_bound(ke), a_tran.upper_bound(ke));

174

Complexity: Logarithmic.

175

The insert, insert_range, and emplace members shall not affect the validity of iterators and references to the container, and the erase members shall invalidate only iterators and references to the erased elements.

176

The extract members invalidate only iterators to the removed element; pointers and references to the removed element remain valid.

However, accessing the element through such pointers and references while the element is owned by a node_type is undefined behavior.

References and pointers to an element obtained while it is owned by a node_type are invalidated if the element is successfully inserted.

177

The fundamental property of iterators of associative containers is that they iterate through the containers in the non-descending order of keys where non-descending is defined by the comparison that was used to construct them.

For any two dereferenceable iterators i and j such that distance from i to j is positive, the following condition holds: value_comp(*j, *i) == false

178

For associative containers with unique keys the stronger condition holds: value_comp(*i, *j) != false

179

When an associative container is constructed by passing a comparison object the container shall not store a pointer or reference to the passed object, even if that object is passed by reference.

When an associative container is copied, through either a copy constructor or an assignment operator, the target container shall then use the comparison object from the container being copied, as if that comparison object had been passed to the target container in its constructor.

180

The member function templates find, count, contains, lower_bound, upper_bound, equal_range, erase, and extract shall not participate in overload resolution unless the qualified-id Compare::is_transparent is valid and denotes a type ([temp.deduct]).

Additionally, the member function templates extract and erase shall not participate in overload resolution if is_convertible_v<K&&, iterator> || is_convertible_v<K&&, const_iterator> is true, where K is the type substituted as the first template argument.

181

A deduction guide for an associative container shall not participate in overload resolution if any of the following are true:

(181.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(181.2)
It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.
(181.3)
It has a Compare template parameter and a type that qualifies as an allocator is deduced for that parameter.

23.2.7.2 Exception safety guarantees [associative.reqmts.except]

For associative containers, no clear() function throws an exception.

erase(k) does not throw an exception unless that exception is thrown by the container's Compare object (if any).

For associative containers, if an exception is thrown by any operation from within an insert or emplace function inserting a single element, the insertion has no effect.

For associative containers, no swap function throws an exception unless that exception is thrown by the swap of the container's Compare object (if any).

23.2.8 Unordered associative containers [unord.req]

23.2.8.1 General [unord.req.general]

Unordered associative containers provide an ability for fast retrieval of data based on keys.

The worst-case complexity for most operations is linear, but the average case is much faster.

The library provides four unordered associative containers: unordered_set, unordered_map, unordered_multiset, and unordered_multimap.

Unordered associative containers conform to the requirements for Containers ([container.requirements]), except that the expressions a == b and a != b have different semantics than for the other container types.

Each unordered associative container is parameterized by Key, by a function object type Hash that meets the Cpp17Hash requirements ([hash.requirements]) and acts as a hash function for argument values of type Key, and by a binary predicate Pred that induces an equivalence relation on values of type Key.

Additionally, unordered_map and unordered_multimap associate an arbitrary mapped type T with the Key.

The container's object of type Hash — denoted by hash — is called the hash function of the container.

The container's object of type Pred — denoted by pred — is called the key equality predicate of the container.

Two values k1 and k2 are considered equivalent if the container's key equality predicate pred(k1, k2) is valid and returns true when passed those values.

If k1 and k2 are equivalent, the container's hash function shall return the same value for both.

[Note 1:

Thus, when an unordered associative container is instantiated with a non-default Pred parameter it usually needs a non-default Hash parameter as well.

— end note]

For any two keys k1 and k2 in the same container, calling pred(k1, k2) shall always return the same value.

For any key k in a container, calling hash(k) shall always return the same value.

An unordered associative container supports unique keys if it may contain at most one element for each key.

Otherwise, it supports equivalent keys.

unordered_set and unordered_map support unique keys.

unordered_multiset and unordered_multimap support equivalent keys.

In containers that support equivalent keys, elements with equivalent keys are adjacent to each other in the iteration order of the container.

Thus, although the absolute order of elements in an unordered container is not specified, its elements are grouped into equivalent-key groups such that all elements of each group have equivalent keys.

Mutating operations on unordered containers shall preserve the relative order of elements within each equivalent-key group unless otherwise specified.

For unordered_set and unordered_multiset the value type is the same as the key type.

For unordered_map and unordered_multimap it is pair<const Key, T>.

For unordered containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators.

It is unspecified whether or not iterator and const_iterator are the same type.

[Note 2:

iterator and const_iterator have identical semantics in this case, and iterator is convertible to const_iterator.

Users can avoid violating the one-definition rule by always using const_iterator in their function parameter lists.

— end note]

The elements of an unordered associative container are organized into buckets.

Keys with the same hash code appear in the same bucket.

The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound.

Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements.

For unordered_multiset and unordered_multimap, rehashing preserves the relative ordering of equivalent elements.

In this subclause,

(10.1)
X denotes an unordered associative container class,
(10.2)
a denotes a value of type X,
(10.3)
a2 denotes a value of a type with nodes compatible with type X (Table 70),
(10.4)
b denotes a value of type X or const X,
(10.5)
a_uniq denotes a value of type X when X supports unique keys,
(10.6)
a_eq denotes a value of type X when X supports equivalent keys,
(10.7)
a_tran denotes a value of type X or const X when the qualified-ids X::key_equal::is_transparent and X::hasher::is_transparent are both valid and denote types ([temp.deduct]),
(10.8)
i and j denote input iterators that refer to value_type,
(10.9)
[i, j) denotes a valid range,
(10.10)
rg denotes a value of a type R that models container-compatible-range<value_type>,
(10.11)
p and q2 denote valid constant iterators to a,
(10.12)
q and q1 denote valid dereferenceable constant iterators to a,
(10.13)
r denotes a valid dereferenceable iterator to a,
(10.14)
[q1, q2) denotes a valid range in a,
(10.15)
il denotes a value of type initializer_list<value_type>,
(10.16)
t denotes a value of type X::value_type,
(10.17)
k denotes a value of type key_type,
(10.18)
hf denotes a value of type hasher or const hasher,
(10.19)
eq denotes a value of type key_equal or const key_equal,
(10.20)
ke is a value such that
- (10.20.1)
  eq(r1, ke) == eq(ke, r1),
- (10.20.2)
  hf(r1) == hf(ke) if eq(r1, ke) is true, and
- (10.20.3)
  if any two of eq(r1, ke), eq(r2, ke), and eq(r1, r2) are true, then all three are true,
where r1 and r2 are keys of elements in a_tran,
(10.21)
kx is a value such that
- (10.21.1)
  eq(r1, kx) == eq(kx, r1),
- (10.21.2)
  hf(r1) == hf(kx) if eq(r1, kx) is true,
- (10.21.3)
  if any two of eq(r1, kx), eq(r2, kx), and eq(r1, r2) are true, then all three are true, and
- (10.21.4)
  kx is not convertible to either iterator or const_iterator,
where r1 and r2 are keys of elements in a_tran,
(10.22)
n denotes a value of type size_type,
(10.23)
z denotes a value of type float, and
(10.24)
nh denotes an rvalue of type X::node_type.

A type X meets the unordered associative container requirements if X meets all the requirements of an allocator-aware container ([container.alloc.reqmts]) and the following types, statements, and expressions are well-formed and have the specified semantics, except that for unordered_map and unordered_multimap, the requirements placed on value_type in [container.reqmts] apply instead to key_type and mapped_type.

[Note 3:

For example, key_type and mapped_type sometimes need to be Cpp17CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not Cpp17CopyAssignable.

— end note]

🔗

typename X::key_type

Result: Key.

🔗

typename X::mapped_type

Result: T.

Remarks: For unordered_map and unordered_multimap only.

🔗

typename X::value_type

Result: Key for unordered_set and unordered_multiset only; pair<const Key, T> for unordered_map and unordered_multimap only.

Preconditions: value_type is Cpp17Erasable from X.

🔗

typename X::hasher

Result: Hash.

Preconditions: Hash is a unary function object type such that the expression hf(k) has type size_t.

🔗

typename X::key_equal

Result: Pred.

Preconditions: Pred meets the Cpp17CopyConstructible requirements.

Pred is a binary predicate that takes two arguments of type Key.

Pred is an equivalence relation.

🔗

typename X::local_iterator

Result: An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::iterator's.

[Note 4:

A local_iterator object can be used to iterate through a single bucket, but cannot be used to iterate across buckets.

— end note]

🔗

typename X::const_local_iterator

Result: An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::const_iterator's.

[Note 5:

A const_local_iterator object can be used to iterate through a single bucket, but cannot be used to iterate across buckets.

— end note]

🔗

typename X::node_type

Result: A specialization of a node-handle class template ([container.node]), such that the public nested types are the same types as the corresponding types in X.

🔗

X(n, hf, eq)

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate.

Complexity:

O (n)

🔗

X(n, hf)

Preconditions: key_equal meets the Cpp17DefaultConstructible requirements.

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate.

Complexity:

O (n)

🔗

X(n)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate.

Complexity:

O (n)

🔗

X a = X();
X a;

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

Effects: Constructs an empty container with an unspecified number of buckets, using hasher() as the hash function and key_equal() as the key equality predicate.

Complexity: Constant.

🔗

X(i, j, n, hf, eq)

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate, and inserts elements from [i, j) into it.

Complexity: Average case

O (N)

(N is distance(i, j)), worst case

O (N^{2})

🔗

X(i, j, n, hf)

Preconditions: key_equal meets the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.

Complexity: Average case

O (N)

(N is distance(i, j)), worst case

O (N^{2})

🔗

X(i, j, n)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.

Complexity: Average case

O (N)

(N is distance(i, j)), worst case

O (N^{2})

🔗

X(i, j)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container with an unspecified number of buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it.

Complexity: Average case

O (N)

(N is distance(i, j)), worst case

O (N^{2})

🔗

X(from_range, rg, n, hf, eq)

Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate, and inserts elements from rg into it.

Complexity: Average case

O (N)

(N is ranges::distance(rg)), worst case

O (N^{2})

🔗

X(from_range, rg, n, hf)

Preconditions: key_equal meets the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate, and inserts elements from rg into it.

Complexity: Average case

O (N)

(N is ranges::distance(rg)), worst case

O (N^{2})

🔗

X(from_range, rg, n)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from rg into it.

Complexity: Average case

O (N)

(N is ranges::distance(rg)), worst case

O (N^{2})

🔗

X(from_range, rg)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

Complexity: Average case

O (N)

(N is ranges::distance(rg)), worst case

O (N^{2})

🔗

X(il)

Effects: Equivalent to X(il.begin(), il.end()).

🔗

X(il, n)

Effects: Equivalent to X(il.begin(), il.end(), n).

🔗

X(il, n, hf)

Effects: Equivalent to X(il.begin(), il.end(), n, hf).

🔗

X(il, n, hf, eq)

Effects: Equivalent to X(il.begin(), il.end(), n, hf, eq).

🔗

X(b)

Effects: In addition to the container requirements ([container.reqmts]), copies the hash function, predicate, and maximum load factor.

Complexity: Average case linear in b.size(), worst case quadratic.

🔗

a = b

Result: X&

Effects: In addition to the container requirements, copies the hash function, predicate, and maximum load factor.

Complexity: Average case linear in b.size(), worst case quadratic.

🔗

a = il

Result: X&

Preconditions: value_type is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Effects: Assigns the range [il.begin(), il.end()) into a.

All existing elements of a are either assigned to or destroyed.

Complexity: Average case linear in il.size(), worst case quadratic.

🔗

b.hash_function()

Result: hasher

Returns: b's hash function.

Complexity: Constant.

🔗

b.key_eq()

Result: key_equal

Returns: b's key equality predicate.

Complexity: Constant.

🔗

a_uniq.emplace(args)

Result: pair<iterator, bool>

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of t.

Returns: The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t.

Complexity: Average case

O (1)

, worst case

O (a_uniq.size())

🔗

a_eq.emplace(args)

Result: iterator

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

Effects: Inserts a value_type object t constructed with std::forward<Args>(args)....

Returns: An iterator pointing to the newly inserted element.

Complexity: Average case

O (1)

, worst case

O (a_eq.size())

🔗

a.emplace_hint(p, args)

Result: iterator

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

Effects: Equivalent to a.emplace(std::forward<Args>(args)...).

Returns: An iterator pointing to the element with the key equivalent to the newly inserted element.

The const_iterator p is a hint pointing to where the search should start.

Implementations are permitted to ignore the hint.

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a_uniq.insert(t)

Result: pair<iterator, bool>

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

Effects: Inserts t if and only if there is no element in the container with key equivalent to the key of t.

Returns: The bool component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of t.

Complexity: Average case

O (1)

, worst case

O (a_uniq.size())

🔗

a_eq.insert(t)

Result: iterator

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

100

Effects: Inserts t.

101

Returns: An iterator pointing to the newly inserted element.

102

Complexity: Average case

O (1)

, worst case

O (a_eq.size())

🔗

a.insert(p, t)

103

Result: iterator

104

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

105

Effects: Equivalent to a.insert(t).

The iterator p is a hint pointing to where the search should start.

Implementations are permitted to ignore the hint.

106

Returns: An iterator pointing to the element with the key equivalent to that of t.

107

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.insert(i, j)

108

Result: void

109

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Neither i nor j are iterators into a.

110

Effects: Equivalent to a.insert(t) for each element in [i,j).

111

Complexity: Average case

O (N)

, where N is distance(i, j), worst case

O (N (a.size() + 1))

🔗

a.insert_range(rg)

112

Result: void

113

Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

rg and a do not overlap.

114

Effects: Equivalent to a.insert(t) for each element t in rg.

115

Complexity: Average case

O (N)

, where N is ranges::distance(rg), worst case

O (N (a.size() + 1))

🔗

a.insert(il)

116

Effects: Equivalent to a.insert(il.begin(), il.end()).

🔗

a_uniq.insert(nh)

117

Result: insert_return_type

118

Preconditions: nh is empty or a_uniq.get_allocator() == nh.get_allocator() is true.

119

Effects: If nh is empty, has no effect.

Otherwise, inserts the element owned by nh if and only if there is no element in the container with a key equivalent to nh.key().

120

Postconditions: If nh is empty, inserted is false, position is end(), and node is empty.

121

Complexity: Average case

O (1)

, worst case

O (a_uniq.size())

🔗

a_eq.insert(nh)

122

Result: iterator

123

Preconditions: nh is empty or a_eq.get_allocator() == nh.get_allocator() is true.

124

Effects: If nh is empty, has no effect and returns a_eq.end().

Otherwise, inserts the element owned by nh and returns an iterator pointing to the newly inserted element.

125

Postconditions: nh is empty.

126

Complexity: Average case

O (1)

, worst case

O (a_eq.size())

🔗

a.insert(q, nh)

127

Result: iterator

128

Preconditions: nh is empty or a.get_allocator() == nh.get_allocator() is true.

129

Effects: If nh is empty, has no effect and returns a.end().

The iterator q is a hint pointing to where the search should start.

Implementations are permitted to ignore the hint.

130

Postconditions: nh is empty if insertion succeeds, unchanged if insertion fails.

131

Returns: An iterator pointing to the element with key equivalent to nh.key().

132

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.extract(k)

133

Result: node_type

134

Effects: Removes an element in the container with key equivalent to k.

135

Returns: A node_type owning the element if found, otherwise an empty node_type.

136

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a_tran.extract(kx)

137

Result: node_type

138

Effects: Removes an element in the container with key equivalent to kx.

139

Returns: A node_type owning the element if found, otherwise an empty node_type.

140

Complexity: Average case

O (1)

, worst case

O (a_tran.size())

🔗

a.extract(q)

141

Result: node_type

142

Effects: Removes the element pointed to by q.

143

Returns: A node_type owning that element.

144

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.merge(a2)

145

Result: void

146

Preconditions: a.get_allocator() == a2.get_allocator().

147

Effects: Attempts to extract each element in a2 and insert it into a using the hash function and key equality predicate of a.

In containers with unique keys, if there is an element in a with key equivalent to the key of an element from a2, then that element is not extracted from a2.

148

Postconditions: Pointers and references to the transferred elements of a2 refer to those same elements but as members of a.

Iterators referring to the transferred elements and all iterators referring to a will be invalidated, but iterators to elements remaining in a2 will remain valid.

149

Complexity: Average case

O (N)

, where N is a2.size(), worst case

O (N *a.size() + N)

🔗

a.erase(k)

150

Result: size_type

151

Effects: Erases all elements with key equivalent to k.

152

Returns: The number of elements erased.

153

Complexity: Average case

O (a.count(k))

, worst case

O (a.size())

🔗

a_tran.erase(kx)

154

Result: size_type

155

Effects: Erases all elements with key equivalent to kx.

156

Returns: The number of elements erased.

157

Complexity: Average case

O (a_tran.count(kx))

, worst case

O (a_tran.size())

🔗

a.erase(q)

158

Result: iterator

159

Effects: Erases the element pointed to by q.

160

Returns: The iterator immediately following q prior to the erasure.

161

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.erase(r)

162

Result: iterator

163

Effects: Erases the element pointed to by r.

164

Returns: The iterator immediately following r prior to the erasure.

165

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.erase(q1, q2)

166

Result: iterator

167

Effects: Erases all elements in the range [q1, q2).

168

Returns: The iterator immediately following the erased elements prior to the erasure.

169

Complexity: Average case linear in distance(q1, q2), worst case

O (a.size())

🔗

a.clear()

170

Result: void

171

Effects: Erases all elements in the container.

172

Postconditions: a.empty() is true.

173

Complexity: Linear in a.size().

🔗

b.find(k)

174

Result: iterator; const_iterator for constant b.

175

Returns: An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.

176

Complexity: Average case

O (1)

, worst case

O (b.size())

🔗

a_tran.find(ke)

177

Result: iterator; const_iterator for constant a_tran.

178

Returns: An iterator pointing to an element with key equivalent to ke, or a_tran.end() if no such element exists.

179

Complexity: Average case

O (1)

, worst case

O (a_tran.size())

🔗

b.count(k)

180

Result: size_type

181

Returns: The number of elements with key equivalent to k.

182

Complexity: Average case

O (b.count(k))

, worst case

O (b.size())

🔗

a_tran.count(ke)

183

Result: size_type

184

Returns: The number of elements with key equivalent to ke.

185

Complexity: Average case

O (a_tran.count(ke))

, worst case

O (a_tran.size())

🔗

b.contains(k)

186

Effects: Equivalent to b.find(k) != b.end().

🔗

a_tran.contains(ke)

187

Effects: Equivalent to a_tran.find(ke) != a_tran.end().

🔗

b.equal_range(k)

188

Result: pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant b.

189

Returns: A range containing all elements with keys equivalent to k.

Returns make_pair(b.end(), b.end()) if no such elements exist.

190

Complexity: Average case

O (b.count(k))

, worst case

O (b.size())

🔗

a_tran.equal_range(ke)

191

Result: pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant a_tran.

192

Returns: A range containing all elements with keys equivalent to ke.

Returns make_pair(a_tran.end(), a_tran.end()) if no such elements exist.

193

Complexity: Average case

O (a_tran.count(ke))

, worst case

O (a_tran.size())

🔗

b.bucket_count()

194

Result: size_type

195

Returns: The number of buckets that b contains.

196

Complexity: Constant.

🔗

b.max_bucket_count()

197

Result: size_type

198

Returns: An upper bound on the number of buckets that b can ever contain.

199

Complexity: Constant.

🔗

b.bucket(k)

200

Result: size_type

201

Preconditions: b.bucket_count() > 0.

202

Returns: The index of the bucket in which elements with keys equivalent to k would be found, if any such element existed.

The return value is in the range [0, b.bucket_count()).

203

Complexity: Constant.

🔗

a_tran.bucket(ke)

204

Result: size_type

205

Preconditions: a_tran.bucket_count() > 0.

206

Postconditions: The return value is in the range [0, a_tran.bucket_count()).

207

Returns: The index of the bucket in which elements with keys equivalent to ke would be found, if any such element existed.

208

Complexity: Constant.

🔗

b.bucket_size(n)

209

Result: size_type

210

Preconditions: n shall be in the range [0, b.bucket_count()).

211

Returns: The number of elements in the

n^{th}

bucket.

212

Complexity:

O (b.bucket_size(n))

🔗

b.begin(n)

213

Result: local_iterator; const_local_iterator for constant b.

214

Preconditions: n is in the range [0, b.bucket_count()).

215

Returns: An iterator referring to the first element in the bucket.

If the bucket is empty, then b.begin(n) == b.end(n).

216

Complexity: Constant.

🔗

b.end(n)

217

Result: local_iterator; const_local_iterator for constant b.

218

Preconditions: n is in the range [0, b.bucket_count()).

219

Returns: An iterator which is the past-the-end value for the bucket.

220

Complexity: Constant.

🔗

b.cbegin(n)

221

Result: const_local_iterator

222

Preconditions: n shall be in the range [0, b.bucket_count()).

223

Returns: An iterator referring to the first element in the bucket.

If the bucket is empty, then b.cbegin(n) == b.cend(n).

224

Complexity: Constant.

🔗

b.cend(n)

225

Result: const_local_iterator

226

Preconditions: n is in the range [0, b.bucket_count()).

227

Returns: An iterator which is the past-the-end value for the bucket.

228

Complexity: Constant.

🔗

b.load_factor()

229

Result: float

230

Returns: The average number of elements per bucket.

231

Complexity: Constant.

🔗

b.max_load_factor()

232

Result: float

233

Returns: A positive number that the container attempts to keep the load factor less than or equal to.

The container automatically increases the number of buckets as necessary to keep the load factor below this number.

234

Complexity: Constant.

🔗

a.max_load_factor(z)

235

Result: void

236

Preconditions: z is positive.

May change the container's maximum load factor, using z as a hint.

237

Complexity: Constant.

🔗

a.rehash(n)

238

Result: void

239

Postconditions: a.bucket_count() >= a.size() / a.max_load_factor() and a.bucket_count() >= n.

240

Complexity: Average case linear in a.size(), worst case quadratic.

🔗

a.reserve(n)

241

Effects: Equivalent to a.rehash(ceil(n / a.max_load_factor())).

242

Two unordered containers a and b compare equal if a.size() == b.size() and, for every equivalent-key group [Ea1, Ea2) obtained from a.equal_range(Ea1), there exists an equivalent-key group [Eb1, Eb2) obtained from b.equal_range(Ea1), such that is_permutation(Ea1, Ea2, Eb1, Eb2) returns true.

For unordered_set and unordered_map, the complexity of operator== (i.e., the number of calls to the == operator of the value_type, to the predicate returned by key_eq(), and to the hasher returned by hash_function()) is proportional to N in the average case and to

N^{2}

in the worst case, where N is a.size().

For unordered_multiset and unordered_multimap, the complexity of operator== is proportional to

\sum E_{i}^{2}

in the average case and to

N^{2}

in the worst case, where N is a.size(), and

E_{i}

is the size of the

i^{th}

equivalent-key group in a.

However, if the respective elements of each corresponding pair of equivalent-key groups

E a_{i}

and

E b_{i}

are arranged in the same order (as is commonly the case, e.g., if a and b are unmodified copies of the same container), then the average-case complexity for unordered_multiset and unordered_multimap becomes proportional to N (but worst-case complexity remains

O (N^{2})

, e.g., for a pathologically bad hash function).

The behavior of a program that uses operator== or operator!= on unordered containers is undefined unless the Pred function object has the same behavior for both containers and the equality comparison function for Key is a refinement197 of the partition into equivalent-key groups produced by Pred.

243

The iterator types iterator and const_iterator of an unordered associative container are of at least the forward iterator category.

For unordered associative containers where the key type and value type are the same, both iterator and const_iterator are constant iterators.

244

The insert, insert_range, and emplace members shall not affect the validity of references to container elements, but may invalidate all iterators to the container.

The erase members shall invalidate only iterators and references to the erased elements, and preserve the relative order of the elements that are not erased.

245

The insert, insert_range, and emplace members shall not affect the validity of iterators if (N + n) <= z * B, where N is the number of elements in the container prior to the insert operation, n is the number of elements inserted, B is the container's bucket count, and z is the container's maximum load factor.

246

The extract members invalidate only iterators to the removed element, and preserve the relative order of the elements that are not erased; pointers and references to the removed element remain valid.

However, accessing the element through such pointers and references while the element is owned by a node_type is undefined behavior.

References and pointers to an element obtained while it is owned by a node_type are invalidated if the element is successfully inserted.

247

The member function templates find, count, equal_range, contains, extract, erase, and bucket shall not participate in overload resolution unless the qualified-ids Pred::is_transparent and Hash::is_transparent are both valid and denote types ([temp.deduct]).

248

A deduction guide for an unordered associative container shall not participate in overload resolution if any of the following are true:

(248.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(248.2)
It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.
(248.3)
It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.
(248.4)
It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

197)197)

Equality comparison is a refinement of partitioning if no two objects that compare equal fall into different partitions.

23.2.8.2 Exception safety guarantees [unord.req.except]

For unordered associative containers, no clear() function throws an exception.

erase(k) does not throw an exception unless that exception is thrown by the container's Hash or Pred object (if any).

For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an insert or emplace function inserting a single element, the insertion has no effect.

For unordered associative containers, no swap function throws an exception unless that exception is thrown by the swap of the container's Hash or Pred object (if any).

For unordered associative containers, if an exception is thrown from within a rehash() function other than by the container's hash function or comparison function, the rehash() function has no effect.

23.3 Sequence containers [sequences]

23.3.1 General [sequences.general]

The headers <array>, <deque>, <forward_list>, <inplace_vector>, <list>, and <vector> define class templates that meet the requirements for sequence containers.

The following exposition-only alias template may appear in deduction guides for sequence containers: template<class InputIterator> using iter-value-type = typename iterator_traits<InputIterator>::value_type; // exposition only

23.3.2 Header <array> synopsis [array.syn]

🔗

// mostly freestanding #include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [array], class template array template<class T, size_t N> struct array; // partially freestanding template<class T, size_t N> constexpr bool operator==(const array<T, N>& x, const array<T, N>& y); template<class T, size_t N> constexpr synth-three-way-result<T> operator<=>(const array<T, N>& x, const array<T, N>& y); // [array.special], specialized algorithms template<class T, size_t N> constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y))); // [array.creation], array creation functions template<class T, size_t N> constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]); template<class T, size_t N> constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]); // [array.tuple], tuple interface template<class T> struct tuple_size; template<size_t I, class T> struct tuple_element; template<class T, size_t N> struct tuple_size<array<T, N>>; template<size_t I, class T, size_t N> struct tuple_element<I, array<T, N>>; template<size_t I, class T, size_t N> constexpr T& get(array<T, N>&) noexcept; template<size_t I, class T, size_t N> constexpr T&& get(array<T, N>&&) noexcept; template<size_t I, class T, size_t N> constexpr const T& get(const array<T, N>&) noexcept; template<size_t I, class T, size_t N> constexpr const T&& get(const array<T, N>&&) noexcept; }

23.3.3 Class template array [array]

23.3.3.1 Overview [array.overview]

The header <array> defines a class template for storing fixed-size sequences of objects.

An array is a contiguous container .

An instance of array<T, N> stores N elements of type T, so that size() == N is an invariant.

An array is an aggregate that can be list-initialized with up to N elements whose types are convertible to T.

An array meets all of the requirements of a container ([container.reqmts]) and of a reversible container ([container.rev.reqmts]), except that a default constructed array object is not empty if

N > 0

An array meets some of the requirements of a sequence container .

Descriptions are provided here only for operations on array that are not described in one of these tables and for operations where there is additional semantic information.

array<T, N> is a structural type ([temp.param]) if T is a structural type.

Two values a1 and a2 of type array<T, N> are template-argument-equivalent if and only if each pair of corresponding elements in a1 and a2 are template-argument-equivalent.

The types iterator and const_iterator meet the constexpr iterator requirements.

🔗

namespace std { template<class T, size_t N> struct array { // types using value_type = T; using pointer = T*; using const_pointer = const T*; using reference = T&; using const_reference = const T&; using size_type = size_t; using difference_type = ptrdiff_t; using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; // no explicit construct/copy/destroy for aggregate type constexpr void fill(const T& u); constexpr void swap(array&) noexcept(is_nothrow_swappable_v<T>); // iterators constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; constexpr reverse_iterator rbegin() noexcept; constexpr const_reverse_iterator rbegin() const noexcept; constexpr reverse_iterator rend() noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept; // capacity constexpr bool empty() const noexcept; constexpr size_type size() const noexcept; constexpr size_type max_size() const noexcept; // element access constexpr reference operator[](size_type n); constexpr const_reference operator[](size_type n) const; constexpr reference at(size_type n); // freestanding-deleted constexpr const_reference at(size_type n) const; // freestanding-deleted constexpr reference front(); constexpr const_reference front() const; constexpr reference back(); constexpr const_reference back() const; constexpr T* data() noexcept; constexpr const T* data() const noexcept; }; template<class T, class... U> array(T, U...) -> array<T, 1 + sizeof...(U)>; }

23.3.3.2 Constructors, copy, and assignment [array.cons]

An array relies on the implicitly-declared special member functions ([class.default.ctor], [class.dtor], [class.copy.ctor]) to conform to the container requirements table in [container.requirements].

In addition to the requirements specified in the container requirements table, the implicitly-declared move constructor and move assignment operator for array require that T be Cpp17MoveConstructible or Cpp17MoveAssignable, respectively.

🔗

template<class T, class... U>
  array(T, U...) -> array<T, 1 + sizeof...(U)>;

Mandates: (is_same_v<T, U> && ...) is true.

23.3.3.3 Member functions [array.members]

🔗

constexpr size_type size() const noexcept;

Returns: N.

🔗

constexpr T* data() noexcept;
constexpr const T* data() const noexcept;

Returns: A pointer such that [data(), data() + size()) is a valid range.

For a non-empty array, data() == addressof(front()) is true.

🔗

constexpr void fill(const T& u);

Effects: As if by fill_n(begin(), N, u).

🔗

constexpr void swap(array& y) noexcept(is_nothrow_swappable_v<T>);

Effects: Equivalent to swap_ranges(begin(), end(), y.begin()).

[Note 1:

Unlike the swap function for other containers, array::swap takes linear time, can exit via an exception, and does not cause iterators to become associated with the other container.

— end note]

23.3.3.4 Specialized algorithms [array.special]

🔗

template<class T, size_t N>
  constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));

Constraints: N == 0 or is_swappable_v<T> is true.

Effects: As if by x.swap(y).

Complexity: Linear in N.

23.3.3.5 Zero-sized arrays [array.zero]

array shall provide support for the special case N == 0.

In the case that N == 0, begin() == end() == unique value.

The return value of data() is unspecified.

The effect of calling front() or back() for a zero-sized array is undefined.

Member function swap() shall have a non-throwing exception specification.

23.3.3.6 Array creation functions [array.creation]

🔗

template<class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);

Mandates: is_array_v<T> is false and is_constructible_v<remove_cv_t<T>, T&> is true.

Preconditions: T meets the Cpp17CopyConstructible requirements.

Returns: {{ a[0], …, a[N - 1] }}.

🔗

template<class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);

Mandates: is_array_v<T> is false and is_constructible_v<remove_cv_t<T>, T> is true.

Preconditions: T meets the Cpp17MoveConstructible requirements.

Returns: {{ std::move(a[0]), …, std::move(a[N - 1]) }}.

23.3.3.7 Tuple interface [array.tuple]

🔗

template<class T, size_t N>
  struct tuple_size<array<T, N>> : integral_constant<size_t, N> { };

🔗

template<size_t I, class T, size_t N>
  struct tuple_element<I, array<T, N>> {
    using type = T;
  };

Mandates: I < N is true.

🔗

template<size_t I, class T, size_t N>
  constexpr T& get(array<T, N>& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr T&& get(array<T, N>&& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr const T& get(const array<T, N>& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr const T&& get(const array<T, N>&& a) noexcept;

Mandates: I < N is true.

Returns: A reference to the

I^{th}

element of a, where indexing is zero-based.

23.3.4 Header <deque> synopsis [deque.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [deque], class template deque template<class T, class Allocator = allocator<T>> class deque; template<class T, class Allocator> bool operator==(const deque<T, Allocator>& x, const deque<T, Allocator>& y); template<class T, class Allocator> synth-three-way-result<T> operator<=>(const deque<T, Allocator>& x, const deque<T, Allocator>& y); template<class T, class Allocator> void swap(deque<T, Allocator>& x, deque<T, Allocator>& y) noexcept(noexcept(x.swap(y))); // [deque.erasure], erasure template<class T, class Allocator, class U = T> typename deque<T, Allocator>::size_type erase(deque<T, Allocator>& c, const U& value); template<class T, class Allocator, class Predicate> typename deque<T, Allocator>::size_type erase_if(deque<T, Allocator>& c, Predicate pred); namespace pmr { template<class T> using deque = std::deque<T, polymorphic_allocator<T>>; } }

23.3.5 Class template deque [deque]

23.3.5.1 Overview [deque.overview]

A deque is a sequence container that supports random access iterators .

In addition, it supports constant time insert and erase operations at the beginning or the end; insert and erase in the middle take linear time.

That is, a deque is especially optimized for pushing and popping elements at the beginning and end.

Storage management is handled automatically.

A deque meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), and of a sequence container, including the optional sequence container requirements ([sequence.reqmts]).

Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.

namespace std { template<class T, class Allocator = allocator<T>> class deque { public: // types using value_type = T; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; // [deque.cons], construct/copy/destroy deque() : deque(Allocator()) { } explicit deque(const Allocator&); explicit deque(size_type n, const Allocator& = Allocator()); deque(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator> deque(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range<T> R> deque(from_range_t, R&& rg, const Allocator& = Allocator()); deque(const deque& x); deque(deque&&); deque(const deque&, const type_identity_t<Allocator>&); deque(deque&&, const type_identity_t<Allocator>&); deque(initializer_list<T>, const Allocator& = Allocator()); ~deque(); deque& operator=(const deque& x); deque& operator=(deque&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value); deque& operator=(initializer_list<T>); template<class InputIterator> void assign(InputIterator first, InputIterator last); template<container-compatible-range<T> R> void assign_range(R&& rg); void assign(size_type n, const T& t); void assign(initializer_list<T>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // [deque.capacity], capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; void resize(size_type sz); void resize(size_type sz, const T& c); void shrink_to_fit(); // element access reference operator[](size_type n); const_reference operator[](size_type n) const; reference at(size_type n); const_reference at(size_type n) const; reference front(); const_reference front() const; reference back(); const_reference back() const; // [deque.modifiers], modifiers template<class... Args> reference emplace_front(Args&&... args); template<class... Args> reference emplace_back(Args&&... args); template<class... Args> iterator emplace(const_iterator position, Args&&... args); void push_front(const T& x); void push_front(T&& x); template<container-compatible-range<T> R> void prepend_range(R&& rg); void push_back(const T& x); void push_back(T&& x); template<container-compatible-range<T> R> void append_range(R&& rg); iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); iterator insert(const_iterator position, size_type n, const T& x); template<class InputIterator> iterator insert(const_iterator position, InputIterator first, InputIterator last); template<container-compatible-range<T> R> iterator insert_range(const_iterator position, R&& rg); iterator insert(const_iterator position, initializer_list<T>); void pop_front(); void pop_back(); iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void swap(deque&) noexcept(allocator_traits<Allocator>::is_always_equal::value); void clear() noexcept; }; template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>> deque(InputIterator, InputIterator, Allocator = Allocator()) -> deque<iter-value-type<InputIterator>, Allocator>; template<ranges::input_range R, class Allocator = allocator<ranges::range_value_t<R>>> deque(from_range_t, R&&, Allocator = Allocator()) -> deque<ranges::range_value_t<R>, Allocator>; }

23.3.5.2 Constructors, copy, and assignment [deque.cons]

🔗

explicit deque(const Allocator&);

Effects: Constructs an empty deque, using the specified allocator.

Complexity: Constant.

🔗

explicit deque(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into deque.

Effects: Constructs a deque with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

🔗

deque(size_type n, const T& value, const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into deque.

Effects: Constructs a deque with n copies of value, using the specified allocator.

Complexity: Linear in n.

🔗

template<class InputIterator>
  deque(InputIterator first, InputIterator last, const Allocator& = Allocator());

Effects: Constructs a deque equal to the range [first, last), using the specified allocator.

Complexity: Linear in distance(first, last).

🔗

template<container-compatible-range<T> R>
  deque(from_range_t, R&& rg, const Allocator& = Allocator());

Effects: Constructs a deque with the elements of the range rg, using the specified allocator.

Complexity: Linear in ranges::distance(rg).

23.3.5.3 Capacity [deque.capacity]

🔗

void resize(size_type sz);

Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into deque.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() default-inserted elements to the sequence.

🔗

void resize(size_type sz, const T& c);

Preconditions: T is Cpp17CopyInsertable into deque.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() copies of c to the sequence.

🔗

void shrink_to_fit();

Preconditions: T is Cpp17MoveInsertable into deque.

Effects: shrink_to_fit is a non-binding request to reduce memory use but does not change the size of the sequence.

[Note 1:

The request is non-binding to allow latitude for implementation-specific optimizations.

— end note]

If the size is equal to the old capacity, or if an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T, then there are no effects.

Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.

Remarks: If the size is not equal to the old capacity, then invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

23.3.5.4 Modifiers [deque.modifiers]

🔗

iterator insert(const_iterator position, const T& x);
iterator insert(const_iterator position, T&& x);
iterator insert(const_iterator position, size_type n, const T& x);
template<class InputIterator>
  iterator insert(const_iterator position,
                  InputIterator first, InputIterator last);
template<container-compatible-range<T> R>
  iterator insert_range(const_iterator position, R&& rg);
iterator insert(const_iterator position, initializer_list<T>);

template<class... Args> reference emplace_front(Args&&... args);
template<class... Args> reference emplace_back(Args&&... args);
template<class... Args> iterator emplace(const_iterator position, Args&&... args);
void push_front(const T& x);
void push_front(T&& x);
template<container-compatible-range<T> R>
  void prepend_range(R&& rg);
void push_back(const T& x);
void push_back(T&& x);
template<container-compatible-range<T> R>
  void append_range(R&& rg);

Effects: An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque.

An insertion at either end of the deque invalidates all the iterators to the deque, but has no effect on the validity of references to elements of the deque.

Complexity: The complexity is linear in the number of elements inserted plus the lesser of the distances to the beginning and end of the deque.

Inserting a single element at either the beginning or end of a deque always takes constant time and causes a single call to a constructor of T.

Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T, there are no effects.

If an exception is thrown while inserting a single element at either end, there are no effects.

Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T, the effects are unspecified.

🔗

iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);
void pop_front();
void pop_back();

Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements.

An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements.

An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque.

[Note 1:

pop_front and pop_back are erase operations.

— end note]

Throws: Nothing unless an exception is thrown by the assignment operator of T.

Complexity: The number of calls to the destructor of T is the same as the number of elements erased, but the number of calls to the assignment operator of T is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements.

23.3.5.5 Erasure [deque.erasure]

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template<class T, class Allocator, class U = T>
  typename deque<T, Allocator>::size_type
    erase(deque<T, Allocator>& c, const U& value);

Effects: Equivalent to: auto it = remove(c.begin(), c.end(), value); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;

🔗

template<class T, class Allocator, class Predicate>
  typename deque<T, Allocator>::size_type
    erase_if(deque<T, Allocator>& c, Predicate pred);

Effects: Equivalent to: auto it = remove_if(c.begin(), c.end(), pred); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;

23.3.6 Header <forward_list> synopsis [forward.list.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [forward.list], class template forward_list template<class T, class Allocator = allocator<T>> class forward_list; template<class T, class Allocator> bool operator==(const forward_list<T, Allocator>& x, const forward_list<T, Allocator>& y); template<class T, class Allocator> synth-three-way-result<T> operator<=>(const forward_list<T, Allocator>& x, const forward_list<T, Allocator>& y); template<class T, class Allocator> void swap(forward_list<T, Allocator>& x, forward_list<T, Allocator>& y) noexcept(noexcept(x.swap(y))); // [forward.list.erasure], erasure template<class T, class Allocator, class U = T> typename forward_list<T, Allocator>::size_type erase(forward_list<T, Allocator>& c, const U& value); template<class T, class Allocator, class Predicate> typename forward_list<T, Allocator>::size_type erase_if(forward_list<T, Allocator>& c, Predicate pred); namespace pmr { template<class T> using forward_list = std::forward_list<T, polymorphic_allocator<T>>; } }

23.3.7 Class template forward_list [forward.list]

23.3.7.1 Overview [forward.list.overview]

A forward_list is a container that supports forward iterators and allows constant time insert and erase operations anywhere within the sequence, with storage management handled automatically.

Fast random access to list elements is not supported.

[Note 1:

It is intended that forward_list have zero space or time overhead relative to a hand-written C-style singly linked list.

Features that would conflict with that goal have been omitted.

— end note]

A forward_list meets all of the requirements of a container ([container.reqmts]), except that the size() member function is not provided and operator== has linear complexity.

A forward_list also meets all of the requirements for an allocator-aware container ([container.alloc.reqmts]).

In addition, a forward_list provides the assign member functions and several of the optional sequence container requirements ([sequence.reqmts]).

Descriptions are provided here only for operations on forward_list that are not described in that table or for operations where there is additional semantic information.

[Note 2:

Modifying any list requires access to the element preceding the first element of interest, but in a forward_list there is no constant-time way to access a preceding element.

For this reason, erase_after and splice_after take fully-open ranges, not semi-open ranges.

— end note]

namespace std { template<class T, class Allocator = allocator<T>> class forward_list { public: // types using value_type = T; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] // [forward.list.cons], construct/copy/destroy forward_list() : forward_list(Allocator()) { } explicit forward_list(const Allocator&); explicit forward_list(size_type n, const Allocator& = Allocator()); forward_list(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator> forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range<T> R> forward_list(from_range_t, R&& rg, const Allocator& = Allocator()); forward_list(const forward_list& x); forward_list(forward_list&& x); forward_list(const forward_list& x, const type_identity_t<Allocator>&); forward_list(forward_list&& x, const type_identity_t<Allocator>&); forward_list(initializer_list<T>, const Allocator& = Allocator()); ~forward_list(); forward_list& operator=(const forward_list& x); forward_list& operator=(forward_list&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value); forward_list& operator=(initializer_list<T>); template<class InputIterator> void assign(InputIterator first, InputIterator last); template<container-compatible-range<T> R> void assign_range(R&& rg); void assign(size_type n, const T& t); void assign(initializer_list<T>); allocator_type get_allocator() const noexcept; // [forward.list.iter], iterators iterator before_begin() noexcept; const_iterator before_begin() const noexcept; iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cbefore_begin() const noexcept; const_iterator cend() const noexcept; // capacity bool empty() const noexcept; size_type max_size() const noexcept; // [forward.list.access], element access reference front(); const_reference front() const; // [forward.list.modifiers], modifiers template<class... Args> reference emplace_front(Args&&... args); void push_front(const T& x); void push_front(T&& x); template<container-compatible-range<T> R> void prepend_range(R&& rg); void pop_front(); template<class... Args> iterator emplace_after(const_iterator position, Args&&... args); iterator insert_after(const_iterator position, const T& x); iterator insert_after(const_iterator position, T&& x); iterator insert_after(const_iterator position, size_type n, const T& x); template<class InputIterator> iterator insert_after(const_iterator position, InputIterator first, InputIterator last); iterator insert_after(const_iterator position, initializer_list<T> il); template<container-compatible-range<T> R> iterator insert_range_after(const_iterator position, R&& rg); iterator erase_after(const_iterator position); iterator erase_after(const_iterator position, const_iterator last); void swap(forward_list&) noexcept(allocator_traits<Allocator>::is_always_equal::value); void resize(size_type sz); void resize(size_type sz, const value_type& c); void clear() noexcept; // [forward.list.ops], forward_list operations void splice_after(const_iterator position, forward_list& x); void splice_after(const_iterator position, forward_list&& x); void splice_after(const_iterator position, forward_list& x, const_iterator i); void splice_after(const_iterator position, forward_list&& x, const_iterator i); void splice_after(const_iterator position, forward_list& x, const_iterator first, const_iterator last); void splice_after(const_iterator position, forward_list&& x, const_iterator first, const_iterator last); size_type remove(const T& value); template<class Predicate> size_type remove_if(Predicate pred); size_type unique(); template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred); void merge(forward_list& x); void merge(forward_list&& x); template<class Compare> void merge(forward_list& x, Compare comp); template<class Compare> void merge(forward_list&& x, Compare comp); void sort(); template<class Compare> void sort(Compare comp); void reverse() noexcept; }; template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>> forward_list(InputIterator, InputIterator, Allocator = Allocator()) -> forward_list<iter-value-type<InputIterator>, Allocator>; template<ranges::input_range R, class Allocator = allocator<ranges::range_value_t<R>>> forward_list(from_range_t, R&&, Allocator = Allocator()) -> forward_list<ranges::range_value_t<R>, Allocator>; }

An incomplete type T may be used when instantiating forward_list if the allocator meets the allocator completeness requirements .

T shall be complete before any member of the resulting specialization of forward_list is referenced.

23.3.7.2 Constructors, copy, and assignment [forward.list.cons]

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explicit forward_list(const Allocator&);

Effects: Constructs an empty forward_list object using the specified allocator.

Complexity: Constant.

🔗

explicit forward_list(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into forward_list.

Effects: Constructs a forward_list object with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

🔗

forward_list(size_type n, const T& value, const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into forward_list.

Effects: Constructs a forward_list object with n copies of value using the specified allocator.

Complexity: Linear in n.

🔗

template<class InputIterator>
  forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator());

Effects: Constructs a forward_list object equal to the range [first, last).

Complexity: Linear in distance(first, last).

🔗

template<container-compatible-range<T> R>
  forward_list(from_range_t, R&& rg, const Allocator& = Allocator());

Effects: Constructs a forward_list object with the elements of the range rg.

Complexity: Linear in ranges::distance(rg).

23.3.7.3 Iterators [forward.list.iter]

🔗

iterator before_begin() noexcept;
const_iterator before_begin() const noexcept;
const_iterator cbefore_begin() const noexcept;

Effects: cbefore_begin() is equivalent to const_cast<forward_list const&>(*this).before_begin().

Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by begin().

Remarks: before_begin() == end() shall equal false.

23.3.7.4 Element access [forward.list.access]

🔗

reference front();
const_reference front() const;

Returns: *begin()

23.3.7.5 Modifiers [forward.list.modifiers]

The member functions in this subclause do not affect the validity of iterators and references when inserting elements, and when erasing elements invalidate iterators and references to the erased elements only.

If an exception is thrown by any of these member functions there is no effect on the container.

Inserting n elements into a forward_list is linear in n, and the number of calls to the copy or move constructor of T is exactly equal to n.

Erasing n elements from a forward_list is linear in n and the number of calls to the destructor of type T is exactly equal to n.

🔗

template<class... Args> reference emplace_front(Args&&... args);

Effects: Inserts an object of type value_type constructed with value_type(std::forward<Args>(args)...) at the beginning of the list.

🔗

void push_front(const T& x);
void push_front(T&& x);

Effects: Inserts a copy of x at the beginning of the list.

🔗

template<container-compatible-range<T> R>
  void prepend_range(R&& rg);

Effects: Inserts a copy of each element of rg at the beginning of the list.

[Note 1:

The order of elements is not reversed.

— end note]

🔗

void pop_front();

Effects: As if by erase_after(before_begin()).

🔗

iterator insert_after(const_iterator position, const T& x);

Preconditions: T is Cpp17CopyInsertable into forward_list.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts a copy of x after position.

Returns: An iterator pointing to the copy of x.

🔗

iterator insert_after(const_iterator position, T&& x);

Preconditions: T is Cpp17MoveInsertable into forward_list.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts a copy of x after position.

Returns: An iterator pointing to the copy of x.

🔗

iterator insert_after(const_iterator position, size_type n, const T& x);

Preconditions: T is Cpp17CopyInsertable into forward_list.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts n copies of x after position.

Returns: An iterator pointing to the last inserted copy of x, or position if n == 0 is true.

🔗

template<class InputIterator>
  iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

Preconditions: T is Cpp17EmplaceConstructible into forward_list from *first.

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Neither first nor last are iterators in *this.

Effects: Inserts copies of elements in [first, last) after position.

Returns: An iterator pointing to the last inserted element, or position if first == last is true.

🔗

template<container-compatible-range<T> R>
  iterator insert_range_after(const_iterator position, R&& rg);

Preconditions: T is Cpp17EmplaceConstructible into forward_list from *ranges::begin(rg).

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

rg and *this do not overlap.

Effects: Inserts copies of elements in the range rg after position.

Returns: An iterator pointing to the last inserted element, or position if rg is empty.

🔗

iterator insert_after(const_iterator position, initializer_list<T> il);

Effects: Equivalent to: return insert_after(position, il.begin(), il.end());

🔗

template<class... Args>
  iterator emplace_after(const_iterator position, Args&&... args);

Preconditions: T is Cpp17EmplaceConstructible into forward_list from std::forward<Args>(args)....

position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts an object of type value_type direct-non-list-initialized with std::forward<Args>(args)... after position.

Returns: An iterator pointing to the new object.

🔗

iterator erase_after(const_iterator position);

Preconditions: The iterator following position is dereferenceable.

Effects: Erases the element pointed to by the iterator following position.

Returns: An iterator pointing to the element following the one that was erased, or end() if no such element exists.

Throws: Nothing.

🔗

iterator erase_after(const_iterator position, const_iterator last);

Preconditions: All iterators in the range (position, last) are dereferenceable.

Effects: Erases the elements in the range (position, last).

Returns: last.

Throws: Nothing.

🔗

void resize(size_type sz);

Preconditions: T is Cpp17DefaultInsertable into forward_list.

Effects: If sz < distance(begin(), end()), erases the last distance(begin(), end()) - sz elements from the list.

Otherwise, inserts sz - distance(begin(), end()) default-inserted elements at the end of the list.

🔗

void resize(size_type sz, const value_type& c);

Preconditions: T is Cpp17CopyInsertable into forward_list.

Effects: If sz < distance(begin(), end()), erases the last distance(begin(), end()) - sz elements from the list.

Otherwise, inserts sz - distance(begin(), end()) copies of c at the end of the list.

🔗

void clear() noexcept;

Effects: Erases all elements in the range [begin(), end()).

Remarks: Does not invalidate past-the-end iterators.

23.3.7.6 Operations [forward.list.ops]

In this subclause, arguments for a template parameter named Predicate or BinaryPredicate shall meet the corresponding requirements in [algorithms.requirements].

The semantics of i + n, where i is an iterator into the list and n is an integer, are the same as those of next(i, n).

The expression i - n, where i is an iterator into the list and n is an integer, means an iterator j such that j + n == i is true.

For merge and sort, the definitions and requirements in [alg.sorting] apply.

🔗

void splice_after(const_iterator position, forward_list& x);
void splice_after(const_iterator position, forward_list&& x);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

get_allocator() == x.get_allocator() is true.

addressof(x) != this is true.

Effects: Inserts the contents of x after position, and x becomes empty.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity:

O (distance(x.begin(), x.end()))

🔗

void splice_after(const_iterator position, forward_list& x, const_iterator i);
void splice_after(const_iterator position, forward_list&& x, const_iterator i);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

The iterator following i is a dereferenceable iterator in x.

get_allocator() == x.get_allocator() is true.

Effects: Inserts the element following i into *this, following position, and removes it from x.

The result is unchanged if position == i or position == ++i.

Pointers and references to *++i continue to refer to the same element but as a member of *this.

Iterators to *++i continue to refer to the same element, but now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity:

O (1)

🔗

void splice_after(const_iterator position, forward_list& x,
                  const_iterator first, const_iterator last);
void splice_after(const_iterator position, forward_list&& x,
                  const_iterator first, const_iterator last);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

(first, last) is a valid range in x, and all iterators in the range (first, last) are dereferenceable.

position is not an iterator in the range (first, last).

get_allocator() == x.get_allocator() is true.

Effects: Inserts elements in the range (first, last) after position and removes the elements from x.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Complexity:

O (distance(first, last))

🔗

size_type remove(const T& value);
template<class Predicate> size_type remove_if(Predicate pred);

Effects: Erases all the elements in the list referred to by a list iterator i for which the following conditions hold: *i == value (for remove()), pred(*i) is true (for remove_if()).

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

Complexity: Exactly distance(begin(), end()) applications of the corresponding predicate.

Remarks: Stable .

🔗

size_type unique();
template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred);

Let binary_pred be equal_to<>{} for the first overload.

Preconditions: binary_pred is an equivalence relation.

Effects: Erases all but the first element from every consecutive group of equivalent elements.

That is, for a nonempty list, erases all elements referred to by the iterator i in the range [begin() + 1, end()) for which binary_pred(*i, *(i - 1)) is true.

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by the predicate.

Complexity: If empty() is false, exactly distance(begin(), end()) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.

🔗

void merge(forward_list& x);
void merge(forward_list&& x);
template<class Compare> void merge(forward_list& x, Compare comp);
template<class Compare> void merge(forward_list&& x, Compare comp);

Let comp be less<> for the first two overloads.

Preconditions: *this and x are both sorted with respect to the comparator comp, and get_allocator() == x.get_allocator() is true.

Effects: If addressof(x) == this, there are no effects.

Otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()).

The result is a range that is sorted with respect to the comparator comp.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Complexity: At most distance(begin(), end()) + distance(x.begin(), x.end()) - 1 comparisons if addressof(x) != this; otherwise, no comparisons are performed.

Remarks: Stable ([algorithm.stable]).

If addressof(x) != this, x is empty after the merge.

No elements are copied by this operation.

If an exception is thrown other than by a comparison, there are no effects.

🔗

void sort();
template<class Compare> void sort(Compare comp);

Effects: Sorts the list according to the operator< or the comp function object.

If an exception is thrown, the order of the elements in *this is unspecified.

Does not affect the validity of iterators and references.

Complexity: Approximately

N log N

comparisons, where N is distance(begin(), end()).

Remarks: Stable .

🔗

void reverse() noexcept;

Effects: Reverses the order of the elements in the list.

Does not affect the validity of iterators and references.

Complexity: Linear time.

23.3.7.7 Erasure [forward.list.erasure]

🔗

template<class T, class Allocator, class U = T>
  typename forward_list<T, Allocator>::size_type
    erase(forward_list<T, Allocator>& c, const U& value);

Effects: Equivalent to: return erase_if(c, [&](const auto& elem) -> bool { return elem == value; });

🔗

template<class T, class Allocator, class Predicate>
  typename forward_list<T, Allocator>::size_type
    erase_if(forward_list<T, Allocator>& c, Predicate pred);

Effects: Equivalent to: return c.remove_if(pred);

23.3.8 Header <list> synopsis [list.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [list], class template list template<class T, class Allocator = allocator<T>> class list; template<class T, class Allocator> bool operator==(const list<T, Allocator>& x, const list<T, Allocator>& y); template<class T, class Allocator> synth-three-way-result<T> operator<=>(const list<T, Allocator>& x, const list<T, Allocator>& y); template<class T, class Allocator> void swap(list<T, Allocator>& x, list<T, Allocator>& y) noexcept(noexcept(x.swap(y))); // [list.erasure], erasure template<class T, class Allocator, class U = T> typename list<T, Allocator>::size_type erase(list<T, Allocator>& c, const U& value); template<class T, class Allocator, class Predicate> typename list<T, Allocator>::size_type erase_if(list<T, Allocator>& c, Predicate pred); namespace pmr { template<class T> using list = std::list<T, polymorphic_allocator<T>>; } }

23.3.9 Class template list [list]

23.3.9.1 Overview [list.overview]

A list is a sequence container that supports bidirectional iterators and allows constant time insert and erase operations anywhere within the sequence, with storage management handled automatically.

Unlike vectors and deques, fast random access to list elements is not supported, but many algorithms only need sequential access anyway.

A list meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), and of a sequence container, including most of the optional sequence container requirements ([sequence.reqmts]).

The exceptions are the operator[] and at member functions, which are not provided.198

Descriptions are provided here only for operations on list that are not described in one of these tables or for operations where there is additional semantic information.

namespace std { template<class T, class Allocator = allocator<T>> class list { public: // types using value_type = T; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; // [list.cons], construct/copy/destroy list() : list(Allocator()) { } explicit list(const Allocator&); explicit list(size_type n, const Allocator& = Allocator()); list(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator> list(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range<T> R> list(from_range_t, R&& rg, const Allocator& = Allocator()); list(const list& x); list(list&& x); list(const list&, const type_identity_t<Allocator>&); list(list&&, const type_identity_t<Allocator>&); list(initializer_list<T>, const Allocator& = Allocator()); ~list(); list& operator=(const list& x); list& operator=(list&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value); list& operator=(initializer_list<T>); template<class InputIterator> void assign(InputIterator first, InputIterator last); template<container-compatible-range<T> R> void assign_range(R&& rg); void assign(size_type n, const T& t); void assign(initializer_list<T>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // [list.capacity], capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; void resize(size_type sz); void resize(size_type sz, const T& c); // element access reference front(); const_reference front() const; reference back(); const_reference back() const; // [list.modifiers], modifiers template<class... Args> reference emplace_front(Args&&... args); template<class... Args> reference emplace_back(Args&&... args); void push_front(const T& x); void push_front(T&& x); template<container-compatible-range<T> R> void prepend_range(R&& rg); void pop_front(); void push_back(const T& x); void push_back(T&& x); template<container-compatible-range<T> R> void append_range(R&& rg); void pop_back(); template<class... Args> iterator emplace(const_iterator position, Args&&... args); iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); iterator insert(const_iterator position, size_type n, const T& x); template<class InputIterator> iterator insert(const_iterator position, InputIterator first, InputIterator last); template<container-compatible-range<T> R> iterator insert_range(const_iterator position, R&& rg); iterator insert(const_iterator position, initializer_list<T> il); iterator erase(const_iterator position); iterator erase(const_iterator position, const_iterator last); void swap(list&) noexcept(allocator_traits<Allocator>::is_always_equal::value); void clear() noexcept; // [list.ops], list operations void splice(const_iterator position, list& x); void splice(const_iterator position, list&& x); void splice(const_iterator position, list& x, const_iterator i); void splice(const_iterator position, list&& x, const_iterator i); void splice(const_iterator position, list& x, const_iterator first, const_iterator last); void splice(const_iterator position, list&& x, const_iterator first, const_iterator last); size_type remove(const T& value); template<class Predicate> size_type remove_if(Predicate pred); size_type unique(); template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred); void merge(list& x); void merge(list&& x); template<class Compare> void merge(list& x, Compare comp); template<class Compare> void merge(list&& x, Compare comp); void sort(); template<class Compare> void sort(Compare comp); void reverse() noexcept; }; template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>> list(InputIterator, InputIterator, Allocator = Allocator()) -> list<iter-value-type<InputIterator>, Allocator>; template<ranges::input_range R, class Allocator = allocator<ranges::range_value_t<R>>> list(from_range_t, R&&, Allocator = Allocator()) -> list<ranges::range_value_t<R>, Allocator>; }

An incomplete type T may be used when instantiating list if the allocator meets the allocator completeness requirements .

T shall be complete before any member of the resulting specialization of list is referenced.

198)198)

These member functions are only provided by containers whose iterators are random access iterators.

23.3.9.2 Constructors, copy, and assignment [list.cons]

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explicit list(const Allocator&);

Effects: Constructs an empty list, using the specified allocator.

Complexity: Constant.

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explicit list(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into list.

Effects: Constructs a list with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

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list(size_type n, const T& value, const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into list.

Effects: Constructs a list with n copies of value, using the specified allocator.

Complexity: Linear in n.

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template<class InputIterator>
  list(InputIterator first, InputIterator last, const Allocator& = Allocator());

Effects: Constructs a list equal to the range [first, last).

Complexity: Linear in distance(first, last).

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template<container-compatible-range<T> R>
  list(from_range_t, R&& rg, const Allocator& = Allocator());

Effects: Constructs a list object with the elements of the range rg.

Complexity: Linear in ranges::distance(rg).

23.3.9.3 Capacity [list.capacity]

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void resize(size_type sz);

Preconditions: T is Cpp17DefaultInsertable into list.

Effects: If size() < sz, appends sz - size() default-inserted elements to the sequence.

If sz <= size(), equivalent to: list<T>::iterator it = begin(); advance(it, sz); erase(it, end());

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void resize(size_type sz, const T& c);

Preconditions: T is Cpp17CopyInsertable into list.

Effects: As if by: if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) { iterator i = begin(); advance(i, sz); erase(i, end()); } else ; // do nothing

23.3.9.4 Modifiers [list.modifiers]

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iterator insert(const_iterator position, const T& x);
iterator insert(const_iterator position, T&& x);
iterator insert(const_iterator position, size_type n, const T& x);
template<class InputIterator>
  iterator insert(const_iterator position, InputIterator first,
                  InputIterator last);
template<container-compatible-range<T> R>
  iterator insert_range(const_iterator position, R&& rg);
iterator insert(const_iterator position, initializer_list<T>);

template<class... Args> reference emplace_front(Args&&... args);
template<class... Args> reference emplace_back(Args&&... args);
template<class... Args> iterator emplace(const_iterator position, Args&&... args);
void push_front(const T& x);
void push_front(T&& x);
template<container-compatible-range<T> R>
  void prepend_range(R&& rg);
void push_back(const T& x);
void push_back(T&& x);
template<container-compatible-range<T> R>
  void append_range(R&& rg);

Complexity: Insertion of a single element into a list takes constant time and exactly one call to a constructor of T.

Insertion of multiple elements into a list is linear in the number of elements inserted, and the number of calls to the copy constructor or move constructor of T is exactly equal to the number of elements inserted.

Remarks: Does not affect the validity of iterators and references.

If an exception is thrown, there are no effects.

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iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);
void pop_front();
void pop_back();
void clear() noexcept;

Effects: Invalidates only the iterators and references to the erased elements.

Throws: Nothing.

Complexity: Erasing a single element is a constant time operation with a single call to the destructor of T.

Erasing a range in a list is linear time in the size of the range and the number of calls to the destructor of type T is exactly equal to the size of the range.

23.3.9.5 Operations [list.ops]

Since lists allow fast insertion and erasing from the middle of a list, certain operations are provided specifically for them.199

In this subclause, arguments for a template parameter named Predicate or BinaryPredicate shall meet the corresponding requirements in [algorithms.requirements].

The semantics of i + n and i - n, where i is an iterator into the list and n is an integer, are the same as those of next(i, n) and prev(i, n), respectively.

For merge and sort, the definitions and requirements in [alg.sorting] apply.

list provides three splice operations that destructively move elements from one list to another.

The behavior of splice operations is undefined if get_allocator() != x.get_allocator().

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void splice(const_iterator position, list& x);
void splice(const_iterator position, list&& x);

Preconditions: addressof(x) != this is true.

Effects: Inserts the contents of x before position and x becomes empty.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity: Constant time.

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void splice(const_iterator position, list& x, const_iterator i);
void splice(const_iterator position, list&& x, const_iterator i);

Preconditions: i is a valid dereferenceable iterator of x.

Effects: Inserts an element pointed to by i from list x before position and removes the element from x.

The result is unchanged if position == i or position == ++i.

Pointers and references to *i continue to refer to this same element but as a member of *this.

Iterators to *i (including i itself) continue to refer to the same element, but now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity: Constant time.

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void splice(const_iterator position, list& x, const_iterator first,
            const_iterator last);
void splice(const_iterator position, list&& x, const_iterator first,
            const_iterator last);

Preconditions: [first, last) is a valid range in x.

position is not an iterator in the range [first, last).

Effects: Inserts elements in the range [first, last) before position and removes the elements from x.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity: Constant time if addressof(x) == this; otherwise, linear time.

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size_type remove(const T& value);
template<class Predicate> size_type remove_if(Predicate pred);

Effects: Erases all the elements in the list referred to by a list iterator i for which the following conditions hold: *i == value, pred(*i) != false.

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by *i == value or pred(*i) != false.

Complexity: Exactly size() applications of the corresponding predicate.

Remarks: Stable .

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size_type unique();
template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred);

Let binary_pred be equal_to<>{} for the first overload.

Preconditions: binary_pred is an equivalence relation.

Effects: Erases all but the first element from every consecutive group of equivalent elements.

That is, for a nonempty list, erases all elements referred to by the iterator i in the range [begin() + 1, end()) for which binary_pred(*i, *(i - 1)) is true.

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by the predicate.

Complexity: If empty() is false, exactly size() - 1 applications of the corresponding predicate, otherwise no applications of the predicate.

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void merge(list& x);
void merge(list&& x);
template<class Compare> void merge(list& x, Compare comp);
template<class Compare> void merge(list&& x, Compare comp);

Let comp be less<> for the first two overloads.

Preconditions: *this and x are both sorted with respect to the comparator comp, and get_allocator() == x.get_allocator() is true.

Effects: If addressof(x) == this, there are no effects.

Otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()).

The result is a range that is sorted with respect to the comparator comp.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Complexity: At most size() + x.size() - 1 comparisons if addressof(x) != this; otherwise, no comparisons are performed.

Remarks: Stable ([algorithm.stable]).

If addressof(x) != this, x is empty after the merge.

No elements are copied by this operation.

If an exception is thrown other than by a comparison, there are no effects.

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void reverse() noexcept;

Effects: Reverses the order of the elements in the list.

Does not affect the validity of iterators and references.

Complexity: Linear time.

🔗

void sort();
template<class Compare> void sort(Compare comp);

Effects: Sorts the list according to the operator< or a Compare function object.

If an exception is thrown, the order of the elements in *this is unspecified.

Does not affect the validity of iterators and references.

Complexity: Approximately

N log N

comparisons, where N == size().

Remarks: Stable .

199)199)

As specified in [allocator.requirements], the requirements in this Clause apply only to lists whose allocators compare equal.

23.3.9.6 Erasure [list.erasure]

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template<class T, class Allocator, class U = T>
  typename list<T, Allocator>::size_type
    erase(list<T, Allocator>& c, const U& value);

Effects: Equivalent to: return erase_if(c, [&](const auto& elem) -> bool { return elem == value; });

🔗

template<class T, class Allocator, class Predicate>
  typename list<T, Allocator>::size_type
    erase_if(list<T, Allocator>& c, Predicate pred);

Effects: Equivalent to: return c.remove_if(pred);

23.3.10 Header <vector> synopsis [vector.syn]

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#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [vector], class template vector template<class T, class Allocator = allocator<T>> class vector; template<class T, class Allocator> constexpr bool operator==(const vector<T, Allocator>& x, const vector<T, Allocator>& y); template<class T, class Allocator> constexpr synth-three-way-result<T> operator<=>(const vector<T, Allocator>& x, const vector<T, Allocator>& y); template<class T, class Allocator> constexpr void swap(vector<T, Allocator>& x, vector<T, Allocator>& y) noexcept(noexcept(x.swap(y))); // [vector.erasure], erasure template<class T, class Allocator, class U = T> constexpr typename vector<T, Allocator>::size_type erase(vector<T, Allocator>& c, const U& value); template<class T, class Allocator, class Predicate> constexpr typename vector<T, Allocator>::size_type erase_if(vector<T, Allocator>& c, Predicate pred); namespace pmr { template<class T> using vector = std::vector<T, polymorphic_allocator<T>>; } // [vector.bool], specialization of vector for bool // [vector.bool.pspc], partial class template specialization vector<bool, Allocator> template<class Allocator> class vector<bool, Allocator>; template<class T> constexpr bool is-vector-bool-reference = see below; // exposition only // hash support template<class T> struct hash; template<class Allocator> struct hash<vector<bool, Allocator>>; // [vector.bool.fmt], formatter specialization for vector<bool> template<class T, class charT> requires is-vector-bool-reference<T> struct formatter<T, charT>; }

23.3.11 Class template vector [vector]

23.3.11.1 Overview [vector.overview]

A vector is a sequence container that supports (amortized) constant time insert and erase operations at the end; insert and erase in the middle take linear time.

Storage management is handled automatically, though hints can be given to improve efficiency.

A vector meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), of a sequence container, including most of the optional sequence container requirements ([sequence.reqmts]), and, for an element type other than bool, of a contiguous container .

The exceptions are the push_front, prepend_range, pop_front, and emplace_front member functions, which are not provided.

Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.

The types iterator and const_iterator meet the constexpr iterator requirements ([iterator.requirements.general]).

namespace std { template<class T, class Allocator = allocator<T>> class vector { public: // types using value_type = T; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; // [vector.cons], construct/copy/destroy constexpr vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { } constexpr explicit vector(const Allocator&) noexcept; constexpr explicit vector(size_type n, const Allocator& = Allocator()); constexpr vector(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator> constexpr vector(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range<T> R> constexpr vector(from_range_t, R&& rg, const Allocator& = Allocator()); constexpr vector(const vector& x); constexpr vector(vector&&) noexcept; constexpr vector(const vector&, const type_identity_t<Allocator>&); constexpr vector(vector&&, const type_identity_t<Allocator>&); constexpr vector(initializer_list<T>, const Allocator& = Allocator()); constexpr ~vector(); constexpr vector& operator=(const vector& x); constexpr vector& operator=(vector&& x) noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value); constexpr vector& operator=(initializer_list<T>); template<class InputIterator> constexpr void assign(InputIterator first, InputIterator last); template<container-compatible-range<T> R> constexpr void assign_range(R&& rg); constexpr void assign(size_type n, const T& u); constexpr void assign(initializer_list<T>); constexpr allocator_type get_allocator() const noexcept; // iterators constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; constexpr reverse_iterator rbegin() noexcept; constexpr const_reverse_iterator rbegin() const noexcept; constexpr reverse_iterator rend() noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept; // [vector.capacity], capacity constexpr bool empty() const noexcept; constexpr size_type size() const noexcept; constexpr size_type max_size() const noexcept; constexpr size_type capacity() const noexcept; constexpr void resize(size_type sz); constexpr void resize(size_type sz, const T& c); constexpr void reserve(size_type n); constexpr void shrink_to_fit(); // element access constexpr reference operator[](size_type n); constexpr const_reference operator[](size_type n) const; constexpr reference at(size_type n); constexpr const_reference at(size_type n) const; constexpr reference front(); constexpr const_reference front() const; constexpr reference back(); constexpr const_reference back() const; // [vector.data], data access constexpr T* data() noexcept; constexpr const T* data() const noexcept; // [vector.modifiers], modifiers template<class... Args> constexpr reference emplace_back(Args&&... args); constexpr void push_back(const T& x); constexpr void push_back(T&& x); template<container-compatible-range<T> R> constexpr void append_range(R&& rg); constexpr void pop_back(); template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args); constexpr iterator insert(const_iterator position, const T& x); constexpr iterator insert(const_iterator position, T&& x); constexpr iterator insert(const_iterator position, size_type n, const T& x); template<class InputIterator> constexpr iterator insert(const_iterator position, InputIterator first, InputIterator last); template<container-compatible-range<T> R> constexpr iterator insert_range(const_iterator position, R&& rg); constexpr iterator insert(const_iterator position, initializer_list<T> il); constexpr iterator erase(const_iterator position); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void swap(vector&) noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value || allocator_traits<Allocator>::is_always_equal::value); constexpr void clear() noexcept; }; template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>> vector(InputIterator, InputIterator, Allocator = Allocator()) -> vector<iter-value-type<InputIterator>, Allocator>; template<ranges::input_range R, class Allocator = allocator<ranges::range_value_t<R>>> vector(from_range_t, R&&, Allocator = Allocator()) -> vector<ranges::range_value_t<R>, Allocator>; }

An incomplete type T may be used when instantiating vector if the allocator meets the allocator completeness requirements .

T shall be complete before any member of the resulting specialization of vector is referenced.

23.3.11.2 Constructors [vector.cons]

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constexpr explicit vector(const Allocator&) noexcept;

Effects: Constructs an empty vector, using the specified allocator.

Complexity: Constant.

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constexpr explicit vector(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into vector.

Effects: Constructs a vector with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

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constexpr vector(size_type n, const T& value,
                 const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into vector.

Effects: Constructs a vector with n copies of value, using the specified allocator.

Complexity: Linear in n.

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template<class InputIterator>
  constexpr vector(InputIterator first, InputIterator last,
                   const Allocator& = Allocator());

Effects: Constructs a vector equal to the range [first, last), using the specified allocator.

Complexity: Makes only N calls to the copy constructor of T (where N is the distance between first and last) and no reallocations if iterators first and last are of forward, bidirectional, or random access categories.

It makes order N calls to the copy constructor of T and order

log N

reallocations if they are just input iterators.

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template<container-compatible-range<T> R>
  constexpr vector(from_range_t, R&& rg, const Allocator& = Allocator());

Effects: Constructs a vector object with the elements of the range rg, using the specified allocator.

Complexity: Initializes exactly N elements from the results of dereferencing successive iterators of rg, where N is ranges::distance(rg).

Performs no reallocations if R models ranges::forward_range or ranges::sized_range; otherwise, performs order

log N

reallocations and order N calls to the copy or move constructor of T.

23.3.11.3 Capacity [vector.capacity]

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constexpr size_type capacity() const noexcept;

Returns: The total number of elements that the vector can hold without requiring reallocation.

Complexity: Constant time.

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constexpr void reserve(size_type n);

Preconditions: T is Cpp17MoveInsertable into vector.

Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation accordingly.

After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise.

Reallocation happens at this point if and only if the current capacity is less than the argument of reserve().

If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable type, there are no effects.

Throws: length_error if n > max_size().200

Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence.

Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

[Note 1:

If no reallocation happens, they remain valid.

— end note]

No reallocation shall take place during insertions that happen after a call to reserve() until an insertion would make the size of the vector greater than the value of capacity().

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constexpr void shrink_to_fit();

Preconditions: T is Cpp17MoveInsertable into vector.

Effects: shrink_to_fit is a non-binding request to reduce capacity() to size().

[Note 2:

The request is non-binding to allow latitude for implementation-specific optimizations.

— end note]

It does not increase capacity(), but may reduce capacity() by causing reallocation.

If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T, there are no effects.

Complexity: If reallocation happens, linear in the size of the sequence.

Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator.

[Note 3:

If no reallocation happens, they remain valid.

— end note]

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constexpr void swap(vector& x)
  noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
           allocator_traits<Allocator>::is_always_equal::value);

Effects: Exchanges the contents and capacity() of *this with that of x.

Complexity: Constant time.

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constexpr void resize(size_type sz);

Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into vector.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() default-inserted elements to the sequence.

Remarks: If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T, there are no effects.

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constexpr void resize(size_type sz, const T& c);

Preconditions: T is Cpp17CopyInsertable into vector.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() copies of c to the sequence.

Remarks: If an exception is thrown, there are no effects.

200)200)

reserve() uses Allocator::allocate() which can throw an appropriate exception.

23.3.11.4 Data [vector.data]

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constexpr T*       data() noexcept;
constexpr const T* data() const noexcept;

Returns: A pointer such that [data(), data() + size()) is a valid range.

For a non-empty vector, data() == addressof(front()) is true.

Complexity: Constant time.

23.3.11.5 Modifiers [vector.modifiers]

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constexpr iterator insert(const_iterator position, const T& x);
constexpr iterator insert(const_iterator position, T&& x);
constexpr iterator insert(const_iterator position, size_type n, const T& x);
template<class InputIterator>
  constexpr iterator insert(const_iterator position, InputIterator first, InputIterator last);
template<container-compatible-range<T> R>
  constexpr iterator insert_range(const_iterator position, R&& rg);
constexpr iterator insert(const_iterator position, initializer_list<T>);

template<class... Args> constexpr reference emplace_back(Args&&... args);
template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args);
constexpr void push_back(const T& x);
constexpr void push_back(T&& x);
template<container-compatible-range<T> R>
  constexpr void append_range(R&& rg);

Complexity: If reallocation happens, linear in the number of elements of the resulting vector; otherwise, linear in the number of elements inserted plus the distance to the end of the vector.

Remarks: Causes reallocation if the new size is greater than the old capacity.

Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

If no reallocation happens, then references, pointers, and iterators before the insertion point remain valid but those at or after the insertion point, including the past-the-end iterator, are invalidated.

If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T or by any InputIterator operation, there are no effects.

If an exception is thrown while inserting a single element at the end and T is Cpp17CopyInsertable or is_nothrow_move_constructible_v<T> is true, there are no effects.

Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T, the effects are unspecified.

🔗

constexpr iterator erase(const_iterator position);
constexpr iterator erase(const_iterator first, const_iterator last);
constexpr void pop_back();

Effects: Invalidates iterators and references at or after the point of the erase.

Throws: Nothing unless an exception is thrown by the assignment operator or move assignment operator of T.

Complexity: The destructor of T is called the number of times equal to the number of the elements erased, but the assignment operator of T is called the number of times equal to the number of elements in the vector after the erased elements.

23.3.11.6 Erasure [vector.erasure]

🔗

template<class T, class Allocator, class U = T>
  constexpr typename vector<T, Allocator>::size_type
    erase(vector<T, Allocator>& c, const U& value);

Effects: Equivalent to: auto it = remove(c.begin(), c.end(), value); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;

🔗

template<class T, class Allocator, class Predicate>
  constexpr typename vector<T, Allocator>::size_type
    erase_if(vector<T, Allocator>& c, Predicate pred);

Effects: Equivalent to: auto it = remove_if(c.begin(), c.end(), pred); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;

23.3.12 Specialization of vector for bool [vector.bool]

23.3.12.1 Partial class template specialization vector<bool, Allocator> [vector.bool.pspc]

To optimize space allocation, a partial specialization of vector for bool elements is provided: namespace std { template<class Allocator> class vector<bool, Allocator> { public: // types using value_type = bool; using allocator_type = Allocator; using pointer = implementation-defined; using const_pointer = implementation-defined; using const_reference = bool; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; // bit reference class reference { public: constexpr reference(const reference&) = default; constexpr ~reference(); constexpr operator bool() const noexcept; constexpr reference& operator=(bool x) noexcept; constexpr reference& operator=(const reference& x) noexcept; constexpr const reference& operator=(bool x) const noexcept; constexpr void flip() noexcept; // flips the bit }; // construct/copy/destroy constexpr vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { } constexpr explicit vector(const Allocator&) noexcept; constexpr explicit vector(size_type n, const Allocator& = Allocator()); constexpr vector(size_type n, const bool& value, const Allocator& = Allocator()); template<class InputIterator> constexpr vector(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range<bool> R> constexpr vector(from_range_t, R&& rg, const Allocator& = Allocator()); constexpr vector(const vector& x); constexpr vector(vector&& x) noexcept; constexpr vector(const vector&, const type_identity_t<Allocator>&); constexpr vector(vector&&, const type_identity_t<Allocator>&); constexpr vector(initializer_list<bool>, const Allocator& = Allocator()); constexpr ~vector(); constexpr vector& operator=(const vector& x); constexpr vector& operator=(vector&& x) noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value); constexpr vector& operator=(initializer_list<bool>); template<class InputIterator> constexpr void assign(InputIterator first, InputIterator last); template<container-compatible-range<bool> R> constexpr void assign_range(R&& rg); constexpr void assign(size_type n, const bool& t); constexpr void assign(initializer_list<bool>); constexpr allocator_type get_allocator() const noexcept; // iterators constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; constexpr reverse_iterator rbegin() noexcept; constexpr const_reverse_iterator rbegin() const noexcept; constexpr reverse_iterator rend() noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept; // capacity constexpr bool empty() const noexcept; constexpr size_type size() const noexcept; constexpr size_type max_size() const noexcept; constexpr size_type capacity() const noexcept; constexpr void resize(size_type sz, bool c = false); constexpr void reserve(size_type n); constexpr void shrink_to_fit(); // element access constexpr reference operator[](size_type n); constexpr const_reference operator[](size_type n) const; constexpr reference at(size_type n); constexpr const_reference at(size_type n) const; constexpr reference front(); constexpr const_reference front() const; constexpr reference back(); constexpr const_reference back() const; // modifiers template<class... Args> constexpr reference emplace_back(Args&&... args); constexpr void push_back(const bool& x); template<container-compatible-range<bool> R> constexpr void append_range(R&& rg); constexpr void pop_back(); template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args); constexpr iterator insert(const_iterator position, const bool& x); constexpr iterator insert(const_iterator position, size_type n, const bool& x); template<class InputIterator> constexpr iterator insert(const_iterator position, InputIterator first, InputIterator last); template<container-compatible-range<bool> R> constexpr iterator insert_range(const_iterator position, R&& rg); constexpr iterator insert(const_iterator position, initializer_list<bool> il); constexpr iterator erase(const_iterator position); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void swap(vector&) noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value || allocator_traits<Allocator>::is_always_equal::value); static constexpr void swap(reference x, reference y) noexcept; constexpr void flip() noexcept; // flips all bits constexpr void clear() noexcept; }; }

Unless described below, all operations have the same requirements and semantics as the primary vector template, except that operations dealing with the bool value type map to bit values in the container storage and allocator_traits::construct is not used to construct these values.

There is no requirement that the data be stored as a contiguous allocation of bool values.

A space-optimized representation of bits is recommended instead.

reference is a class that simulates the behavior of references of a single bit in vector<bool>.

The conversion function returns true when the bit is set, and false otherwise.

The assignment operators set the bit when the argument is (convertible to) true and clear it otherwise.

flip reverses the state of the bit.

🔗

constexpr void flip() noexcept;

Effects: Replaces each element in the container with its complement.

🔗

static constexpr void swap(reference x, reference y) noexcept;

Effects: Exchanges the contents of x and y as if by: bool b = x; x = y; y = b;

🔗

template<class Allocator> struct hash<vector<bool, Allocator>>;

The specialization is enabled ([unord.hash]).

🔗

template<class T>
  constexpr bool is-vector-bool-reference = see below;

The expression is-vector-bool-reference<T> is true if T denotes the type vector<bool, Alloc>::reference for some type Alloc and vector<bool, Alloc> is not a program-defined specialization.

23.3.12.2 Formatter specialization for vector<bool> [vector.bool.fmt]

🔗

namespace std { template<class T, class charT> requires is-vector-bool-reference<T> struct formatter<T, charT> { private: formatter<bool, charT> underlying_; // exposition only public: template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(const T& ref, FormatContext& ctx) const; }; }

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Equivalent to: return underlying_.parse(ctx);

🔗

template<class FormatContext>
  typename FormatContext::iterator
    format(const T& ref, FormatContext& ctx) const;

Equivalent to: return underlying_.format(ref, ctx);

23.3.13 Header <inplace_vector> synopsis [inplace.vector.syn]

🔗

// mostly freestanding #include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [inplace.vector], class template inplace_vector template<class T, size_t N> class inplace_vector; // partially freestanding // [inplace.vector.erasure], erasure template<class T, size_t N, class U = T> constexpr typename inplace_vector<T, N>::size_type erase(inplace_vector<T, N>& c, const U& value); template<class T, size_t N, class Predicate> constexpr typename inplace_vector<T, N>::size_type erase_if(inplace_vector<T, N>& c, Predicate pred); }

23.3.14 Class template inplace_vector [inplace.vector]

23.3.14.1 Overview [inplace.vector.overview]

An inplace_vector is a contiguous container.

Its capacity is fixed and its elements are stored within the inplace_vector object itself.

An inplace_vector meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of a contiguous container, and of a sequence container, including most of the optional sequence container requirements ([sequence.reqmts]).

The exceptions are the push_front, prepend_range, pop_front, and emplace_front member functions, which are not provided.

Descriptions are provided here only for operations on inplace_vector that are not described in one of these tables or for operations where there is additional semantic information.

For any N, inplace_vector<T, N>::iterator and inplace_vector<T, N>::const_iterator meet the constexpr iterator requirements.

For any

N > 0

, if T is not trivially copyable or is_trivially_default_constructible_v<T> is false, then no inplace_vector<T, N> member functions are usable in constant expressions.

Any member function of inplace_vector<T, N> that would cause the size to exceed N throws an exception of type bad_alloc.

Let IV denote a specialization of inplace_vector<T, N>.

If N is zero, then IV is trivially copyable and empty, and std::is_trivially_default_constructible_v<IV> is true.

Otherwise:

(6.1)
If is_trivially_copy_constructible_v<T> is true, then IV has a trivial copy constructor.
(6.2)
If is_trivially_move_constructible_v<T> is true, then IV has a trivial move constructor.
(6.3)
If is_trivially_destructible_v<T> is true, then:
- (6.3.1)
 IV has a trivial destructor.
- (6.3.2)
 If is_trivially_copy_constructible_v<T> && is_trivially_copy_assignable_v<T> is true, then IV has a trivial copy assignment operator.
- (6.3.3)
 If is_trivially_move_constructible_v<T> && is_trivially_move_assignable_v<T> is true, then IV has a trivial move assignment operator.

namespace std { template<class T, size_t N> class inplace_vector { public: // types: using value_type = T; using pointer = T*; using const_pointer = const T*; using reference = value_type&; using const_reference = const value_type&; using size_type = size_t; using difference_type = ptrdiff_t; using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; // [inplace.vector.cons], construct/copy/destroy constexpr inplace_vector() noexcept; constexpr explicit inplace_vector(size_type n); // freestanding-deleted constexpr inplace_vector(size_type n, const T& value); // freestanding-deleted template<class InputIterator> constexpr inplace_vector(InputIterator first, InputIterator last); // freestanding-deleted template<container-compatible-range<T> R> constexpr inplace_vector(from_range_t, R&& rg); // freestanding-deleted constexpr inplace_vector(const inplace_vector&); constexpr inplace_vector(inplace_vector&&) noexcept(N == 0 || is_nothrow_move_constructible_v<T>); constexpr inplace_vector(initializer_list<T> il); // freestanding-deleted constexpr ~inplace_vector(); constexpr inplace_vector& operator=(const inplace_vector& other); constexpr inplace_vector& operator=(inplace_vector&& other) noexcept(N == 0 || (is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>)); constexpr inplace_vector& operator=(initializer_list<T>); // freestanding-deleted template<class InputIterator> constexpr void assign(InputIterator first, InputIterator last); // freestanding-deleted template<container-compatible-range<T> R> constexpr void assign_range(R&& rg); // freestanding-deleted constexpr void assign(size_type n, const T& u); // freestanding-deleted constexpr void assign(initializer_list<T> il); // freestanding-deleted // iterators constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; constexpr reverse_iterator rbegin() noexcept; constexpr const_reverse_iterator rbegin() const noexcept; constexpr reverse_iterator rend() noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept; // [inplace.vector.capacity], size/capacity constexpr bool empty() const noexcept; constexpr size_type size() const noexcept; static constexpr size_type max_size() noexcept; static constexpr size_type capacity() noexcept; constexpr void resize(size_type sz); // freestanding-deleted constexpr void resize(size_type sz, const T& c); // freestanding-deleted static constexpr void reserve(size_type n); // freestanding-deleted static constexpr void shrink_to_fit() noexcept; // element access constexpr reference operator[](size_type n); constexpr const_reference operator[](size_type n) const; constexpr reference at(size_type n); // freestanding-deleted constexpr const_reference at(size_type n) const; // freestanding-deleted constexpr reference front(); constexpr const_reference front() const; constexpr reference back(); constexpr const_reference back() const; // [inplace.vector.data], data access constexpr T* data() noexcept; constexpr const T* data() const noexcept; // [inplace.vector.modifiers], modifiers template<class... Args> constexpr reference emplace_back(Args&&... args); // freestanding-deleted constexpr reference push_back(const T& x); // freestanding-deleted constexpr reference push_back(T&& x); // freestanding-deleted template<container-compatible-range<T> R> constexpr void append_range(R&& rg); // freestanding-deleted constexpr void pop_back(); template<class... Args> constexpr pointer try_emplace_back(Args&&... args); constexpr pointer try_push_back(const T& x); constexpr pointer try_push_back(T&& x); template<container-compatible-range<T> R> constexpr ranges::borrowed_iterator_t<R> try_append_range(R&& rg); template<class... Args> constexpr reference unchecked_emplace_back(Args&&... args); constexpr reference unchecked_push_back(const T& x); constexpr reference unchecked_push_back(T&& x); template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args); // freestanding-deleted constexpr iterator insert(const_iterator position, const T& x); // freestanding-deleted constexpr iterator insert(const_iterator position, T&& x); // freestanding-deleted constexpr iterator insert(const_iterator position, size_type n, // freestanding-deleted const T& x); template<class InputIterator> constexpr iterator insert(const_iterator position, // freestanding-deleted InputIterator first, InputIterator last); template<container-compatible-range<T> R> constexpr iterator insert_range(const_iterator position, R&& rg); // freestanding-deleted constexpr iterator insert(const_iterator position, // freestanding-deleted initializer_list<T> il); constexpr iterator erase(const_iterator position); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void swap(inplace_vector& x) noexcept(N == 0 || (is_nothrow_swappable_v<T> && is_nothrow_move_constructible_v<T>)); constexpr void clear() noexcept; constexpr friend bool operator==(const inplace_vector& x, const inplace_vector& y); constexpr friend synth-three-way-result<T> operator<=>(const inplace_vector& x, const inplace_vector& y); constexpr friend void swap(inplace_vector& x, inplace_vector& y) noexcept(N == 0 || (is_nothrow_swappable_v<T> && is_nothrow_move_constructible_v<T>)) { x.swap(y); } }; }

23.3.14.2 Constructors [inplace.vector.cons]

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constexpr explicit inplace_vector(size_type n);

Preconditions: T is Cpp17DefaultInsertable into inplace_vector.

Effects: Constructs an inplace_vector with n default-inserted elements.

Complexity: Linear in n.

🔗

constexpr inplace_vector(size_type n, const T& value);

Preconditions: T is Cpp17CopyInsertable into inplace_vector.

Effects: Constructs an inplace_vector with n copies of value.

Complexity: Linear in n.

🔗

template<class InputIterator>
  constexpr inplace_vector(InputIterator first, InputIterator last);

Effects: Constructs an inplace_vector equal to the range [first, last).

Complexity: Linear in distance(first, last).

🔗

template<container-compatible-range<T> R>
  constexpr inplace_vector(from_range_t, R&& rg);

Effects: Constructs an inplace_vector with the elements of the range rg.

Complexity: Linear in ranges::distance(rg).

23.3.14.3 Size and capacity [inplace.vector.capacity]

🔗

static constexpr size_type capacity() noexcept;
static constexpr size_type max_size() noexcept;

Returns: N.

🔗

constexpr void resize(size_type sz);

Preconditions: T is Cpp17DefaultInsertable into inplace_vector.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() default-inserted elements to the sequence.

Remarks: If an exception is thrown, there are no effects on *this.

🔗

constexpr void resize(size_type sz, const T& c);

Preconditions: T is Cpp17CopyInsertable into inplace_vector.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() copies of c to the sequence.

Remarks: If an exception is thrown, there are no effects on *this.

23.3.14.4 Data [inplace.vector.data]

🔗

constexpr       T* data()       noexcept;
constexpr const T* data() const noexcept;

Returns: A pointer such that [data(), data() + size()) is a valid range.

For a non-empty inplace_vector, data() == addressof(front()) is true.

Complexity: Constant time.

23.3.14.5 Modifiers [inplace.vector.modifiers]

🔗

constexpr iterator insert(const_iterator position, const T& x);
constexpr iterator insert(const_iterator position, T&& x);
constexpr iterator insert(const_iterator position, size_type n, const T& x);
template<class InputIterator>
  constexpr iterator insert(const_iterator position, InputIterator first, InputIterator last);
template<container-compatible-range<T> R>
  constexpr iterator insert_range(const_iterator position, R&& rg);
constexpr iterator insert(const_iterator position, initializer_list<T> il);

template<class... Args>
  constexpr iterator emplace(const_iterator position, Args&&... args);
template<container-compatible-range<T> R>
  constexpr void append_range(R&& rg);

Let n be the value of size() before this call for the append_range overload, and distance(begin, position) otherwise.

Complexity: Linear in the number of elements inserted plus the distance to the end of the vector.

Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T or by any InputIterator operation, there are no effects.

Otherwise, if an exception is thrown, then size() ≥ n and elements in the range begin() + [0, n) are not modified.

🔗

constexpr reference push_back(const T& x);
constexpr reference push_back(T&& x);
template<class... Args>
  constexpr reference emplace_back(Args&&... args);

Returns: back().

Throws: bad_alloc or any exception thrown by the initialization of the inserted element.

Complexity: Constant.

Remarks: If an exception is thrown, there are no effects on *this.

🔗

template<class... Args>
  constexpr pointer try_emplace_back(Args&&... args);
constexpr pointer try_push_back(const T& x);
constexpr pointer try_push_back(T&& x);

Let vals denote a pack:

(8.1)
std::forward<Args>(args)... for the first overload,
(8.2)
x for the second overload,
(8.3)
std::move(x) for the third overload.

Preconditions: value_type is Cpp17EmplaceConstructible into inplace_vector from vals....

Effects: If size() < capacity() is true, appends an object of type T direct-non-list-initialized with vals....

Otherwise, there are no effects.

Returns: nullptr if size() == capacity() is true, otherwise addressof(back()).

Throws: Nothing unless an exception is thrown by the initialization of the inserted element.

Complexity: Constant.

Remarks: If an exception is thrown, there are no effects on *this.

🔗

template<container-compatible-range<T> R>
  constexpr ranges::borrowed_iterator_t<R> try_append_range(R&& rg);

Preconditions: value_type is Cpp17EmplaceConstructible into inplace_vector from

*ranges::begin(rg).

Effects: Appends copies of initial elements in rg before end(), until all elements are inserted or size() == capacity() is true.

Each iterator in the range rg is dereferenced at most once.

Returns: An iterator pointing to the first element of rg that was not inserted into *this, or ranges::end(rg) if no such element exists.

Complexity: Linear in the number of elements inserted.

Remarks: Let n be the value of size() prior to this call.

If an exception is thrown after the insertion of k elements, then size() equals

n + k

, elements in the range begin() + [0, n) are not modified, and elements in the range begin() + [n,

n + k

) correspond to the inserted elements.

🔗

template<class... Args>
  constexpr reference unchecked_emplace_back(Args&&... args);

Preconditions: size() < capacity() is true.

Effects: Equivalent to: return *try_emplace_back(std::forward<Args>(args)...);

🔗

constexpr reference unchecked_push_back(const T& x);
constexpr reference unchecked_push_back(T&& x);

Preconditions: size() < capacity() is true.

Effects: Equivalent to: return *try_push_back(std::forward<decltype(x)>(x));

🔗

static constexpr void reserve(size_type n);

Effects: None.

Throws: bad_alloc if n > capacity() is true.

🔗

static constexpr void shrink_to_fit() noexcept;

Effects: None.

🔗

constexpr iterator erase(const_iterator position);
constexpr iterator erase(const_iterator first, const_iterator last);
constexpr void pop_back();

Effects: Invalidates iterators and references at or after the point of the erase.

Throws: Nothing unless an exception is thrown by the assignment operator or move assignment operator of T.

23.3.14.6 Erasure [inplace.vector.erasure]

🔗

template<class T, size_t N, class U = T>
  constexpr size_t erase(inplace_vector<T, N>& c, const U& value);

Effects: Equivalent to: auto it = remove(c.begin(), c.end(), value); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;

🔗

template<class T, size_t N, class Predicate>
  constexpr size_t erase_if(inplace_vector<T, N>& c, Predicate pred);

Effects: Equivalent to: auto it = remove_if(c.begin(), c.end(), pred); auto r = distance(it, c.end()); c.erase(it, c.end()); return r;

23.4 Associative containers [associative]

23.4.1 General [associative.general]

The header <map> defines the class templates map and multimap; the header <set> defines the class templates set and multiset.

The following exposition-only alias templates may appear in deduction guides for associative containers: template<class InputIterator> using iter-value-type = typename iterator_traits<InputIterator>::value_type; // exposition only template<class InputIterator> using iter-key-type = remove_const_t< tuple_element_t<0, iter-value-type<InputIterator>>>; // exposition only template<class InputIterator> using iter-mapped-type = tuple_element_t<1, iter-value-type<InputIterator>>; // exposition only template<class InputIterator> using iter-to-alloc-type = pair< add_const_t<tuple_element_t<0, iter-value-type<InputIterator>>>, tuple_element_t<1, iter-value-type<InputIterator>>>; // exposition only template<ranges::input_range Range> using range-key-type = remove_const_t<typename ranges::range_value_t<Range>::first_type>; // exposition only template<ranges::input_range Range> using range-mapped-type = typename ranges::range_value_t<Range>::second_type; // exposition only template<ranges::input_range Range> using range-to-alloc-type = pair<add_const_t<typename ranges::range_value_t<Range>::first_type>, typename ranges::range_value_t<Range>::second_type>; // exposition only

23.4.2 Header <map> synopsis [associative.map.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [map], class template map template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> class map; template<class Key, class T, class Compare, class Allocator> bool operator==(const map<Key, T, Compare, Allocator>& x, const map<Key, T, Compare, Allocator>& y); template<class Key, class T, class Compare, class Allocator> synth-three-way-result<pair<const Key, T>> operator<=>(const map<Key, T, Compare, Allocator>& x, const map<Key, T, Compare, Allocator>& y); template<class Key, class T, class Compare, class Allocator> void swap(map<Key, T, Compare, Allocator>& x, map<Key, T, Compare, Allocator>& y) noexcept(noexcept(x.swap(y))); // [map.erasure], erasure for map template<class Key, class T, class Compare, class Allocator, class Predicate> typename map<Key, T, Compare, Allocator>::size_type erase_if(map<Key, T, Compare, Allocator>& c, Predicate pred); // [multimap], class template multimap template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> class multimap; template<class Key, class T, class Compare, class Allocator> bool operator==(const multimap<Key, T, Compare, Allocator>& x, const multimap<Key, T, Compare, Allocator>& y); template<class Key, class T, class Compare, class Allocator> synth-three-way-result<pair<const Key, T>> operator<=>(const multimap<Key, T, Compare, Allocator>& x, const multimap<Key, T, Compare, Allocator>& y); template<class Key, class T, class Compare, class Allocator> void swap(multimap<Key, T, Compare, Allocator>& x, multimap<Key, T, Compare, Allocator>& y) noexcept(noexcept(x.swap(y))); // [multimap.erasure], erasure for multimap template<class Key, class T, class Compare, class Allocator, class Predicate> typename multimap<Key, T, Compare, Allocator>::size_type erase_if(multimap<Key, T, Compare, Allocator>& c, Predicate pred); namespace pmr { template<class Key, class T, class Compare = less<Key>> using map = std::map<Key, T, Compare, polymorphic_allocator<pair<const Key, T>>>; template<class Key, class T, class Compare = less<Key>> using multimap = std::multimap<Key, T, Compare, polymorphic_allocator<pair<const Key, T>>>; } }

23.4.3 Class template map [map]

23.4.3.1 Overview [map.overview]

A map is an associative container that supports unique keys (i.e., contains at most one of each key value) and provides for fast retrieval of values of another type T based on the keys.

The map class supports bidirectional iterators.

A map meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of an allocator-aware container ([container.alloc.reqmts]).

and of an associative container ([associative.reqmts]).

A map also provides most operations described in [associative.reqmts] for unique keys.

This means that a map supports the a_uniq operations in [associative.reqmts] but not the a_eq operations.

For a map<Key,T> the key_type is Key and the value_type is pair<const Key,T>.

Descriptions are provided here only for operations on map that are not described in one of those tables or for operations where there is additional semantic information.

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namespace std { template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> class map { public: // types using key_type = Key; using mapped_type = T; using value_type = pair<const Key, T>; using key_compare = Compare; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using node_type = unspecified; using insert_return_type = insert-return-type<iterator, node_type>; class value_compare { protected: Compare comp; value_compare(Compare c) : comp(c) {} public: bool operator()(const value_type& x, const value_type& y) const { return comp(x.first, y.first); } }; // [map.cons], construct/copy/destroy map() : map(Compare()) { } explicit map(const Compare& comp, const Allocator& = Allocator()); template<class InputIterator> map(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator()); template<container-compatible-range<value_type> R> map(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator()); map(const map& x); map(map&& x); explicit map(const Allocator&); map(const map&, const type_identity_t<Allocator>&); map(map&&, const type_identity_t<Allocator>&); map(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template<class InputIterator> map(InputIterator first, InputIterator last, const Allocator& a) : map(first, last, Compare(), a) { } template<container-compatible-range<value_type> R> map(from_range_t, R&& rg, const Allocator& a)) : map(from_range, std::forward<R>(rg), Compare(), a) { } map(initializer_list<value_type> il, const Allocator& a) : map(il, Compare(), a) { } ~map(); map& operator=(const map& x); map& operator=(map&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Compare>); map& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [map.access], element access mapped_type& operator[](const key_type& x); mapped_type& operator[](key_type&& x); template<class K> mapped_type& operator[](K&& x); mapped_type& at(const key_type& x); const mapped_type& at(const key_type& x) const; template<class K> mapped_type& at(const K& x); template<class K> const mapped_type& at(const K& x) const; // [map.modifiers], modifiers template<class... Args> pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); pair<iterator, bool> insert(const value_type& x); pair<iterator, bool> insert(value_type&& x); template<class P> pair<iterator, bool> insert(P&& x); iterator insert(const_iterator position, const value_type& x); iterator insert(const_iterator position, value_type&& x); template<class P> iterator insert(const_iterator position, P&&); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); insert_return_type insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); template<class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template<class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template<class K, class... Args> pair<iterator, bool> try_emplace(K&& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args); template<class K, class... Args> iterator try_emplace(const_iterator hint, K&& k, Args&&... args); template<class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template<class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template<class K, class M> pair<iterator, bool> insert_or_assign(K&& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj); template<class K, class M> iterator insert_or_assign(const_iterator hint, K&& k, M&& obj); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(map&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Compare>); void clear() noexcept; template<class C2> void merge(map<Key, T, C2, Allocator>& source); template<class C2> void merge(map<Key, T, C2, Allocator>&& source); template<class C2> void merge(multimap<Key, T, C2, Allocator>& source); template<class C2> void merge(multimap<Key, T, C2, Allocator>&& source); // observers key_compare key_comp() const; value_compare value_comp() const; // map operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; }; template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>, class Allocator = allocator<iter-to-alloc-type<InputIterator>>> map(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator()) -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare, Allocator>; template<ranges::input_range R, class Compare = less<range-key-type<R>, class Allocator = allocator<range-to-alloc-type<R>>> map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> map<range-key-type<R>, range-mapped-type<R>, Compare, Allocator>; template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator()) -> map<Key, T, Compare, Allocator>; template<class InputIterator, class Allocator> map(InputIterator, InputIterator, Allocator) -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, less<iter-key-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> map(from_range_t, R&&, Allocator) -> map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, Allocator>; template<class Key, class T, class Allocator> map(initializer_list<pair<Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>; }

23.4.3.2 Constructors, copy, and assignment [map.cons]

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explicit map(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty map using the specified comparison object and allocator.

Complexity: Constant.

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template<class InputIterator>
  map(InputIterator first, InputIterator last,
      const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty map using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted with respect to comp and otherwise

N log N

, where N is last - first.

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template<container-compatible-range<value_type> R>
  map(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty map using the specified comparison object and allocator, and inserts elements from the range rg.

Complexity: Linear in N if rg is already sorted with respect to comp and otherwise

N log N

, where N is ranges::distance(rg).

23.4.3.3 Element access [map.access]

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mapped_type& operator[](const key_type& x);

Effects: Equivalent to: return try_emplace(x).first->second;

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mapped_type& operator[](key_type&& x);

Effects: Equivalent to: return try_emplace(std::move(x)).first->second;

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template<class K> mapped_type& operator[](K&& x);

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

Effects: Equivalent to: return try_emplace(std::forward<K>(x)).first->second;

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mapped_type&       at(const key_type& x);
const mapped_type& at(const key_type& x) const;

Returns: A reference to the mapped_type corresponding to x in *this.

Throws: An exception object of type out_of_range if no such element is present.

Complexity: Logarithmic.

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template<class K> mapped_type&       at(const K& x);
template<class K> const mapped_type& at(const K& x) const;

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

Preconditions: The expression find(x) is well-formed and has well-defined behavior.

Returns: A reference to find(x)->second.

Throws: An exception object of type out_of_range if find(x) == end() is true.

Complexity: Logarithmic.

23.4.3.4 Modifiers [map.modifiers]

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template<class P>
  pair<iterator, bool> insert(P&& x);
template<class P>
  iterator insert(const_iterator position, P&& x);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: The first form is equivalent to return emplace(std::forward(x)).

The second form is equivalent to return emplace_hint(position, std::forward(x)).

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template<class... Args>
  pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into map from piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class... Args>
  pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into map from piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class K, class... Args>
  pair<iterator, bool> try_emplace(K&& k, Args&&... args);
template<class K, class... Args>
  iterator try_emplace(const_iterator hint, K&& k, Args&&... args);

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

For the first overload, is_convertible_v<K&&, const_iterator> and is_convertible_v<K&&, iterator> are both false.

Preconditions: value_type is Cpp17EmplaceConstructible into map from piecewise_construct, forward_as_tuple(std::forward<K>(k)), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise, let r be equal_range(k).

Constructs an object u of type value_type with piecewise_construct, forward_as_tuple(std::forward<K>(k)), forward_as_tuple(std::forward<Args>(args)...).

If equal_range(u.first) == r is false, the behavior is undefined.

Inserts u into *this.

Returns: For the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class M>
  pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into map from k, std::forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with k, std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class M>
  pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into map from std::move(k), std::forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with std::move(k), std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class K, class M>
  pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
template<class K, class M>
  iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into map from std::forward<K>(k), std::
forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>
(obj) to e.second.

Otherwise, let r be equal_range(k).

Constructs an object u of type value_type with std::forward<K>(k), std::forward<M>(obj).

If equal_range(u.first) == r is false, the behavior is undefined.

Inserts u into *this.

Returns: For the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

23.4.3.5 Erasure [map.erasure]

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template<class Key, class T, class Compare, class Allocator, class Predicate>
  typename map<Key, T, Compare, Allocator>::size_type
    erase_if(map<Key, T, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.4.4 Class template multimap [multimap]

23.4.4.1 Overview [multimap.overview]

A multimap is an associative container that supports equivalent keys (i.e., possibly containing multiple copies of the same key value) and provides for fast retrieval of values of another type T based on the keys.

The multimap class supports bidirectional iterators.

A multimap meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), and of an associative container ([associative.reqmts]).

A multimap also provides most operations described in [associative.reqmts] for equal keys.

This means that a multimap supports the a_eq operations in [associative.reqmts] but not the a_uniq operations.

For a multimap<Key,T> the key_type is Key and the value_type is pair<const Key,T>.

Descriptions are provided here only for operations on multimap that are not described in one of those tables or for operations where there is additional semantic information.

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namespace std { template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> class multimap { public: // types using key_type = Key; using mapped_type = T; using value_type = pair<const Key, T>; using key_compare = Compare; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using node_type = unspecified; class value_compare { protected: Compare comp; value_compare(Compare c) : comp(c) { } public: bool operator()(const value_type& x, const value_type& y) const { return comp(x.first, y.first); } }; // [multimap.cons], construct/copy/destroy multimap() : multimap(Compare()) { } explicit multimap(const Compare& comp, const Allocator& = Allocator()); template<class InputIterator> multimap(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator()); template<container-compatible-range<value_type> R> multimap(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator()); multimap(const multimap& x); multimap(multimap&& x); explicit multimap(const Allocator&); multimap(const multimap&, const type_identity_t<Allocator>&); multimap(multimap&&, const type_identity_t<Allocator>&); multimap(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template<class InputIterator> multimap(InputIterator first, InputIterator last, const Allocator& a) : multimap(first, last, Compare(), a) { } template<container-compatible-range<value_type> R> multimap(from_range_t, R&& rg, const Allocator& a)) : multimap(from_range, std::forward<R>(rg), Compare(), a) { } multimap(initializer_list<value_type> il, const Allocator& a) : multimap(il, Compare(), a) { } ~multimap(); multimap& operator=(const multimap& x); multimap& operator=(multimap&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Compare>); multimap& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [multimap.modifiers], modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& x); iterator insert(value_type&& x); template<class P> iterator insert(P&& x); iterator insert(const_iterator position, const value_type& x); iterator insert(const_iterator position, value_type&& x); template<class P> iterator insert(const_iterator position, P&& x); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); iterator insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(multimap&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Compare>); void clear() noexcept; template<class C2> void merge(multimap<Key, T, C2, Allocator>& source); template<class C2> void merge(multimap<Key, T, C2, Allocator>&& source); template<class C2> void merge(map<Key, T, C2, Allocator>& source); template<class C2> void merge(map<Key, T, C2, Allocator>&& source); // observers key_compare key_comp() const; value_compare value_comp() const; // map operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; }; template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>, class Allocator = allocator<iter-to-alloc-type<InputIterator>>> multimap(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator()) -> multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare, Allocator>; template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<range-to-alloc-type<R>>> multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> multimap<range-key-type<R>, range-mapped-type<R>, Compare, Allocator>; template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> multimap(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator()) -> multimap<Key, T, Compare, Allocator>; template<class InputIterator, class Allocator> multimap(InputIterator, InputIterator, Allocator) -> multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, less<iter-key-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> multimap(from_range_t, R&&, Allocator) -> multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, Allocator>; template<class Key, class T, class Allocator> multimap(initializer_list<pair<Key, T>>, Allocator) -> multimap<Key, T, less<Key>, Allocator>; }

23.4.4.2 Constructors [multimap.cons]

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explicit multimap(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty multimap using the specified comparison object and allocator.

Complexity: Constant.

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template<class InputIterator>
  multimap(InputIterator first, InputIterator last,
           const Compare& comp = Compare(),
           const Allocator& = Allocator());

Effects: Constructs an empty multimap using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted with respect to comp and otherwise

N log N

, where N is last - first.

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template<container-compatible-range<value_type> R>
  multimap(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty multimap using the specified comparison object and allocator, and inserts elements from the range rg.

Complexity: Linear in N if rg is already sorted with respect to comp and otherwise

N log N

, where N is ranges::distance(rg).

23.4.4.3 Modifiers [multimap.modifiers]

🔗

template<class P> iterator insert(P&& x);
template<class P> iterator insert(const_iterator position, P&& x);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: The first form is equivalent to return emplace(std::forward(x)).

The second form is equivalent to return emplace_hint(position, std::forward(x)).

23.4.4.4 Erasure [multimap.erasure]

🔗

template<class Key, class T, class Compare, class Allocator, class Predicate>
  typename multimap<Key, T, Compare, Allocator>::size_type
    erase_if(multimap<Key, T, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.4.5 Header <set> synopsis [associative.set.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [set], class template set template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>> class set; template<class Key, class Compare, class Allocator> bool operator==(const set<Key, Compare, Allocator>& x, const set<Key, Compare, Allocator>& y); template<class Key, class Compare, class Allocator> synth-three-way-result<Key> operator<=>(const set<Key, Compare, Allocator>& x, const set<Key, Compare, Allocator>& y); template<class Key, class Compare, class Allocator> void swap(set<Key, Compare, Allocator>& x, set<Key, Compare, Allocator>& y) noexcept(noexcept(x.swap(y))); // [set.erasure], erasure for set template<class Key, class Compare, class Allocator, class Predicate> typename set<Key, Compare, Allocator>::size_type erase_if(set<Key, Compare, Allocator>& c, Predicate pred); // [multiset], class template multiset template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>> class multiset; template<class Key, class Compare, class Allocator> bool operator==(const multiset<Key, Compare, Allocator>& x, const multiset<Key, Compare, Allocator>& y); template<class Key, class Compare, class Allocator> synth-three-way-result<Key> operator<=>(const multiset<Key, Compare, Allocator>& x, const multiset<Key, Compare, Allocator>& y); template<class Key, class Compare, class Allocator> void swap(multiset<Key, Compare, Allocator>& x, multiset<Key, Compare, Allocator>& y) noexcept(noexcept(x.swap(y))); // [multiset.erasure], erasure for multiset template<class Key, class Compare, class Allocator, class Predicate> typename multiset<Key, Compare, Allocator>::size_type erase_if(multiset<Key, Compare, Allocator>& c, Predicate pred); namespace pmr { template<class Key, class Compare = less<Key>> using set = std::set<Key, Compare, polymorphic_allocator<Key>>; template<class Key, class Compare = less<Key>> using multiset = std::multiset<Key, Compare, polymorphic_allocator<Key>>; } }

23.4.6 Class template set [set]

23.4.6.1 Overview [set.overview]

A set is an associative container that supports unique keys (i.e., contains at most one of each key value) and provides for fast retrieval of the keys themselves.

The set class supports bidirectional iterators.

A set meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of an allocator-aware container ([container.alloc.reqmts]).

and of an associative container ([associative.reqmts]).

A set also provides most operations described in [associative.reqmts] for unique keys.

This means that a set supports the a_uniq operations in [associative.reqmts] but not the a_eq operations.

For a set<Key> both the key_type and value_type are Key.

Descriptions are provided here only for operations on set that are not described in one of these tables and for operations where there is additional semantic information.

namespace std { template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>> class set { public: // types using key_type = Key; using key_compare = Compare; using value_type = Key; using value_compare = Compare; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using node_type = unspecified; using insert_return_type = insert-return-type<iterator, node_type>; // [set.cons], construct/copy/destroy set() : set(Compare()) { } explicit set(const Compare& comp, const Allocator& = Allocator()); template<class InputIterator> set(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator()); template<container-compatible-range<value_type> R> set(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator()); set(const set& x); set(set&& x); explicit set(const Allocator&); set(const set&, const type_identity_t<Allocator>&); set(set&&, const type_identity_t<Allocator>&); set(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template<class InputIterator> set(InputIterator first, InputIterator last, const Allocator& a) : set(first, last, Compare(), a) { } template<container-compatible-range<value_type> R> set(from_range_t, R&& rg, const Allocator& a)) : set(from_range, std::forward<R>(rg), Compare(), a) { } set(initializer_list<value_type> il, const Allocator& a) : set(il, Compare(), a) { } ~set(); set& operator=(const set& x); set& operator=(set&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Compare>); set& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [set.modifiers], modifiers template<class... Args> pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); pair<iterator,bool> insert(const value_type& x); pair<iterator,bool> insert(value_type&& x); template<class K> pair<iterator, bool> insert(K&& x); iterator insert(const_iterator position, const value_type& x); iterator insert(const_iterator position, value_type&& x); template<class K> iterator insert(const_iterator position, K&& x); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); insert_return_type insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(set&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Compare>); void clear() noexcept; template<class C2> void merge(set<Key, C2, Allocator>& source); template<class C2> void merge(set<Key, C2, Allocator>&& source); template<class C2> void merge(multiset<Key, C2, Allocator>& source); template<class C2> void merge(multiset<Key, C2, Allocator>&& source); // observers key_compare key_comp() const; value_compare value_comp() const; // set operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; }; template<class InputIterator, class Compare = less<iter-value-type<InputIterator>>, class Allocator = allocator<iter-value-type<InputIterator>>> set(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator()) -> set<iter-value-type<InputIterator>, Compare, Allocator>; template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> set<ranges::range_value_t<R>, Compare, Allocator>; template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>> set(initializer_list<Key>, Compare = Compare(), Allocator = Allocator()) -> set<Key, Compare, Allocator>; template<class InputIterator, class Allocator> set(InputIterator, InputIterator, Allocator) -> set<iter-value-type<InputIterator>, less<iter-value-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> set(from_range_t, R&&, Allocator) -> set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, Allocator>; template<class Key, class Allocator> set(initializer_list<Key>, Allocator) -> set<Key, less<Key>, Allocator>; }

23.4.6.2 Constructors, copy, and assignment [set.cons]

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explicit set(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty set using the specified comparison object and allocator.

Complexity: Constant.

🔗

template<class InputIterator>
  set(InputIterator first, InputIterator last,
      const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted with respect to comp and otherwise

N log N

, where N is last - first.

🔗

template<container-compatible-range<value_type> R>
  set(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range rg.

Complexity: Linear in N if rg is already sorted with respect to comp and otherwise

N log N

, where N is ranges::distance(rg).

23.4.6.3 Erasure [set.erasure]

🔗

template<class Key, class Compare, class Allocator, class Predicate>
  typename set<Key, Compare, Allocator>::size_type
    erase_if(set<Key, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.4.6.4 Modifiers [set.modifiers]

🔗

template<class K> pair<iterator, bool> insert(K&& x);
template<class K> iterator insert(const_iterator hint, K&& x);

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

For the second overload, is_convertible_v<K&&, const_iterator> and is_convertible_v<K&&, iterator> are both false.

Preconditions: value_type is Cpp17EmplaceConstructible into set from std::forward<K>(x).

Effects: If the set already contains an element that is equivalent to x, there is no effect.

Otherwise, let r be equal_range(x).

Constructs an object u of type value_type with std::forward<K>(x).

If equal_range(u) == r is false, the behavior is undefined.

Inserts u into *this.

Returns: For the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the set element that is equivalent to x.

Complexity: Logarithmic.

23.4.7 Class template multiset [multiset]

23.4.7.1 Overview [multiset.overview]

A multiset is an associative container that supports equivalent keys (i.e., possibly contains multiple copies of the same key value) and provides for fast retrieval of the keys themselves.

The multiset class supports bidirectional iterators.

A multiset meets all of the requirements of a container ([container.reqmts]), of a reversible container ([container.rev.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), of an associative container ([associative.reqmts]).

multiset also provides most operations described in [associative.reqmts] for duplicate keys.

This means that a multiset supports the a_eq operations in [associative.reqmts] but not the a_uniq operations.

For a multiset<Key> both the key_type and value_type are Key.

Descriptions are provided here only for operations on multiset that are not described in one of these tables and for operations where there is additional semantic information.

namespace std { template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>> class multiset { public: // types using key_type = Key; using key_compare = Compare; using value_type = Key; using value_compare = Compare; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using node_type = unspecified; // [multiset.cons], construct/copy/destroy multiset() : multiset(Compare()) { } explicit multiset(const Compare& comp, const Allocator& = Allocator()); template<class InputIterator> multiset(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator()); template<container-compatible-range<value_type> R> multiset(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator()); multiset(const multiset& x); multiset(multiset&& x); explicit multiset(const Allocator&); multiset(const multiset&, const type_identity_t<Allocator>&); multiset(multiset&&, const type_identity_t<Allocator>&); multiset(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template<class InputIterator> multiset(InputIterator first, InputIterator last, const Allocator& a) : multiset(first, last, Compare(), a) { } template<container-compatible-range<value_type> R> multiset(from_range_t, R&& rg, const Allocator& a)) : multiset(from_range, std::forward<R>(rg), Compare(), a) { } multiset(initializer_list<value_type> il, const Allocator& a) : multiset(il, Compare(), a) { } ~multiset(); multiset& operator=(const multiset& x); multiset& operator=(multiset&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Compare>); multiset& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& x); iterator insert(value_type&& x); iterator insert(const_iterator position, const value_type& x); iterator insert(const_iterator position, value_type&& x); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); iterator insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(multiset&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Compare>); void clear() noexcept; template<class C2> void merge(multiset<Key, C2, Allocator>& source); template<class C2> void merge(multiset<Key, C2, Allocator>&& source); template<class C2> void merge(set<Key, C2, Allocator>& source); template<class C2> void merge(set<Key, C2, Allocator>&& source); // observers key_compare key_comp() const; value_compare value_comp() const; // set operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; }; template<class InputIterator, class Compare = less<iter-value-type<InputIterator>>, class Allocator = allocator<iter-value-type<InputIterator>>> multiset(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator()) -> multiset<iter-value-type<InputIterator>, Compare, Allocator>; template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> multiset<ranges::range_value_t<R>, Compare, Allocator>; template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>> multiset(initializer_list<Key>, Compare = Compare(), Allocator = Allocator()) -> multiset<Key, Compare, Allocator>; template<class InputIterator, class Allocator> multiset(InputIterator, InputIterator, Allocator) -> multiset<iter-value-type<InputIterator>, less<iter-value-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> multiset(from_range_t, R&&, Allocator) -> multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, Allocator>; template<class Key, class Allocator> multiset(initializer_list<Key>, Allocator) -> multiset<Key, less<Key>, Allocator>; }

23.4.7.2 Constructors [multiset.cons]

🔗

explicit multiset(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty multiset using the specified comparison object and allocator.

Complexity: Constant.

🔗

template<class InputIterator>
  multiset(InputIterator first, InputIterator last,
           const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty multiset using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted with respect to comp and otherwise

N log N

, where N is last - first.

🔗

template<container-compatible-range<value_type> R>
  multiset(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty multiset using the specified comparison object and allocator, and inserts elements from the range rg.

Complexity: Linear in N if rg is already sorted with respect to comp and otherwise

N log N

, where N is ranges::distance(rg).

23.4.7.3 Erasure [multiset.erasure]

🔗

template<class Key, class Compare, class Allocator, class Predicate>
  typename multiset<Key, Compare, Allocator>::size_type
    erase_if(multiset<Key, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.5 Unordered associative containers [unord]

23.5.1 General [unord.general]

The header <unordered_map> defines the class templates unordered_map and unordered_multimap; the header <unordered_set> defines the class templates unordered_set and unordered_multiset.

The exposition-only alias templates iter-value-type, iter-key-type, iter-mapped-type, iter-to-alloc-type, range-key-type, range-mapped-type, and range-to-alloc-type defined in [associative.general] may appear in deduction guides for unordered containers.

23.5.2 Header <unordered_map> synopsis [unord.map.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [unord.map], class template unordered_map template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Alloc = allocator<pair<const Key, T>>> class unordered_map; // [unord.multimap], class template unordered_multimap template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Alloc = allocator<pair<const Key, T>>> class unordered_multimap; template<class Key, class T, class Hash, class Pred, class Alloc> bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& a, const unordered_map<Key, T, Hash, Pred, Alloc>& b); template<class Key, class T, class Hash, class Pred, class Alloc> bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& a, const unordered_multimap<Key, T, Hash, Pred, Alloc>& b); template<class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x, unordered_map<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); template<class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x, unordered_multimap<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); // [unord.map.erasure], erasure for unordered_map template<class K, class T, class H, class P, class A, class Predicate> typename unordered_map<K, T, H, P, A>::size_type erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred); // [unord.multimap.erasure], erasure for unordered_multimap template<class K, class T, class H, class P, class A, class Predicate> typename unordered_multimap<K, T, H, P, A>::size_type erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred); namespace pmr { template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>> using unordered_map = std::unordered_map<Key, T, Hash, Pred, polymorphic_allocator<pair<const Key, T>>>; template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>> using unordered_multimap = std::unordered_multimap<Key, T, Hash, Pred, polymorphic_allocator<pair<const Key, T>>>; } }

23.5.3 Class template unordered_map [unord.map]

23.5.3.1 Overview [unord.map.overview]

An unordered_map is an unordered associative container that supports unique keys (an unordered_map contains at most one of each key value) and that associates values of another type mapped_type with the keys.

The unordered_map class supports forward iterators.

An unordered_map meets all of the requirements of a container ([container.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), and of an unordered associative container ([unord.req]).

It provides the operations described in the preceding requirements table for unique keys; that is, an unordered_map supports the a_uniq operations in that table, not the a_eq operations.

For an unordered_map<Key, T> the key_type is Key, the mapped_type is T, and the value_type is pair<const Key, T>.

Subclause [unord.map] only describes operations on unordered_map that are not described in one of the requirement tables, or for which there is additional semantic information.

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namespace std { template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> class unordered_map { public: // types using key_type = Key; using mapped_type = T; using value_type = pair<const Key, T>; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using local_iterator = implementation-defined; // see [container.requirements] using const_local_iterator = implementation-defined; // see [container.requirements] using node_type = unspecified; using insert_return_type = insert-return-type<iterator, node_type>; // [unord.map.cnstr], construct/copy/destroy unordered_map(); explicit unordered_map(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<container-compatible-range<value_type> R> unordered_map(from_range_t, R&& rg, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(const unordered_map&); unordered_map(unordered_map&&); explicit unordered_map(const Allocator&); unordered_map(const unordered_map&, const type_identity_t<Allocator>&); unordered_map(unordered_map&&, const type_identity_t<Allocator>&); unordered_map(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(size_type n, const allocator_type& a) : unordered_map(n, hasher(), key_equal(), a) { } unordered_map(size_type n, const hasher& hf, const allocator_type& a) : unordered_map(n, hf, key_equal(), a) { } template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_map(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(f, l, n, hf, key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_map(from_range_t, R&& rg, size_type n, const allocator_type& a) : unordered_map(from_range, std::forward<R>(rg), n, hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_map(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { } unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_map(il, n, hasher(), key_equal(), a) { } unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(il, n, hf, key_equal(), a) { } ~unordered_map(); unordered_map& operator=(const unordered_map&); unordered_map& operator=(unordered_map&&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); unordered_map& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [unord.map.modifiers], modifiers template<class... Args> pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); pair<iterator, bool> insert(const value_type& obj); pair<iterator, bool> insert(value_type&& obj); template<class P> pair<iterator, bool> insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class P> iterator insert(const_iterator hint, P&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); insert_return_type insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); template<class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template<class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template<class K, class... Args> pair<iterator, bool> try_emplace(K&& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args); template<class K, class... Args> iterator try_emplace(const_iterator hint, K&& k, Args&&... args); template<class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template<class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template<class K, class M> pair<iterator, bool> insert_or_assign(K&& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj); template<class K, class M> iterator insert_or_assign(const_iterator hint, K&& k, M&& obj); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(unordered_map&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Hash> && is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // map operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; pair<iterator, iterator> equal_range(const key_type& k); pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> pair<iterator, iterator> equal_range(const K& k); template<class K> pair<const_iterator, const_iterator> equal_range(const K& k) const; // [unord.map.elem], element access mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k); template<class K> mapped_type& operator[](K&& k); mapped_type& at(const key_type& k); const mapped_type& at(const key_type& k) const; template<class K> mapped_type& at(const K& k); template<class K> const mapped_type& at(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; template<class InputIterator, class Hash = hash<iter-key-type<InputIterator>>, class Pred = equal_to<iter-key-type<InputIterator>>, class Allocator = allocator<iter-to-alloc-type<InputIterator>>> unordered_map(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash, Pred, Allocator>; template<ranges::input_range R, class Hash = hash<range-key-type<R>>, class Pred = equal_to<range-key-type<R>>, class Allocator = allocator<range-to-alloc-type<R>>> unordered_map(from_range_t, R&&, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_map<range-key-type<R>, range-mapped-type<R>, Hash, Pred, Allocator>; template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> unordered_map(initializer_list<pair<Key, T>>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_map<Key, T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_map(InputIterator, InputIterator, typename see below::size_type, Allocator) -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, hash<iter-key-type<InputIterator>>, equal_to<iter-key-type<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_map(InputIterator, InputIterator, Allocator) -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, hash<iter-key-type<InputIterator>>, equal_to<iter-key-type<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_map(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash, equal_to<iter-key-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_map(from_range_t, R&&, typename see below::size_type, Allocator) -> unordered_map<range-key-type<R>, range-mapped-type<R>, hash<range-key-type<R>>, equal_to<range-key-type<R>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_map(from_range_t, R&&, Allocator) -> unordered_map<range-key-type<R>, range-mapped-type<R>, hash<range-key-type<R>>, equal_to<range-key-type<R>>, Allocator>; template<ranges::input_range R, class Hash, class Allocator> unordered_map(from_range_t, R&&, typename see below::size_type, Hash, Allocator) -> unordered_map<range-key-type<R>, range-mapped-type<R>, Hash, equal_to<range-key-type<R>>, Allocator>; template<class Key, class T, class Allocator> unordered_map(initializer_list<pair<Key, T>>, typename see below::size_type, Allocator) -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Allocator> unordered_map(initializer_list<pair<Key, T>>, Allocator) -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_map(initializer_list<pair<Key, T>>, typename see below::size_type, Hash, Allocator) -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>; }

A size_type parameter type in an unordered_map deduction guide refers to the size_type member type of the type deduced by the deduction guide.

23.5.3.2 Constructors [unord.map.cnstr]

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unordered_map() : unordered_map(size_type(see below)) { }
explicit unordered_map(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

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template<class InputIterator>
  unordered_map(InputIterator f, InputIterator l,
                size_type n = see below,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());
template<container-compatible-range<value_type> R>
  unordered_map(from_range_t, R&& rg,
                size_type n = see below,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());
unordered_map(initializer_list<value_type> il,
              size_type n = see below,
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l), rg, or il, respectively.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

23.5.3.3 Element access [unord.map.elem]

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mapped_type& operator[](const key_type& k);

Effects: Equivalent to: return try_emplace(k).first->second;

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mapped_type& operator[](key_type&& k);

Effects: Equivalent to: return try_emplace(std::move(k)).first->second;

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template<class K> mapped_type& operator[](K&& k);

Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

Effects: Equivalent to: return try_emplace(std::forward<K>(k)).first->second;

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mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;

Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.

Throws: An exception object of type out_of_range if no such element is present.

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template<class K> mapped_type&       at(const K& k);
template<class K> const mapped_type& at(const K& k) const;

Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

Preconditions: The expression find(k) is well-formed and has well-defined behavior.

Returns: A reference to find(k)->second.

Throws: An exception object of type out_of_range if find(k) == end() is true.

23.5.3.4 Modifiers [unord.map.modifiers]

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template<class P>
  pair<iterator, bool> insert(P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace(std::forward(obj));

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template<class P>
  iterator insert(const_iterator hint, P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace_hint(hint, std::forward(obj));

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template<class... Args>
  pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class... Args>
  pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class K, class... Args>
  pair<iterator, bool> try_emplace(K&& k, Args&&... args);
template<class K, class... Args>
  iterator try_emplace(const_iterator hint, K&& k, Args&&... args);

Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

For the first overload, is_convertible_v<K&&, const_iterator> and is_convertible_v<K&&, iterator> are both false.

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from piecewise_construct, forward_as_tuple(std::forward<K>(k)), forward_as_tuple(std::forward<Args>
(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise, let h be hash_function()(k).

Constructs an object u of type value_type with piecewise_construct, forward_as_tuple(std::forward<K>(k)), forward_as_tuple(std::forward<Args>(args)...).

If hash_function()(u.first) != h || contains(u.first) is true, the behavior is undefined.

Inserts u into *this.

Returns: For the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class M>
  pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from k, std::forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with k, std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class M>
  pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from std::move(k), std::forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with std::move(k), std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

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template<class K, class M>
  pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
template<class K, class M>
  iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from std::forward<K>
(k), std::forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>
(obj) to e.second.

Otherwise, let h be hash_function()(k).

Constructs an object u of type value_type with std::forward<K>(k), std::forward<M>(obj).

If hash_function()(u.first) != h || contains(u.first) is true, the behavior is undefined.

Inserts u into *this.

Returns: For the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

23.5.3.5 Erasure [unord.map.erasure]

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template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_map<K, T, H, P, A>::size_type
    erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.5.4 Class template unordered_multimap [unord.multimap]

23.5.4.1 Overview [unord.multimap.overview]

An unordered_multimap is an unordered associative container that supports equivalent keys (an instance of unordered_multimap may contain multiple copies of each key value) and that associates values of another type mapped_type with the keys.

The unordered_multimap class supports forward iterators.

An unordered_multimap meets all of the requirements of a container ([container.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), and of an unordered associative container ([unord.req]).

It provides the operations described in the preceding requirements table for equivalent keys; that is, an unordered_multimap supports the a_eq operations in that table, not the a_uniq operations.

For an unordered_multimap<Key, T> the key_type is Key, the mapped_type is T, and the value_type is pair<const Key, T>.

Subclause [unord.multimap] only describes operations on unordered_multimap that are not described in one of the requirement tables, or for which there is additional semantic information.

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namespace std { template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> class unordered_multimap { public: // types using key_type = Key; using mapped_type = T; using value_type = pair<const Key, T>; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using local_iterator = implementation-defined; // see [container.requirements] using const_local_iterator = implementation-defined; // see [container.requirements] using node_type = unspecified; // [unord.multimap.cnstr], construct/copy/destroy unordered_multimap(); explicit unordered_multimap(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<container-compatible-range<value_type> R> unordered_multimap(from_range_t, R&& rg, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multimap(const unordered_multimap&); unordered_multimap(unordered_multimap&&); explicit unordered_multimap(const Allocator&); unordered_multimap(const unordered_multimap&, const type_identity_t<Allocator>&); unordered_multimap(unordered_multimap&&, const type_identity_t<Allocator>&); unordered_multimap(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multimap(size_type n, const allocator_type& a) : unordered_multimap(n, hasher(), key_equal(), a) { } unordered_multimap(size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(n, hf, key_equal(), a) { } template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multimap(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(f, l, n, hf, key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_multimap(from_range_t, R&& rg, size_type n, const allocator_type& a) : unordered_multimap(from_range, std::forward<R>(rg), n, hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_multimap(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multimap(il, n, hasher(), key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(il, n, hf, key_equal(), a) { } ~unordered_multimap(); unordered_multimap& operator=(const unordered_multimap&); unordered_multimap& operator=(unordered_multimap&&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); unordered_multimap& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [unord.multimap.modifiers], modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& obj); iterator insert(value_type&& obj); template<class P> iterator insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class P> iterator insert(const_iterator hint, P&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); iterator insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(unordered_multimap&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Hash> && is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_map<Key, T, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // map operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; pair<iterator, iterator> equal_range(const key_type& k); pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> pair<iterator, iterator> equal_range(const K& k); template<class K> pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; template<class InputIterator, class Hash = hash<iter-key-type<InputIterator>>, class Pred = equal_to<iter-key-type<InputIterator>>, class Allocator = allocator<iter-to-alloc-type<InputIterator>>> unordered_multimap(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash, Pred, Allocator>; template<ranges::input_range R, class Hash = hash<range-key-type<R>>, class Pred = equal_to<range-key-type<R>>, class Allocator = allocator<range-to-alloc-type<R>>> unordered_multimap(from_range_t, R&&, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multimap<range-key-type<R>, range-mapped-type<R>, Hash, Pred, Allocator>; template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> unordered_multimap(initializer_list<pair<Key, T>>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multimap<Key, T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Allocator) -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, hash<iter-key-type<InputIterator>>, equal_to<iter-key-type<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_multimap(InputIterator, InputIterator, Allocator) -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, hash<iter-key-type<InputIterator>>, equal_to<iter-key-type<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash, equal_to<iter-key-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_multimap(from_range_t, R&&, typename see below::size_type, Allocator) -> unordered_multimap<range-key-type<R>, range-mapped-type<R>, hash<range-key-type<R>>, equal_to<range-key-type<R>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_multimap(from_range_t, R&&, Allocator) -> unordered_multimap<range-key-type<R>, range-mapped-type<R>, hash<range-key-type<R>>, equal_to<range-key-type<R>>, Allocator>; template<ranges::input_range R, class Hash, class Allocator> unordered_multimap(from_range_t, R&&, typename see below::size_type, Hash, Allocator) -> unordered_multimap<range-key-type<R>, range-mapped-type<R>, Hash, equal_to<range-key-type<R>>, Allocator>; template<class Key, class T, class Allocator> unordered_multimap(initializer_list<pair<Key, T>>, typename see below::size_type, Allocator) -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Allocator> unordered_multimap(initializer_list<pair<Key, T>>, Allocator) -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_multimap(initializer_list<pair<Key, T>>, typename see below::size_type, Hash, Allocator) -> unordered_multimap<Key, T, Hash, equal_to<Key>, Allocator>; }

A size_type parameter type in an unordered_multimap deduction guide refers to the size_type member type of the type deduced by the deduction guide.

23.5.4.2 Constructors [unord.multimap.cnstr]

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unordered_multimap() : unordered_multimap(size_type(see below)) { }
explicit unordered_multimap(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

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template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l,
                     size_type n = see below,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
template<container-compatible-range<value_type> R>
  unordered_multimap(from_range_t, R&& rg,
                     size_type n = see below,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
unordered_multimap(initializer_list<value_type> il,
                   size_type n = see below,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l), rg, or il, respectively.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

23.5.4.3 Modifiers [unord.multimap.modifiers]

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template<class P>
  iterator insert(P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace(std::forward(obj));

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template<class P>
  iterator insert(const_iterator hint, P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace_hint(hint, std::forward(obj));

23.5.4.4 Erasure [unord.multimap.erasure]

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template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_multimap<K, T, H, P, A>::size_type
    erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.5.5 Header <unordered_set> synopsis [unord.set.syn]

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#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [unord.set], class template unordered_set template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Alloc = allocator<Key>> class unordered_set; // [unord.multiset], class template unordered_multiset template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Alloc = allocator<Key>> class unordered_multiset; template<class Key, class Hash, class Pred, class Alloc> bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& a, const unordered_set<Key, Hash, Pred, Alloc>& b); template<class Key, class Hash, class Pred, class Alloc> bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& a, const unordered_multiset<Key, Hash, Pred, Alloc>& b); template<class Key, class Hash, class Pred, class Alloc> void swap(unordered_set<Key, Hash, Pred, Alloc>& x, unordered_set<Key, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); template<class Key, class Hash, class Pred, class Alloc> void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x, unordered_multiset<Key, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); // [unord.set.erasure], erasure for unordered_set template<class K, class H, class P, class A, class Predicate> typename unordered_set<K, H, P, A>::size_type erase_if(unordered_set<K, H, P, A>& c, Predicate pred); // [unord.multiset.erasure], erasure for unordered_multiset template<class K, class H, class P, class A, class Predicate> typename unordered_multiset<K, H, P, A>::size_type erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred); namespace pmr { template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>> using unordered_set = std::unordered_set<Key, Hash, Pred, polymorphic_allocator<Key>>; template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>> using unordered_multiset = std::unordered_multiset<Key, Hash, Pred, polymorphic_allocator<Key>>; } }

23.5.6 Class template unordered_set [unord.set]

23.5.6.1 Overview [unord.set.overview]

An unordered_set is an unordered associative container that supports unique keys (an unordered_set contains at most one of each key value) and in which the elements' keys are the elements themselves.

The unordered_set class supports forward iterators.

An unordered_set meets all of the requirements of a container ([container.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), of an unordered associative container ([unord.req]).

It provides the operations described in the preceding requirements table for unique keys; that is, an unordered_set supports the a_uniq operations in that table, not the a_eq operations.

For an unordered_set<Key> the key_type and the value_type are both Key.

The iterator and const_iterator types are both constant iterator types.

It is unspecified whether they are the same type.

Subclause [unord.set] only describes operations on unordered_set that are not described in one of the requirement tables, or for which there is additional semantic information.

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namespace std { template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<Key>> class unordered_set { public: // types using key_type = Key; using value_type = Key; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using local_iterator = implementation-defined; // see [container.requirements] using const_local_iterator = implementation-defined; // see [container.requirements] using node_type = unspecified; using insert_return_type = insert-return-type<iterator, node_type>; // [unord.set.cnstr], construct/copy/destroy unordered_set(); explicit unordered_set(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<container-compatible-range<value_type> R> unordered_set(from_range_t, R&& rg, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_set(const unordered_set&); unordered_set(unordered_set&&); explicit unordered_set(const Allocator&); unordered_set(const unordered_set&, const type_identity_t<Allocator>&); unordered_set(unordered_set&&, const type_identity_t<Allocator>&); unordered_set(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_set(size_type n, const allocator_type& a) : unordered_set(n, hasher(), key_equal(), a) { } unordered_set(size_type n, const hasher& hf, const allocator_type& a) : unordered_set(n, hf, key_equal(), a) { } template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_set(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(f, l, n, hf, key_equal(), a) { } unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_set(il, n, hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_set(from_range_t, R&& rg, size_type n, const allocator_type& a) : unordered_set(from_range, std::forward<R>(rg), n, hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_set(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { } unordered_set(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(il, n, hf, key_equal(), a) { } ~unordered_set(); unordered_set& operator=(const unordered_set&); unordered_set& operator=(unordered_set&&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); unordered_set& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [unord.set.modifiers], modifiers template<class... Args> pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); pair<iterator, bool> insert(const value_type& obj); pair<iterator, bool> insert(value_type&& obj); template<class K> pair<iterator, bool> insert(K&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class K> iterator insert(const_iterator hint, K&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); insert_return_type insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(unordered_set&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Hash> && is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // set operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; pair<iterator, iterator> equal_range(const key_type& k); pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> pair<iterator, iterator> equal_range(const K& k); template<class K> pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; template<class InputIterator, class Hash = hash<iter-value-type<InputIterator>>, class Pred = equal_to<iter-value-type<InputIterator>>, class Allocator = allocator<iter-value-type<InputIterator>>> unordered_set(InputIterator, InputIterator, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_set<iter-value-type<InputIterator>, Hash, Pred, Allocator>; template<ranges::input_range R, class Hash = hash<ranges::range_value_t<R>>, class Pred = equal_to<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> unordered_set(from_range_t, R&&, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_set<ranges::range_value_t<R>, Hash, Pred, Allocator>; template<class T, class Hash = hash<T>, class Pred = equal_to<T>, class Allocator = allocator<T>> unordered_set(initializer_list<T>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_set<T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_set(InputIterator, InputIterator, typename see below::size_type, Allocator) -> unordered_set<iter-value-type<InputIterator>, hash<iter-value-type<InputIterator>>, equal_to<iter-value-type<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_set(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) -> unordered_set<iter-value-type<InputIterator>, Hash, equal_to<iter-value-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_set(from_range_t, R&&, typename see below::size_type, Allocator) -> unordered_set<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>, equal_to<ranges::range_value_t<R>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_set(from_range_t, R&&, Allocator) -> unordered_set<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>, equal_to<ranges::range_value_t<R>>, Allocator>; template<ranges::input_range R, class Hash, class Allocator> unordered_set(from_range_t, R&&, typename see below::size_type, Hash, Allocator) -> unordered_set<ranges::range_value_t<R>, Hash, equal_to<ranges::range_value_t<R>>, Allocator>; template<class T, class Allocator> unordered_set(initializer_list<T>, typename see below::size_type, Allocator) -> unordered_set<T, hash<T>, equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_set(initializer_list<T>, typename see below::size_type, Hash, Allocator) -> unordered_set<T, Hash, equal_to<T>, Allocator>; }

A size_type parameter type in an unordered_set deduction guide refers to the size_type member type of the type deduced by the deduction guide.

23.5.6.2 Constructors [unord.set.cnstr]

🔗

unordered_set() : unordered_set(size_type(see below)) { }
explicit unordered_set(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

🔗

template<class InputIterator>
  unordered_set(InputIterator f, InputIterator l,
                size_type n = see below,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());
template<container-compatible-range<value_type> R>
  unordered_multiset(from_range_t, R&& rg,
                     size_type n = see below,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
unordered_set(initializer_list<value_type> il,
              size_type n = see below,
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l), rg, or il, respectively.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

23.5.6.3 Erasure [unord.set.erasure]

🔗

template<class K, class H, class P, class A, class Predicate>
  typename unordered_set<K, H, P, A>::size_type
    erase_if(unordered_set<K, H, P, A>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.5.6.4 Modifiers [unord.set.modifiers]

🔗

template<class K> pair<iterator, bool> insert(K&& obj);
template<class K> iterator insert(const_iterator hint, K&& obj);

Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

For the second overload, is_convertible_v<K&&, const_iterator> and is_convertible_v<K&&, iterator> are both false.

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_set from std::forward<K>
(obj).

Effects: If the set already contains an element that is equivalent to obj, there is no effect.

Otherwise, let h be hash_function()(obj).

Constructs an object u of type value_type with std::forward<K>(obj).

If hash_function()(u) != h || contains(u) is true, the behavior is undefined.

Inserts u into *this.

Returns: For the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the set element that is equivalent to obj.

Complexity: Average case constant, worst case linear.

23.5.7 Class template unordered_multiset [unord.multiset]

23.5.7.1 Overview [unord.multiset.overview]

An unordered_multiset is an unordered associative container that supports equivalent keys (an instance of unordered_multiset may contain multiple copies of the same key value) and in which each element's key is the element itself.

The unordered_multiset class supports forward iterators.

An unordered_multiset meets all of the requirements of a container ([container.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), and of an unordered associative container ([unord.req]).

It provides the operations described in the preceding requirements table for equivalent keys; that is, an unordered_multiset supports the a_eq operations in that table, not the a_uniq operations.

For an unordered_multiset<Key> the key_type and the value_type are both Key.

The iterator and const_iterator types are both constant iterator types.

It is unspecified whether they are the same type.

Subclause [unord.multiset] only describes operations on unordered_multiset that are not described in one of the requirement tables, or for which there is additional semantic information.

🔗

namespace std { template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<Key>> class unordered_multiset { public: // types using key_type = Key; using value_type = Key; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = implementation-defined; // see [container.requirements] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using local_iterator = implementation-defined; // see [container.requirements] using const_local_iterator = implementation-defined; // see [container.requirements] using node_type = unspecified; // [unord.multiset.cnstr], construct/copy/destroy unordered_multiset(); explicit unordered_multiset(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<container-compatible-range<value_type> R> unordered_multiset(from_range_t, R&& rg, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multiset(const unordered_multiset&); unordered_multiset(unordered_multiset&&); explicit unordered_multiset(const Allocator&); unordered_multiset(const unordered_multiset&, const type_identity_t<Allocator>&); unordered_multiset(unordered_multiset&&, const type_identity_t<Allocator>&); unordered_multiset(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multiset(size_type n, const allocator_type& a) : unordered_multiset(n, hasher(), key_equal(), a) { } unordered_multiset(size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(n, hf, key_equal(), a) { } template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multiset(f, l, n, hasher(), key_equal(), a) { } template<class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(f, l, n, hf, key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_multiset(from_range_t, R&& rg, size_type n, const allocator_type& a) : unordered_multiset(from_range, std::forward<R>(rg), n, hasher(), key_equal(), a) { } template<container-compatible-range<value_type> R> unordered_multiset(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multiset(il, n, hasher(), key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(il, n, hf, key_equal(), a) { } ~unordered_multiset(); unordered_multiset& operator=(const unordered_multiset&); unordered_multiset& operator=(unordered_multiset&&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); unordered_multiset& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& obj); iterator insert(value_type&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); iterator insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(unordered_multiset&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Hash> && is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // set operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; pair<iterator, iterator> equal_range(const key_type& k); pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> pair<iterator, iterator> equal_range(const K& k); template<class K> pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); void reserve(size_type n); }; template<class InputIterator, class Hash = hash<iter-value-type<InputIterator>>, class Pred = equal_to<iter-value-type<InputIterator>>, class Allocator = allocator<iter-value-type<InputIterator>>> unordered_multiset(InputIterator, InputIterator, see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multiset<iter-value-type<InputIterator>, Hash, Pred, Allocator>; template<ranges::input_range R, class Hash = hash<ranges::range_value_t<R>>, class Pred = equal_to<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> unordered_multiset(from_range_t, R&&, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multiset<ranges::range_value_t<R>, Hash, Pred, Allocator>; template<class T, class Hash = hash<T>, class Pred = equal_to<T>, class Allocator = allocator<T>> unordered_multiset(initializer_list<T>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multiset<T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Allocator) -> unordered_multiset<iter-value-type<InputIterator>, hash<iter-value-type<InputIterator>>, equal_to<iter-value-type<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator) -> unordered_multiset<iter-value-type<InputIterator>, Hash, equal_to<iter-value-type<InputIterator>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_multiset(from_range_t, R&&, typename see below::size_type, Allocator) -> unordered_multiset<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>, equal_to<ranges::range_value_t<R>>, Allocator>; template<ranges::input_range R, class Allocator> unordered_multiset(from_range_t, R&&, Allocator) -> unordered_multiset<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>, equal_to<ranges::range_value_t<R>>, Allocator>; template<ranges::input_range R, class Hash, class Allocator> unordered_multiset(from_range_t, R&&, typename see below::size_type, Hash, Allocator) -> unordered_multiset<ranges::range_value_t<R>, Hash, equal_to<ranges::range_value_t<R>>, Allocator>; template<class T, class Allocator> unordered_multiset(initializer_list<T>, typename see below::size_type, Allocator) -> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_multiset(initializer_list<T>, typename see below::size_type, Hash, Allocator) -> unordered_multiset<T, Hash, equal_to<T>, Allocator>; }

A size_type parameter type in an unordered_multiset deduction guide refers to the size_type member type of the type deduced by the deduction guide.

23.5.7.2 Constructors [unord.multiset.cnstr]

🔗

unordered_multiset() : unordered_multiset(size_type(see below)) { }
explicit unordered_multiset(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

🔗

template<class InputIterator>
  unordered_multiset(InputIterator f, InputIterator l,
                     size_type n = see below,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
template<container-compatible-range<value_type> R>
  unordered_multiset(from_range_t, R&& rg,
                     size_type n = see below,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
unordered_multiset(initializer_list<value_type> il,
                   size_type n = see below,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l), rg, or il, respectively.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

23.5.7.3 Erasure [unord.multiset.erasure]

🔗

template<class K, class H, class P, class A, class Predicate>
  typename unordered_multiset<K, H, P, A>::size_type
    erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);

Effects: Equivalent to: auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size();

23.6 Container adaptors [container.adaptors]

23.6.1 General [container.adaptors.general]

The headers <queue>, <stack>, <flat_map>, and <flat_set> define the container adaptors queue and priority_queue, stack, flat_map and flat_multimap, and flat_set and flat_multiset, respectively.

Each container adaptor takes one or more template parameters named Container, KeyContainer, or MappedContainer that denote the types of containers that the container adaptor adapts.

Each container adaptor has at least one constructor that takes a reference argument to one or more such template parameters.

For each constructor reference argument to a container C, the constructor copies the container into the container adaptor.

If C takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor.

Otherwise, normal copy or move construction is used for the container argument.

For the container adaptors that take a single container template parameter Container, the first template parameter T of the container adaptor shall denote the same type as Container::value_type.

For container adaptors, no swap function throws an exception unless that exception is thrown by the swap of the adaptor's Container, KeyContainer, MappedContainer, or Compare object (if any).

A constructor template of a container adaptor shall not participate in overload resolution if it has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

For container adaptors that have them, the insert, emplace, and erase members affect the validity of iterators, references, and pointers to the adaptor's container(s) in the same way that the containers' respective insert, emplace, and erase members do.

[Example 1:

A call to flat_map<Key, T>::insert can invalidate all iterators to the flat_map.

— end example]

A deduction guide for a container adaptor shall not participate in overload resolution if any of the following are true:

(6.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(6.2)
It has a Compare template parameter and a type that qualifies as an allocator is deduced for that parameter.
(6.3)
It has a Container, KeyContainer, or MappedContainer template parameter and a type that qualifies as an allocator is deduced for that parameter.
(6.4)
It has no Container, KeyContainer, or MappedContainer template parameter, and it has an Allocator template parameter, and a type that does not qualify as an allocator is deduced for that parameter.
(6.5)
It has both Container and Allocator template parameters, and uses_allocator_v<Container, Allocator> is false.
(6.6)
It has both KeyContainer and Allocator template parameters, and uses_allocator_v<KeyContainer, Allocator> is false.
(6.7)
It has both KeyContainer and Compare template parameters, and is_invocable_v<const Compare&, const typename KeyContainer::value_type&, const typename KeyContainer::value_type&> is not a valid expression or is false.
(6.8)
It has both MappedContainer and Allocator template parameters, and uses_allocator_v<MappedContainer, Allocator> is false.

The exposition-only alias template iter-value-type defined in [sequences.general] and the exposition-only alias templates iter-key-type, iter-mapped-type, range-key-type, and range-mapped-type defined in [associative.general] may appear in deduction guides for container adaptors.

The following exposition-only alias template may appear in deduction guides for container adaptors: template<class Allocator, class T> using alloc-rebind = // exposition only typename allocator_traits<Allocator>::template rebind_alloc<T>;

23.6.2 Header <queue> synopsis [queue.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [queue], class template queue template<class T, class Container = deque<T>> class queue; template<class T, class Container> bool operator==(const queue<T, Container>& x, const queue<T, Container>& y); template<class T, class Container> bool operator!=(const queue<T, Container>& x, const queue<T, Container>& y); template<class T, class Container> bool operator< (const queue<T, Container>& x, const queue<T, Container>& y); template<class T, class Container> bool operator> (const queue<T, Container>& x, const queue<T, Container>& y); template<class T, class Container> bool operator<=(const queue<T, Container>& x, const queue<T, Container>& y); template<class T, class Container> bool operator>=(const queue<T, Container>& x, const queue<T, Container>& y); template<class T, three_way_comparable Container> compare_three_way_result_t<Container> operator<=>(const queue<T, Container>& x, const queue<T, Container>& y); template<class T, class Container> void swap(queue<T, Container>& x, queue<T, Container>& y) noexcept(noexcept(x.swap(y))); template<class T, class Container, class Alloc> struct uses_allocator<queue<T, Container>, Alloc>; // [container.adaptors.format], formatter specialization for queue template<class charT, class T, formattable<charT> Container> struct formatter<queue<T, Container>, charT>; // [priority.queue], class template priority_queue template<class T, class Container = vector<T>, class Compare = less<typename Container::value_type>> class priority_queue; template<class T, class Container, class Compare> void swap(priority_queue<T, Container, Compare>& x, priority_queue<T, Container, Compare>& y) noexcept(noexcept(x.swap(y))); template<class T, class Container, class Compare, class Alloc> struct uses_allocator<priority_queue<T, Container, Compare>, Alloc>; // [container.adaptors.format], formatter specialization for priority_queue template<class charT, class T, formattable<charT> Container, class Compare> struct formatter<priority_queue<T, Container, Compare>, charT>; }

23.6.3 Class template queue [queue]

23.6.3.1 Definition [queue.defn]

Any sequence container supporting operations front(), back(), push_back() and pop_front() can be used to instantiate queue.

In particular, list and deque can be used.

namespace std { template<class T, class Container = deque<T>> class queue { public: using value_type = typename Container::value_type; using reference = typename Container::reference; using const_reference = typename Container::const_reference; using size_type = typename Container::size_type; using container_type = Container; protected: Container c; public: queue() : queue(Container()) {} explicit queue(const Container&); explicit queue(Container&&); template<class InputIterator> queue(InputIterator first, InputIterator last); template<container-compatible-range<T> R> queue(from_range_t, R&& rg); template<class Alloc> explicit queue(const Alloc&); template<class Alloc> queue(const Container&, const Alloc&); template<class Alloc> queue(Container&&, const Alloc&); template<class Alloc> queue(const queue&, const Alloc&); template<class Alloc> queue(queue&&, const Alloc&); template<class InputIterator, class Alloc> queue(InputIterator first, InputIterator last, const Alloc&); template<container-compatible-range<T> R, class Alloc> queue(from_range_t, R&& rg, const Alloc&); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } reference front() { return c.front(); } const_reference front() const { return c.front(); } reference back() { return c.back(); } const_reference back() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void push(value_type&& x) { c.push_back(std::move(x)); } template<container-compatible-range<T> R> void push_range(R&& rg); template<class... Args> decltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); } void pop() { c.pop_front(); } void swap(queue& q) noexcept(is_nothrow_swappable_v<Container>) { using std::swap; swap(c, q.c); } }; template<class Container> queue(Container) -> queue<typename Container::value_type, Container>; template<class InputIterator> queue(InputIterator, InputIterator) -> queue<iter-value-type<InputIterator>>; template<ranges::input_range R> queue(from_range_t, R&&) -> queue<ranges::range_value_t<R>>; template<class Container, class Allocator> queue(Container, Allocator) -> queue<typename Container::value_type, Container>; template<class InputIterator, class Allocator> queue(InputIterator, InputIterator, Allocator) -> queue<iter-value-type<InputIterator>, deque<iter-value-type<InputIterator>, Allocator>>; template<ranges::input_range R, class Allocator> queue(from_range_t, R&&, Allocator) -> queue<ranges::range_value_t<R>, deque<ranges::range_value_t<R>, Allocator>>; template<class T, class Container, class Alloc> struct uses_allocator<queue<T, Container>, Alloc> : uses_allocator<Container, Alloc>::type { }; }

23.6.3.2 Constructors [queue.cons]

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explicit queue(const Container& cont);

Effects: Initializes c with cont.

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explicit queue(Container&& cont);

Effects: Initializes c with std::move(cont).

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template<class InputIterator>
  queue(InputIterator first, InputIterator last);

Effects: Initializes c with first as the first argument and last as the second argument.

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template<container-compatible-range<T> R>
  queue(from_range_t, R&& rg);

Effects: Initializes c with ranges::to<Container>(std::forward<R>(rg)).

23.6.3.3 Constructors with allocators [queue.cons.alloc]

If uses_allocator_v<container_type, Alloc> is false the constructors in this subclause shall not participate in overload resolution.

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template<class Alloc> explicit queue(const Alloc& a);

Effects: Initializes c with a.

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template<class Alloc> queue(const container_type& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument.

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template<class Alloc> queue(container_type&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument.

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template<class Alloc> queue(const queue& q, const Alloc& a);

Effects: Initializes c with q.c as the first argument and a as the second argument.

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template<class Alloc> queue(queue&& q, const Alloc& a);

Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument.

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template<class InputIterator, class Alloc>
  queue(InputIterator first, InputIterator last, const Alloc& alloc);

Effects: Initializes c with first as the first argument, last as the second argument, and alloc as the third argument.

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template<container-compatible-range<T> R, class Alloc>
  queue(from_range_t, R&& rg, const Alloc& a);

Effects: Initializes c with ranges::to<Container>(std::forward<R>(rg), a).

23.6.3.4 Modifiers [queue.mod]

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template<container-compatible-range<T> R>
  void push_range(R&& rg);

Effects: Equivalent to c.append_range(std::forward<R>(rg)) if that is a valid expression, otherwise ranges::copy(rg, back_inserter(c)).

23.6.3.5 Operators [queue.ops]

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template<class T, class Container>
  bool operator==(const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c == y.c.

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template<class T, class Container>
  bool operator!=(const queue<T, Container>& x,  const queue<T, Container>& y);

Returns: x.c != y.c.

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template<class T, class Container>
  bool operator< (const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c < y.c.

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template<class T, class Container>
  bool operator> (const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c > y.c.

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template<class T, class Container>
  bool operator<=(const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c <= y.c.

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template<class T, class Container>
    bool operator>=(const queue<T, Container>& x,
                    const queue<T, Container>& y);

Returns: x.c >= y.c.

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template<class T, three_way_comparable Container>
  compare_three_way_result_t<Container>
    operator<=>(const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c <=> y.c.

23.6.3.6 Specialized algorithms [queue.special]

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template<class T, class Container>
  void swap(queue<T, Container>& x, queue<T, Container>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_swappable_v<Container> is true.

Effects: As if by x.swap(y).

23.6.4 Class template priority_queue [priority.queue]

23.6.4.1 Overview [priqueue.overview]

Any sequence container with random access iterator and supporting operations front(), push_back() and pop_back() can be used to instantiate priority_queue.

In particular, vector and deque can be used.

Instantiating priority_queue also involves supplying a function or function object for making priority comparisons; the library assumes that the function or function object defines a strict weak ordering .

namespace std { template<class T, class Container = vector<T>, class Compare = less<typename Container::value_type>> class priority_queue { public: using value_type = typename Container::value_type; using reference = typename Container::reference; using const_reference = typename Container::const_reference; using size_type = typename Container::size_type; using container_type = Container; using value_compare = Compare; protected: Container c; Compare comp; public: priority_queue() : priority_queue(Compare()) {} explicit priority_queue(const Compare& x) : priority_queue(x, Container()) {} priority_queue(const Compare& x, const Container&); priority_queue(const Compare& x, Container&&); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare()); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container&); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, Container&&); template<container-compatible-range<T> R> priority_queue(from_range_t, R&& rg, const Compare& x = Compare()); template<class Alloc> explicit priority_queue(const Alloc&); template<class Alloc> priority_queue(const Compare&, const Alloc&); template<class Alloc> priority_queue(const Compare&, const Container&, const Alloc&); template<class Alloc> priority_queue(const Compare&, Container&&, const Alloc&); template<class Alloc> priority_queue(const priority_queue&, const Alloc&); template<class Alloc> priority_queue(priority_queue&&, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Compare&, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Compare&, const Container&, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Compare&, Container&&, const Alloc&); template<container-compatible-range<T> R, class Alloc> priority_queue(from_range_t, R&& rg, const Compare&, const Alloc&); template<container-compatible-range<T> R, class Alloc> priority_queue(from_range_t, R&& rg, const Alloc&); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } const_reference top() const { return c.front(); } void push(const value_type& x); void push(value_type&& x); template<container-compatible-range<T> R> void push_range(R&& rg); template<class... Args> void emplace(Args&&... args); void pop(); void swap(priority_queue& q) noexcept(is_nothrow_swappable_v<Container> && is_nothrow_swappable_v<Compare>) { using std::swap; swap(c, q.c); swap(comp, q.comp); } }; template<class Compare, class Container> priority_queue(Compare, Container) -> priority_queue<typename Container::value_type, Container, Compare>; template<class InputIterator, class Compare = less<iter-value-type<InputIterator>>, class Container = vector<iter-value-type<InputIterator>>> priority_queue(InputIterator, InputIterator, Compare = Compare(), Container = Container()) -> priority_queue<iter-value-type<InputIterator>, Container, Compare>; template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>> priority_queue(from_range_t, R&&, Compare = Compare()) -> priority_queue<ranges::range_value_t<R>, vector<ranges::range_value_t<R>>, Compare>; template<class Compare, class Container, class Allocator> priority_queue(Compare, Container, Allocator) -> priority_queue<typename Container::value_type, Container, Compare>; template<class InputIterator, class Allocator> priority_queue(InputIterator, InputIterator, Allocator) -> priority_queue<iter-value-type<InputIterator>, vector<iter-value-type<InputIterator>, Allocator>, less<iter-value-type<InputIterator>>>; template<class InputIterator, class Compare, class Allocator> priority_queue(InputIterator, InputIterator, Compare, Allocator) -> priority_queue<iter-value-type<InputIterator>, vector<iter-value-type<InputIterator>, Allocator>, Compare>; template<class InputIterator, class Compare, class Container, class Allocator> priority_queue(InputIterator, InputIterator, Compare, Container, Allocator) -> priority_queue<typename Container::value_type, Container, Compare>; template<ranges::input_range R, class Compare, class Allocator> priority_queue(from_range_t, R&&, Compare, Allocator) -> priority_queue<ranges::range_value_t<R>, vector<ranges::range_value_t<R>, Allocator>, Compare>; template<ranges::input_range R, class Allocator> priority_queue(from_range_t, R&&, Allocator) -> priority_queue<ranges::range_value_t<R>, vector<ranges::range_value_t<R>, Allocator>>; // no equality is provided template<class T, class Container, class Compare, class Alloc> struct uses_allocator<priority_queue<T, Container, Compare>, Alloc> : uses_allocator<Container, Alloc>::type { }; }

23.6.4.2 Constructors [priqueue.cons]

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priority_queue(const Compare& x, const Container& y);
priority_queue(const Compare& x, Container&& y);

Preconditions: x defines a strict weak ordering ([alg.sorting]).

Effects: Initializes comp with x and c with y (copy constructing or move constructing as appropriate); calls make_heap(c.begin(), c.end(), comp).

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template<class InputIterator>
  priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare());

Preconditions: x defines a strict weak ordering ([alg.sorting]).

Effects: Initializes c with first as the first argument and last as the second argument, and initializes comp with x; then calls make_heap(c.begin(), c.end(), comp).

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template<class InputIterator>
  priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container& y);
template<class InputIterator>
  priority_queue(InputIterator first, InputIterator last, const Compare& x, Container&& y);

Preconditions: x defines a strict weak ordering ([alg.sorting]).

Effects: Initializes comp with x and c with y (copy constructing or move constructing as appropriate); calls c.insert(c.end(), first, last); and finally calls make_heap(c.begin(), c.end(), comp).

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template<container-compatible-range<T> R>
  priority_queue(from_range_t, R&& rg, const Compare& x = Compare());

Preconditions: x defines a strict weak ordering ([alg.sorting]).

Effects: Initializes comp with x and c with ranges::to<Container>(std::forward<R>(rg)) and finally calls make_heap(c.begin(), c.end(), comp).

23.6.4.3 Constructors with allocators [priqueue.cons.alloc]

If uses_allocator_v<container_type, Alloc> is false the constructors in this subclause shall not participate in overload resolution.

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template<class Alloc> explicit priority_queue(const Alloc& a);

Effects: Initializes c with a and value-initializes comp.

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template<class Alloc> priority_queue(const Compare& compare, const Alloc& a);

Effects: Initializes c with a and initializes comp with compare.

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template<class Alloc>
  priority_queue(const Compare& compare, const Container& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument, and initializes comp with compare; calls make_heap(c.begin(), c.end(), comp).

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template<class Alloc>
  priority_queue(const Compare& compare, Container&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument, and initializes comp with compare; calls make_heap(c.begin(), c.end(), comp).

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template<class Alloc> priority_queue(const priority_queue& q, const Alloc& a);

Effects: Initializes c with q.c as the first argument and a as the second argument, and initializes comp with q.comp.

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template<class Alloc> priority_queue(priority_queue&& q, const Alloc& a);

Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument, and initializes comp with std::move(q.comp).

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template<class InputIterator, class Alloc>
  priority_queue(InputIterator first, InputIterator last, const Alloc& a);

Effects: Initializes c with first as the first argument, last as the second argument, and a as the third argument, and value-initializes comp; calls make_heap(c.begin(), c.end(), comp).

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template<class InputIterator, class Alloc>
  priority_queue(InputIterator first, InputIterator last, const Compare& compare, const Alloc& a);

Effects: Initializes c with first as the first argument, last as the second argument, and a as the third argument, and initializes comp with compare; calls make_heap(c.begin(), c.end(), comp).

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template<class InputIterator, class Alloc>
  priority_queue(InputIterator first, InputIterator last, const Compare& compare,
                 const Container& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument, and initializes comp with compare; calls c.insert(c.end(), first, last); and finally calls make_heap(c.begin(), c.end(), comp).

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template<class InputIterator, class Alloc>
  priority_queue(InputIterator first, InputIterator last, const Compare& compare, Container&& cont,
                 const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument, and initializes comp with compare; calls c.insert(c.end(), first, last); and finally calls make_heap(c.begin(), c.end(), comp).

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template<container-compatible-range<T> R, class Alloc>
  priority_queue(from_range_t, R&& rg, const Compare& compare, const Alloc& a);

Effects: Initializes comp with compare and c with ranges::to<Container>(std::forward<R>(rg), a); calls make_heap(c.begin(), c.end(), comp).

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template<container-compatible-range<T> R, class Alloc>
  priority_queue(from_range_t, R&& rg, const Alloc& a);

Effects: Initializes c with ranges::to<Container>(std::forward<R>(rg), a) and value-initializes comp; calls make_heap(c.begin(), c.end(), comp).

23.6.4.4 Members [priqueue.members]

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void push(const value_type& x);

Effects: As if by: c.push_back(x); push_heap(c.begin(), c.end(), comp);

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void push(value_type&& x);

Effects: As if by: c.push_back(std::move(x)); push_heap(c.begin(), c.end(), comp);

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template<container-compatible-range<T> R>
  void push_range(R&& rg);

Effects: Inserts all elements of rg in c via c.append_range(std::forward<R>(rg)) if that is a valid expression, or ranges::copy(rg, back_inserter(c)) otherwise.

Then restores the heap property as if by make_heap(c.begin(), c.end(), comp).

Postconditions: is_heap(c.begin(), c.end(), comp) is true.

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template<class... Args> void emplace(Args&&... args);

Effects: As if by: c.emplace_back(std::forward<Args>(args)...); push_heap(c.begin(), c.end(), comp);

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void pop();

Effects: As if by: pop_heap(c.begin(), c.end(), comp); c.pop_back();

23.6.4.5 Specialized algorithms [priqueue.special]

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template<class T, class Container, class Compare>
  void swap(priority_queue<T, Container, Compare>& x,
            priority_queue<T, Container, Compare>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_swappable_v<Container> is true and is_swappable_v<Compare> is true.

Effects: As if by x.swap(y).

23.6.5 Header <stack> synopsis [stack.syn]

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#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [stack], class template stack template<class T, class Container = deque<T>> class stack; template<class T, class Container> bool operator==(const stack<T, Container>& x, const stack<T, Container>& y); template<class T, class Container> bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y); template<class T, class Container> bool operator< (const stack<T, Container>& x, const stack<T, Container>& y); template<class T, class Container> bool operator> (const stack<T, Container>& x, const stack<T, Container>& y); template<class T, class Container> bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y); template<class T, class Container> bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y); template<class T, three_way_comparable Container> compare_three_way_result_t<Container> operator<=>(const stack<T, Container>& x, const stack<T, Container>& y); template<class T, class Container> void swap(stack<T, Container>& x, stack<T, Container>& y) noexcept(noexcept(x.swap(y))); template<class T, class Container, class Alloc> struct uses_allocator<stack<T, Container>, Alloc>; // [container.adaptors.format], formatter specialization for stack template<class charT, class T, formattable<charT> Container> struct formatter<stack<T, Container>, charT>; }

23.6.6 Class template stack [stack]

23.6.6.1 General [stack.general]

Any sequence container supporting operations back(), push_back() and pop_back() can be used to instantiate stack.

In particular, vector, list and deque can be used.

23.6.6.2 Definition [stack.defn]

namespace std { template<class T, class Container = deque<T>> class stack { public: using value_type = typename Container::value_type; using reference = typename Container::reference; using const_reference = typename Container::const_reference; using size_type = typename Container::size_type; using container_type = Container; protected: Container c; public: stack() : stack(Container()) {} explicit stack(const Container&); explicit stack(Container&&); template<class InputIterator> stack(InputIterator first, InputIterator last); template<container-compatible-range<T> R> stack(from_range_t, R&& rg); template<class Alloc> explicit stack(const Alloc&); template<class Alloc> stack(const Container&, const Alloc&); template<class Alloc> stack(Container&&, const Alloc&); template<class Alloc> stack(const stack&, const Alloc&); template<class Alloc> stack(stack&&, const Alloc&); template<class InputIterator, class Alloc> stack(InputIterator first, InputIterator last, const Alloc&); template<container-compatible-range<T> R, class Alloc> stack(from_range_t, R&& rg, const Alloc&); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } reference top() { return c.back(); } const_reference top() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void push(value_type&& x) { c.push_back(std::move(x)); } template<container-compatible-range<T> R> void push_range(R&& rg); template<class... Args> decltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); } void pop() { c.pop_back(); } void swap(stack& s) noexcept(is_nothrow_swappable_v<Container>) { using std::swap; swap(c, s.c); } }; template<class Container> stack(Container) -> stack<typename Container::value_type, Container>; template<class InputIterator> stack(InputIterator, InputIterator) -> stack<iter-value-type<InputIterator>>; template<ranges::input_range R> stack(from_range_t, R&&) -> stack<ranges::range_value_t<R>>; template<class Container, class Allocator> stack(Container, Allocator) -> stack<typename Container::value_type, Container>; template<class InputIterator, class Allocator> stack(InputIterator, InputIterator, Allocator) -> stack<iter-value-type<InputIterator>, deque<iter-value-type<InputIterator>, Allocator>>; template<ranges::input_range R, class Allocator> stack(from_range_t, R&&, Allocator) -> stack<ranges::range_value_t<R>, deque<ranges::range_value_t<R>, Allocator>>; template<class T, class Container, class Alloc> struct uses_allocator<stack<T, Container>, Alloc> : uses_allocator<Container, Alloc>::type { }; }

23.6.6.3 Constructors [stack.cons]

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explicit stack(const Container& cont);

Effects: Initializes c with cont.

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explicit stack(Container&& cont);

Effects: Initializes c with std::move(cont).

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template<class InputIterator>
  stack(InputIterator first, InputIterator last);

Effects: Initializes c with first as the first argument and last as the second argument.

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template<container-compatible-range<T> R>
  stack(from_range_t, R&& rg);

Effects: Initializes c with ranges::to<Container>(std::forward<R>(rg)).

23.6.6.4 Constructors with allocators [stack.cons.alloc]

If uses_allocator_v<container_type, Alloc> is false the constructors in this subclause shall not participate in overload resolution.

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template<class Alloc> explicit stack(const Alloc& a);

Effects: Initializes c with a.

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template<class Alloc> stack(const container_type& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument.

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template<class Alloc> stack(container_type&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument.

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template<class Alloc> stack(const stack& s, const Alloc& a);

Effects: Initializes c with s.c as the first argument and a as the second argument.

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template<class Alloc> stack(stack&& s, const Alloc& a);

Effects: Initializes c with std::move(s.c) as the first argument and a as the second argument.

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template<class InputIterator, class Alloc>
  stack(InputIterator first, InputIterator last, const Alloc& alloc);

Effects: Initializes c with first as the first argument, last as the second argument, and alloc as the third argument.

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template<container-compatible-range<T> R, class Alloc>
  stack(from_range_t, R&& rg, const Alloc& a);

Effects: Initializes c with ranges::to<Container>(std::forward<R>(rg), a).

23.6.6.5 Modifiers [stack.mod]

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template<container-compatible-range<T> R>
  void push_range(R&& rg);

Effects: Equivalent to c.append_range(std::forward<R>(rg)) if that is a valid expression, otherwise ranges::copy(rg, back_inserter(c)).

23.6.6.6 Operators [stack.ops]

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template<class T, class Container>
  bool operator==(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c == y.c.

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template<class T, class Container>
  bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c != y.c.

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template<class T, class Container>
  bool operator< (const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c < y.c.

🔗

template<class T, class Container>
  bool operator> (const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c > y.c.

🔗

template<class T, class Container>
  bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c <= y.c.

🔗

template<class T, class Container>
  bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c >= y.c.

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template<class T, three_way_comparable Container>
  compare_three_way_result_t<Container>
    operator<=>(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c <=> y.c.

23.6.6.7 Specialized algorithms [stack.special]

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template<class T, class Container>
  void swap(stack<T, Container>& x, stack<T, Container>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_swappable_v<Container> is true.

Effects: As if by x.swap(y).

23.6.7 Header <flat_map> synopsis [flat.map.syn]

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#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [flat.map], class template flat_map template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_map; struct sorted_unique_t { explicit sorted_unique_t() = default; }; inline constexpr sorted_unique_t sorted_unique{}; template<class Key, class T, class Compare, class KeyContainer, class MappedContainer, class Allocator> struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>, Allocator>; // [flat.map.erasure], erasure for flat_map template<class Key, class T, class Compare, class KeyContainer, class MappedContainer, class Predicate> typename flat_map<Key, T, Compare, KeyContainer, MappedContainer>::size_type erase_if(flat_map<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred); // [flat.multimap], class template flat_multimap template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_multimap; struct sorted_equivalent_t { explicit sorted_equivalent_t() = default; }; inline constexpr sorted_equivalent_t sorted_equivalent{}; template<class Key, class T, class Compare, class KeyContainer, class MappedContainer, class Allocator> struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>, Allocator>; // [flat.multimap.erasure], erasure for flat_multimap template<class Key, class T, class Compare, class KeyContainer, class MappedContainer, class Predicate> typename flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>::size_type erase_if(flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred); }

23.6.8 Class template flat_map [flat.map]

23.6.8.1 Overview [flat.map.overview]

A flat_map is a container adaptor that provides an associative container interface that supports unique keys (i.e., contains at most one of each key value) and provides for fast retrieval of values of another type T based on the keys.

flat_map supports iterators that meet the Cpp17InputIterator requirements and model the random_access_iterator concept ([iterator.concept.random.access]).

A flat_map meets all of the requirements of a container ([container.reqmts]) and of a reversible container ([container.rev.reqmts]), plus the optional container requirements ([container.opt.reqmts]).

flat_map meets the requirements of an associative container ([associative.reqmts]), except that:

(2.1)
it does not meet the requirements related to node handles ([container.node]),
(2.2)
it does not meet the requirements related to iterator invalidation, and
(2.3)
the time complexity of the operations that insert or erase a single element from the map is linear, including the ones that take an insertion position iterator.

[Note 1:

A flat_map does not meet the additional requirements of an allocator-aware container ([container.alloc.reqmts]).

— end note]

A flat_map also provides most operations described in [associative.reqmts] for unique keys.

This means that a flat_map supports the a_uniq operations in [associative.reqmts] but not the a_eq operations.

For a flat_map<Key, T> the key_type is Key and the value_type is pair<Key, T>.

Descriptions are provided here only for operations on flat_map that are not described in one of those sets of requirements or for operations where there is additional semantic information.

A flat_map maintains the following invariants:

(5.1)
it contains the same number of keys and values;
(5.2)
the keys are sorted with respect to the comparison object; and
(5.3)
the value at offset off within the value container is the value associated with the key at offset off within the key container.

If any member function in [flat.map.defn] exits via an exception the invariants are restored.

[Note 2:

This can result in the flat_map being emptied.

— end note]

Any type C that meets the sequence container requirements ([sequence.reqmts]) can be used to instantiate flat_map, as long as C::iterator meets the Cpp17RandomAccessIterator requirements and invocations of member functions C::size and C::max_size do not exit via an exception.

In particular, vector ([vector]) and deque ([deque]) can be used.

[Note 3:

vector<bool> is not a sequence container.

— end note]

The program is ill-formed if Key is not the same type as KeyContainer::value_type or T is not the same type as MappedContainer::value_type.

The effect of calling a constructor that takes both key_container_type and mapped_container_type arguments with containers of different sizes is undefined.

The effect of calling a constructor or member function that takes a sorted_unique_t argument with a container, containers, or range that is not sorted with respect to key_comp(), or that contains equal elements, is undefined.

23.6.8.2 Definition [flat.map.defn]

namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_map { public: // types using key_type = Key; using mapped_type = T; using value_type = pair<key_type, mapped_type>; using key_compare = Compare; using reference = pair<const key_type&, mapped_type&>; using const_reference = pair<const key_type&, const mapped_type&>; using size_type = size_t; using difference_type = ptrdiff_t; using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using key_container_type = KeyContainer; using mapped_container_type = MappedContainer; class value_compare { private: key_compare comp; // exposition only value_compare(key_compare c) : comp(c) { } // exposition only public: bool operator()(const_reference x, const_reference y) const { return comp(x.first, y.first); } }; struct containers { key_container_type keys; mapped_container_type values; }; // [flat.map.cons], constructors flat_map() : flat_map(key_compare()) { } explicit flat_map(const key_compare& comp) : c(), compare(comp) { } flat_map(key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); flat_map(sorted_unique_t, key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); template<class InputIterator> flat_map(InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(), compare(comp) { insert(first, last); } template<class InputIterator> flat_map(sorted_unique_t s, InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(), compare(comp) { insert(s, first, last); } template<container-compatible-range<value_type> R> flat_map(from_range_t, R&& rg) : flat_map(from_range, std::forward<R>(rg), key_compare()) { } template<container-compatible-range<value_type> R> flat_map(from_range_t, R&& rg, const key_compare& comp) : flat_map(comp) { insert_range(std::forward<R>(rg)); } flat_map(initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_map(il.begin(), il.end(), comp) { } flat_map(sorted_unique_t s, initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_map(s, il.begin(), il.end(), comp) { } // [flat.map.cons.alloc], constructors with allocators template<class Alloc> explicit flat_map(const Alloc& a); template<class Alloc> flat_map(const key_compare& comp, const Alloc& a); template<class Alloc> flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Alloc& a); template<class Alloc> flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_map(sorted_unique_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Alloc& a); template<class Alloc> flat_map(sorted_unique_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_map(const flat_map&, const Alloc& a); template<class Alloc> flat_map(flat_map&&, const Alloc& a); template<class InputIterator, class Alloc> flat_map(InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_map(InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<class InputIterator, class Alloc> flat_map(sorted_unique_t, InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_map(sorted_unique_t, InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_map(from_range_t, R&& rg, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_map(from_range_t, R&& rg, const key_compare& comp, const Alloc& a); template<class Alloc> flat_map(initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_map(initializer_list<value_type> il, const key_compare& comp, const Alloc& a); template<class Alloc> flat_map(sorted_unique_t, initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_map(sorted_unique_t, initializer_list<value_type> il, const key_compare& comp, const Alloc& a); flat_map& operator=(initializer_list<value_type>); // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // [flat.map.capacity], capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [flat.map.access], element access mapped_type& operator[](const key_type& x); mapped_type& operator[](key_type&& x); template<class K> mapped_type& operator[](K&& x); mapped_type& at(const key_type& x); const mapped_type& at(const key_type& x) const; template<class K> mapped_type& at(const K& x); template<class K> const mapped_type& at(const K& x) const; // [flat.map.modifiers], modifiers template<class... Args> pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); pair<iterator, bool> insert(const value_type& x) { return emplace(x); } pair<iterator, bool> insert(value_type&& x) { return emplace(std::move(x)); } iterator insert(const_iterator position, const value_type& x) { return emplace_hint(position, x); } iterator insert(const_iterator position, value_type&& x) { return emplace_hint(position, std::move(x)); } template<class P> pair<iterator, bool> insert(P&& x); template<class P> iterator insert(const_iterator position, P&&); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<class InputIterator> void insert(sorted_unique_t, InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); } void insert(sorted_unique_t s, initializer_list<value_type> il) { insert(s, il.begin(), il.end()); } containers extract() &&; void replace(key_container_type&& key_cont, mapped_container_type&& mapped_cont); template<class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template<class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template<class K, class... Args> pair<iterator, bool> try_emplace(K&& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args); template<class K, class... Args> iterator try_emplace(const_iterator hint, K&& k, Args&&... args); template<class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template<class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template<class K, class M> pair<iterator, bool> insert_or_assign(K&& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template<class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj); template<class K, class M> iterator insert_or_assign(const_iterator hint, K&& k, M&& obj); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(flat_map& y) noexcept; void clear() noexcept; // observers key_compare key_comp() const; value_compare value_comp() const; const key_container_type& keys() const noexcept { return c.keys; } const mapped_container_type& values() const noexcept { return c.values; } // map operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; friend bool operator==(const flat_map& x, const flat_map& y); friend synth-three-way-result<value_type> operator<=>(const flat_map& x, const flat_map& y); friend void swap(flat_map& x, flat_map& y) noexcept { x.swap(y); } private: containers c; // exposition only key_compare compare; // exposition only struct key-equiv { // exposition only key-equiv(key_compare c) : comp(c) { } bool operator()(const_reference x, const_reference y) const { return !comp(x.first, y.first) && !comp(y.first, x.first); } key_compare comp; }; }; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_map(KeyContainer, MappedContainer, Compare = Compare()) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_map(KeyContainer, MappedContainer, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_map(KeyContainer, MappedContainer, Compare, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare = Compare()) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_map(sorted_unique_t, KeyContainer, MappedContainer, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>> flat_map(InputIterator, InputIterator, Compare = Compare()) -> flat_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare>; template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>> flat_map(sorted_unique_t, InputIterator, InputIterator, Compare = Compare()) -> flat_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare>; template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<byte>> flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; template<ranges::input_range R, class Allocator> flat_map(from_range_t, R&&, Allocator) -> flat_map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; template<class Key, class T, class Compare = less<Key>> flat_map(initializer_list<pair<Key, T>>, Compare = Compare()) -> flat_map<Key, T, Compare>; template<class Key, class T, class Compare = less<Key>> flat_map(sorted_unique_t, initializer_list<pair<Key, T>>, Compare = Compare()) -> flat_map<Key, T, Compare>; template<class Key, class T, class Compare, class KeyContainer, class MappedContainer, class Allocator> struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>, Allocator> : bool_constant<uses_allocator_v<KeyContainer, Allocator> && uses_allocator_v<MappedContainer, Allocator>> { }; }

The member type containers has the data members and special members specified above.

It has no base classes or members other than those specified.

23.6.8.3 Constructors [flat.map.cons]

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flat_map(key_container_type key_cont, mapped_container_type mapped_cont,
         const key_compare& comp = key_compare());

Effects: Initializes c.keys with std::move(key_cont), c.values with std::move(mapped_cont), and compare with comp; sorts the range [begin(), end()) with respect to value_comp(); and finally erases the duplicate elements as if by: auto zv = views::zip(c.keys, c.values); auto it = ranges::unique(zv, key-equiv(compare)).begin(); auto dist = distance(zv.begin(), it); c.keys.erase(c.keys.begin() + dist, c.keys.end()); c.values.erase(c.values.begin() + dist, c.values.end());

Complexity: Linear in N if the container arguments are already sorted with respect to value_comp() and otherwise

N log N

, where N is the value of key_cont.size() before this call.

🔗

flat_map(sorted_unique_t, key_container_type key_cont, mapped_container_type mapped_cont,
         const key_compare& comp = key_compare());

Effects: Initializes c.keys with std::move(key_cont), c.values with std::move(mapped_cont), and compare with comp.

Complexity: Constant.

23.6.8.4 Constructors with allocators [flat.map.cons.alloc]

The constructors in this subclause shall not participate in overload resolution unless uses_allocator_v<key_container_type, Alloc> is true and uses_allocator_v<mapped_container_type, Alloc> is true.

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template<class Alloc>
  flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
           const Alloc& a);
template<class Alloc>
  flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
           const key_compare& comp, const Alloc& a);

Effects: Equivalent to flat_map(key_cont, mapped_cont) and flat_map(key_cont, mapped_cont, comp), respectively, except that c.keys and c.values are constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Same as flat_map(key_cont, mapped_cont) and flat_map(key_cont, mapped_cont, comp), respectively.

🔗

template<class Alloc>
  flat_map(sorted_unique_t s, const key_container_type& key_cont,
           const mapped_container_type& mapped_cont, const Alloc& a);
template<class Alloc>
  flat_map(sorted_unique_t s, const key_container_type& key_cont,
           const mapped_container_type& mapped_cont, const key_compare& comp,
           const Alloc& a);

Effects: Equivalent to flat_map(s, key_cont, mapped_cont) and flat_map(s, key_cont, mapped_cont, comp), respectively, except that c.keys and c.values are constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Linear.

🔗

template<class Alloc>
  explicit flat_map(const Alloc& a);
template<class Alloc>
  flat_map(const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_map(const flat_map&, const Alloc& a);
template<class Alloc>
  flat_map(flat_map&&, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_map(InputIterator first, InputIterator last, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_map(InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_map(sorted_unique_t, InputIterator first, InputIterator last, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_map(sorted_unique_t, InputIterator first, InputIterator last,
           const key_compare& comp, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_map(from_range_t, R&& rg, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_map(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_map(initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_map(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_map(sorted_unique_t, initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_map(sorted_unique_t, initializer_list<value_type> il,
           const key_compare& comp, const Alloc& a);

Effects: Equivalent to the corresponding non-allocator constructors except that c.keys and c.values are constructed with uses-allocator construction ([allocator.uses.construction]).

23.6.8.5 Capacity [flat.map.capacity]

🔗

size_type size() const noexcept;

Returns: c.keys.size().

🔗

size_type max_size() const noexcept;

Returns: min<size_type>(c.keys.max_size(), c.values.max_size()).

23.6.8.6 Access [flat.map.access]

🔗

mapped_type& operator[](const key_type& x);

Effects: Equivalent to: return try_emplace(x).first->second;

🔗

mapped_type& operator[](key_type&& x);

Effects: Equivalent to: return try_emplace(std::move(x)).first->second;

🔗

template<class K> mapped_type& operator[](K&& x);

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

Effects: Equivalent to: return try_emplace(std::forward<K>(x)).first->second;

🔗

mapped_type&       at(const key_type& x);
const mapped_type& at(const key_type& x) const;

Returns: A reference to the mapped_type corresponding to x in *this.

Throws: An exception object of type out_of_range if no such element is present.

Complexity: Logarithmic.

🔗

template<class K> mapped_type&       at(const K& x);
template<class K> const mapped_type& at(const K& x) const;

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

Preconditions: The expression find(x) is well-formed and has well-defined behavior.

Returns: A reference to the mapped_type corresponding to x in *this.

Throws: An exception object of type out_of_range if no such element is present.

Complexity: Logarithmic.

23.6.8.7 Modifiers [flat.map.modifiers]

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template<class... Args> pair<iterator, bool> emplace(Args&&... args);

Constraints: is_constructible_v<pair<key_type, mapped_type>, Args...> is true.

Effects: Initializes an object t of type pair<key_type, mapped_type> with std::forward<Args>(args)...; if the map already contains an element whose key is equivalent to t.first, *this is unchanged.

Otherwise, equivalent to: auto key_it = ranges::upper_bound(c.keys, t.first, compare); auto value_it = c.values.begin() + distance(c.keys.begin(), key_it); c.keys.insert(key_it, std::move(t.first)); c.values.insert(value_it, std::move(t.second));

Returns: The bool component of the returned pair is true if and only if the insertion took place, and the iterator component of the pair points to the element with key equivalent to t.first.

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template<class P> pair<iterator, bool> insert(P&& x);
template<class P> iterator insert(const_iterator position, P&& x);

Constraints: is_constructible_v<pair<key_type, mapped_type>, P> is true.

Effects: The first form is equivalent to return emplace(std::forward(x));.

The second form is equivalent to return emplace_hint(position, std::forward(x));.

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template<class InputIterator>
  void insert(InputIterator first, InputIterator last);

Effects: Adds elements to c as if by: for (; first != last; ++first) { value_type value = *first; c.keys.insert(c.keys.end(), std::move(value.first)); c.values.insert(c.values.end(), std::move(value.second)); }

Then, sorts the range of newly inserted elements with respect to value_comp(); merges the resulting sorted range and the sorted range of pre-existing elements into a single sorted range; and finally erases the duplicate elements as if by: auto zv = views::zip(c.keys, c.values); auto it = ranges::unique(zv, key-equiv(compare)).begin(); auto dist = distance(zv.begin(), it); c.keys.erase(c.keys.begin() + dist, c.keys.end()); c.values.erase(c.values.begin() + dist, c.values.end());

Complexity: N +

M log M

, where N is size() before the operation and M is distance(first, last).

Remarks: Since this operation performs an in-place merge, it may allocate memory.

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template<class InputIterator>
  void insert(sorted_unique_t, InputIterator first, InputIterator last);

Then, merges the sorted range of newly added elements and the sorted range of pre-existing elements into a single sorted range; and finally erases the duplicate elements as if by: auto zv = views::zip(c.keys, c.values); auto it = ranges::unique(zv, key-equiv(compare)).begin(); auto dist = distance(zv.begin(), it); c.keys.erase(c.keys.begin() + dist, c.keys.end()); c.values.erase(c.values.begin() + dist, c.values.end());

Complexity: Linear in N, where N is size() after the operation.

Remarks: Since this operation performs an in-place merge, it may allocate memory.

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template<container-compatible-range<value_type> R>
  void insert_range(R&& rg);

Effects: Adds elements to c as if by: for (const auto& e : rg) { c.keys.insert(c.keys.end(), e.first); c.values.insert(c.values.end(), e.second); }

Complexity: N +

M log M

, where N is size() before the operation and M is ranges::distance(rg).

Remarks: Since this operation performs an in-place merge, it may allocate memory.

🔗

template<class... Args>
  pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

Constraints: is_constructible_v<mapped_type, Args...> is true.

Effects: If the map already contains an element whose key is equivalent to k, *this and args... are unchanged.

Otherwise equivalent to: auto key_it = ranges::upper_bound(c.keys, k, compare); auto value_it = c.values.begin() + distance(c.keys.begin(), key_it); c.keys.insert(key_it, std::forward<decltype(k)>(k)); c.values.emplace(value_it, std::forward<Args>(args)...);

Returns: In the first two overloads, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace for the first two overloads, and the same as emplace_hint for the last two overloads.

🔗

template<class K, class... Args>
  pair<iterator, bool> try_emplace(K&& k, Args&&... args);
template<class K, class... Args>
  iterator try_emplace(const_iterator hint, K&& k, Args&&... args);

Constraints:

(19.1)
The qualified-id Compare::is_transparent is valid and denotes a type.
(19.2)
is_constructible_v<key_type, K> is true.
(19.3)
is_constructible_v<mapped_type, Args...> is true.
(19.4)
For the first overload, is_convertible_v<K&&, const_iterator> and is_convertible_v<K&&, iterator> are both false.

Preconditions: The conversion from k into key_type constructs an object u, for which find(k) == find(u) is true.

Effects: If the map already contains an element whose key is equivalent to k, *this and args... are unchanged.

Otherwise equivalent to: auto key_it = ranges::upper_bound(c.keys, k, compare); auto value_it = c.values.begin() + distance(c.keys.begin(), key_it); c.keys.emplace(key_it, std::forward<K>(k)); c.values.emplace(value_it, std::forward<Args>(args)...);

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class M>
  pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

Constraints: is_assignable_v<mapped_type&, M> is true and is_constructible_v<mapped_type, M> is true.

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise, equivalent to try_emplace(std::forward<decltype(k)>(k), std::forward<M>(obj)) for the first two overloads or try_emplace(hint, std::forward<decltype(k)>(k), std::forward<M>(obj)) for the last two overloads.

Returns: In the first two overloads, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace for the first two overloads and the same as emplace_hint for the last two overloads.

🔗

template<class K, class M>
  pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
template<class K, class M>
  iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

Constraints:

(28.1)
The qualified-id Compare::is_transparent is valid and denotes a type.
(28.2)
is_constructible_v<key_type, K> is true.
(28.3)
is_assignable_v<mapped_type&, M> is true.
(28.4)
is_constructible_v<mapped_type, M> is true.

Preconditions: The conversion from k into key_type constructs an object u, for which find(k) == find(u) is true.

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise, equivalent to try_emplace(std::forward<K>(k), std::forward<M>(obj)) for the first overload or try_emplace(hint, std::forward<K>(k), std::forward<M>(obj)) for the second overload.

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

void swap(flat_map& y) noexcept;

Effects: Equivalent to: ranges::swap(compare, y.compare); ranges::swap(c.keys, y.c.keys); ranges::swap(c.values, y.c.values);

🔗

containers extract() &&;

Postconditions: *this is emptied, even if the function exits via an exception.

Returns: std::move(c).

🔗

void replace(key_container_type&& key_cont, mapped_container_type&& mapped_cont);

Preconditions: key_cont.size() == mapped_cont.size() is true, the elements of key_cont are sorted with respect to compare, and key_cont contains no equal elements.

Effects: Equivalent to: c.keys = std::move(key_cont); c.values = std::move(mapped_cont);

23.6.8.8 Erasure [flat.map.erasure]

🔗

template<class Key, class T, class Compare, class KeyContainer, class MappedContainer,
         class Predicate>
  typename flat_map<Key, T, Compare, KeyContainer, MappedContainer>::size_type
    erase_if(flat_map<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred);

Preconditions: Key and T meet the Cpp17MoveAssignable requirements.

Effects: Let E be bool(pred(pair<const Key&, const T&>(e))).

Erases all elements e in c for which E holds.

Returns: The number of elements erased.

Complexity: Exactly c.size() applications of the predicate.

Remarks: Stable ([algorithm.stable]).

If an invocation of erase_if exits via an exception, c is in a valid but unspecified state ([defns.valid]).

[Note 1:

c still meets its invariants, but can be empty.

— end note]

23.6.9 Class template flat_multimap [flat.multimap]

23.6.9.1 Overview [flat.multimap.overview]

A flat_multimap is a container adaptor that provides an associative container interface that supports equivalent keys (i.e., possibly containing multiple copies of the same key value) and provides for fast retrieval of values of another type T based on the keys.

flat_multimap supports iterators that meet the Cpp17InputIterator requirements and model the random_access_iterator concept ([iterator.concept.random.access]).

A flat_multimap meets all of the requirements for a container ([container.reqmts]) and for a reversible container ([container.rev.reqmts]), plus the optional container requirements ([container.opt.reqmts]).

flat_multimap meets the requirements of an associative container ([associative.reqmts]), except that:

(2.1)
it does not meet the requirements related to node handles ([container.node]),
(2.2)
it does not meet the requirements related to iterator invalidation, and
(2.3)
the time complexity of the operations that insert or erase a single element from the map is linear, including the ones that take an insertion position iterator.

[Note 1:

A flat_multimap does not meet the additional requirements of an allocator-aware container ([container.alloc.reqmts]).

— end note]

A flat_multimap also provides most operations described in [associative.reqmts] for equal keys.

This means that a flat_multimap supports the a_eq operations in [associative.reqmts] but not the a_uniq operations.

For a flat_multimap<Key, T> the key_type is Key and the value_type is pair<Key, T>.

Except as otherwise noted, operations on flat_multimap are equivalent to those of flat_map, except that flat_multimap operations do not remove or replace elements with equal keys.

[Example 1:

flat_multimap constructors and emplace do not erase non-unique elements after sorting them.

— end example]

A flat_multimap maintains the following invariants:

(5.1)
it contains the same number of keys and values;
(5.2)
the keys are sorted with respect to the comparison object; and
(5.3)
the value at offset off within the value container is the value associated with the key at offset off within the key container.

If any member function in [flat.multimap.defn] exits via an exception, the invariants are restored.

[Note 2:

This can result in the flat_multimap being emptied.

— end note]

Any type C that meets the sequence container requirements ([sequence.reqmts]) can be used to instantiate flat_multimap, as long as C::iterator meets the Cpp17RandomAccessIterator requirements and invocations of member functions C::size and C::max_size do not exit via an exception.

In particular, vector ([vector]) and deque ([deque]) can be used.

[Note 3:

vector<bool> is not a sequence container.

— end note]

The program is ill-formed if Key is not the same type as KeyContainer::value_type or T is not the same type as MappedContainer::value_type.

The effect of calling a constructor that takes both key_container_type and mapped_container_type arguments with containers of different sizes is undefined.

The effect of calling a constructor or member function that takes a sorted_equivalent_t argument with a container, containers, or range that are not sorted with respect to key_comp() is undefined.

23.6.9.2 Definition [flat.multimap.defn]

namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_multimap { public: // types using key_type = Key; using mapped_type = T; using value_type = pair<key_type, mapped_type>; using key_compare = Compare; using reference = pair<const key_type&, mapped_type&>; using const_reference = pair<const key_type&, const mapped_type&>; using size_type = size_t; using difference_type = ptrdiff_t; using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using key_container_type = KeyContainer; using mapped_container_type = MappedContainer; class value_compare { private: key_compare comp; // exposition only value_compare(key_compare c) : comp(c) { } // exposition only public: bool operator()(const_reference x, const_reference y) const { return comp(x.first, y.first); } }; struct containers { key_container_type keys; mapped_container_type values; }; // [flat.multimap.cons], constructors flat_multimap() : flat_multimap(key_compare()) { } explicit flat_multimap(const key_compare& comp) : c(), compare(comp) { } flat_multimap(key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); flat_multimap(sorted_equivalent_t, key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); template<class InputIterator> flat_multimap(InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(), compare(comp) { insert(first, last); } template<class InputIterator> flat_multimap(sorted_equivalent_t s, InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(), compare(comp) { insert(s, first, last); } template<container-compatible-range<value_type> R> flat_multimap(from_range_t, R&& rg) : flat_multimap(from_range, std::forward<R>(rg), key_compare()) { } template<container-compatible-range<value_type> R> flat_multimap(from_range_t, R&& rg, const key_compare& comp) : flat_multimap(comp) { insert_range(std::forward<R>(rg)); } flat_multimap(initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_multimap(il.begin(), il.end(), comp) { } flat_multimap(sorted_equivalent_t s, initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_multimap(s, il.begin(), il.end(), comp) { } // [flat.multimap.cons.alloc], constructors with allocators template<class Alloc> explicit flat_multimap(const Alloc& a); template<class Alloc> flat_multimap(const key_compare& comp, const Alloc& a); template<class Alloc> flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Alloc& a); template<class Alloc> flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multimap(sorted_equivalent_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Alloc& a); template<class Alloc> flat_multimap(sorted_equivalent_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multimap(const flat_multimap&, const Alloc& a); template<class Alloc> flat_multimap(flat_multimap&&, const Alloc& a); template<class InputIterator, class Alloc> flat_multimap(InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_multimap(InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<class InputIterator, class Alloc> flat_multimap(sorted_equivalent_t, InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_multimap(sorted_equivalent_t, InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_multimap(from_range_t, R&& rg, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_multimap(from_range_t, R&& rg, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multimap(initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_multimap(initializer_list<value_type> il, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multimap(sorted_equivalent_t, initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_multimap(sorted_equivalent_t, initializer_list<value_type> il, const key_compare& comp, const Alloc& a); flat_multimap& operator=(initializer_list<value_type>); // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& x) { return emplace(x); } iterator insert(value_type&& x) { return emplace(std::move(x)); } iterator insert(const_iterator position, const value_type& x) { return emplace_hint(position, x); } iterator insert(const_iterator position, value_type&& x) { return emplace_hint(position, std::move(x)); } template<class P> iterator insert(P&& x); template<class P> iterator insert(const_iterator position, P&&); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<class InputIterator> void insert(sorted_equivalent_t, InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); } void insert(sorted_equivalent_t s, initializer_list<value_type> il) { insert(s, il.begin(), il.end()); } containers extract() &&; void replace(key_container_type&& key_cont, mapped_container_type&& mapped_cont); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(flat_multimap&) noexcept; void clear() noexcept; // observers key_compare key_comp() const; value_compare value_comp() const; const key_container_type& keys() const noexcept { return c.keys; } const mapped_container_type& values() const noexcept { return c.values; } // map operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; friend bool operator==(const flat_multimap& x, const flat_multimap& y); friend synth-three-way-result<value_type> operator<=>(const flat_multimap& x, const flat_multimap& y); friend void swap(flat_multimap& x, flat_multimap& y) noexcept { x.swap(y); } private: containers c; // exposition only key_compare compare; // exposition only }; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_multimap(KeyContainer, MappedContainer, Compare = Compare()) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_multimap(KeyContainer, MappedContainer, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_multimap(KeyContainer, MappedContainer, Compare, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare = Compare()) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>> flat_multimap(InputIterator, InputIterator, Compare = Compare()) -> flat_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare>; template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>> flat_multimap(sorted_equivalent_t, InputIterator, InputIterator, Compare = Compare()) -> flat_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare>; template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<byte>> flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; template<ranges::input_range R, class Allocator> flat_multimap(from_range_t, R&&, Allocator) -> flat_multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; template<class Key, class T, class Compare = less<Key>> flat_multimap(initializer_list<pair<Key, T>>, Compare = Compare()) -> flat_multimap<Key, T, Compare>; template<class Key, class T, class Compare = less<Key>> flat_multimap(sorted_equivalent_t, initializer_list<pair<Key, T>>, Compare = Compare()) -> flat_multimap<Key, T, Compare>; template<class Key, class T, class Compare, class KeyContainer, class MappedContainer, class Allocator> struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>, Allocator> : bool_constant<uses_allocator_v<KeyContainer, Allocator> && uses_allocator_v<MappedContainer, Allocator>> { }; }

The member type containers has the data members and special members specified above.

It has no base classes or members other than those specified.

23.6.9.3 Constructors [flat.multimap.cons]

🔗

flat_multimap(key_container_type key_cont, mapped_container_type mapped_cont,
              const key_compare& comp = key_compare());

Effects: Initializes c.keys with std::move(key_cont), c.values with std::move(mapped_cont), and compare with comp; sorts the range [begin(), end()) with respect to value_comp().

Complexity: Linear in N if the container arguments are already sorted with respect to value_comp() and otherwise

N log N

, where N is the value of key_cont.size() before this call.

🔗

flat_multimap(sorted_equivalent_t, key_container_type key_cont, mapped_container_type mapped_cont,
              const key_compare& comp = key_compare());

Effects: Initializes c.keys with std::move(key_cont), c.values with std::move(mapped_cont), and compare with comp.

Complexity: Constant.

23.6.9.4 Constructors with allocators [flat.multimap.cons.alloc]

The constructors in this subclause shall not participate in overload resolution unless uses_allocator_v<key_container_type, Alloc> is true and uses_allocator_v<mapped_container_type, Alloc> is true.

🔗

template<class Alloc>
  flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
                const Alloc& a);
template<class Alloc>
  flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont,
                const key_compare& comp, const Alloc& a);

Effects: Equivalent to flat_multimap(key_cont, mapped_cont) and flat_multimap(key_cont, mapped_cont, comp), respectively, except that c.keys and c.values are constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Same as flat_multimap(key_cont, mapped_cont) and flat_multimap(key_cont, mapped_cont, comp), respectively.

🔗

template<class Alloc>
  flat_multimap(sorted_equivalent_t s, const key_container_type& key_cont,
                const mapped_container_type& mapped_cont, const Alloc& a);
template<class Alloc>
  flat_multimap(sorted_equivalent_t s, const key_container_type& key_cont,
                const mapped_container_type& mapped_cont, const key_compare& comp,
                const Alloc& a);

Effects: Equivalent to flat_multimap(s, key_cont, mapped_cont) and flat_multimap(s, key_cont, mapped_cont, comp), respectively, except that c.keys and c.values are constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Linear.

🔗

template<class Alloc>
  explicit flat_multimap(const Alloc& a);
template<class Alloc>
  flat_multimap(const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_multimap(const flat_multimap&, const Alloc& a);
template<class Alloc>
  flat_multimap(flat_multimap&&, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multimap(InputIterator first, InputIterator last, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multimap(InputIterator first, InputIterator last, const key_compare& comp,
                const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multimap(sorted_equivalent_t, InputIterator first, InputIterator last,
                const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multimap(sorted_equivalent_t, InputIterator first, InputIterator last,
                const key_compare& comp, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_multimap(from_range_t, R&& rg, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_multimap(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_multimap(initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_multimap(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_multimap(sorted_equivalent_t, initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_multimap(sorted_equivalent_t, initializer_list<value_type> il,
                const key_compare& comp, const Alloc& a);

Effects: Equivalent to the corresponding non-allocator constructors except that c.keys and c.values are constructed with uses-allocator construction ([allocator.uses.construction]).

23.6.9.5 Erasure [flat.multimap.erasure]

🔗

template<class Key, class T, class Compare, class KeyContainer, class MappedContainer,
         class Predicate>
  typename flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>::size_type
    erase_if(flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred);

Preconditions: Key and T meet the Cpp17MoveAssignable requirements.

Effects: Let E be bool(pred(pair<const Key&, const T&>(e))).

Erases all elements e in c for which E holds.

Returns: The number of elements erased.

Complexity: Exactly c.size() applications of the predicate.

Remarks: Stable ([algorithm.stable]).

If an invocation of erase_if exits via an exception, c is in a valid but unspecified state ([defns.valid]).

[Note 1:

c still meets its invariants, but can be empty.

— end note]

23.6.10 Header <flat_set> synopsis [flat.set.syn]

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [flat.set], class template flat_set template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_set; struct sorted_unique_t { explicit sorted_unique_t() = default; }; inline constexpr sorted_unique_t sorted_unique{}; template<class Key, class Compare, class KeyContainer, class Allocator> struct uses_allocator<flat_set<Key, Compare, KeyContainer>, Allocator>; // [flat.set.erasure], erasure for flat_set template<class Key, class Compare, class KeyContainer, class Predicate> typename flat_set<Key, Compare, KeyContainer>::size_type erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred); // [flat.multiset], class template flat_multiset template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_multiset; struct sorted_equivalent_t { explicit sorted_equivalent_t() = default; }; inline constexpr sorted_equivalent_t sorted_equivalent{}; template<class Key, class Compare, class KeyContainer, class Allocator> struct uses_allocator<flat_multiset<Key, Compare, KeyContainer>, Allocator>; // [flat.multiset.erasure], erasure for flat_multiset template<class Key, class Compare, class KeyContainer, class Predicate> typename flat_multiset<Key, Compare, KeyContainer>::size_type erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred); }

23.6.11 Class template flat_set [flat.set]

23.6.11.1 Overview [flat.set.overview]

A flat_set is a container adaptor that provides an associative container interface that supports unique keys (i.e., contains at most one of each key value) and provides for fast retrieval of the keys themselves.

flat_set supports iterators that model the random_access_iterator concept ([iterator.concept.random.access]).

A flat_set meets all of the requirements for a container ([container.reqmts]) and for a reversible container ([container.rev.reqmts]), plus the optional container requirements ([container.opt.reqmts]).

flat_set meets the requirements of an associative container ([associative.reqmts]), except that:

(2.1)
it does not meet the requirements related to node handles ([container.node.overview]),
(2.2)
it does not meet the requirements related to iterator invalidation, and
(2.3)
the time complexity of the operations that insert or erase a single element from the set is linear, including the ones that take an insertion position iterator.

[Note 1:

A flat_set does not meet the additional requirements of an allocator-aware container, as described in [container.alloc.reqmts].

— end note]

A flat_set also provides most operations described in [associative.reqmts] for unique keys.

This means that a flat_set supports the a_uniq operations in [associative.reqmts] but not the a_eq operations.

For a flat_set<Key>, both the key_type and value_type are Key.

Descriptions are provided here only for operations on flat_set that are not described in one of those sets of requirements or for operations where there is additional semantic information.

A flat_set maintains the invariant that the keys are sorted with respect to the comparison object.

If any member function in [flat.set.defn] exits via an exception, the invariant is restored.

[Note 2:

This can result in the flat_set's being emptied.

— end note]

Any sequence container ([sequence.reqmts]) supporting Cpp17RandomAccessIterator can be used to instantiate flat_set.

In particular, vector ([vector]) and deque ([deque]) can be used.

[Note 3:

vector<bool> is not a sequence container.

— end note]

The program is ill-formed if Key is not the same type as KeyContainer::value_type.

The effect of calling a constructor or member function that takes a sorted_unique_t argument with a range that is not sorted with respect to key_comp(), or that contains equal elements, is undefined.

23.6.11.2 Definition [flat.set.defn]

namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_set { public: // types using key_type = Key; using value_type = Key; using key_compare = Compare; using value_compare = Compare; using reference = value_type&; using const_reference = const value_type&; using size_type = typename KeyContainer::size_type; using difference_type = typename KeyContainer::difference_type; using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using container_type = KeyContainer; // [flat.set.cons], constructors flat_set() : flat_set(key_compare()) { } explicit flat_set(const key_compare& comp) : c(), compare(comp) { } explicit flat_set(container_type cont, const key_compare& comp = key_compare()); flat_set(sorted_unique_t, container_type cont, const key_compare& comp = key_compare()) : c(std::move(cont)), compare(comp) { } template<class InputIterator> flat_set(InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(), compare(comp) { insert(first, last); } template<class InputIterator> flat_set(sorted_unique_t, InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(first, last), compare(comp) { } template<container-compatible-range<value_type> R> flat_set(from_range_t, R&& rg) : flat_set(from_range, std::forward<R>(rg), key_compare()) { } template<container-compatible-range<value_type> R> flat_set(from_range_t, R&& rg, const key_compare& comp) : flat_set(comp) { insert_range(std::forward<R>(rg)); } flat_set(initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_set(il.begin(), il.end(), comp) { } flat_set(sorted_unique_t s, initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_set(s, il.begin(), il.end(), comp) { } // [flat.set.cons.alloc], constructors with allocators template<class Alloc> explicit flat_set(const Alloc& a); template<class Alloc> flat_set(const key_compare& comp, const Alloc& a); template<class Alloc> flat_set(const container_type& cont, const Alloc& a); template<class Alloc> flat_set(const container_type& cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_set(sorted_unique_t, const container_type& cont, const Alloc& a); template<class Alloc> flat_set(sorted_unique_t, const container_type& cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_set(const flat_set&, const Alloc& a); template<class Alloc> flat_set(flat_set&&, const Alloc& a); template<class InputIterator, class Alloc> flat_set(InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_set(InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<class InputIterator, class Alloc> flat_set(sorted_unique_t, InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_set(sorted_unique_t, InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_set(from_range_t, R&& rg, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_set(from_range_t, R&& rg, const key_compare& comp, const Alloc& a); template<class Alloc> flat_set(initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_set(initializer_list<value_type> il, const key_compare& comp, const Alloc& a); template<class Alloc> flat_set(sorted_unique_t, initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_set(sorted_unique_t, initializer_list<value_type> il, const key_compare& comp, const Alloc& a); flat_set& operator=(initializer_list<value_type>); // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [flat.set.modifiers], modifiers template<class... Args> pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); pair<iterator, bool> insert(const value_type& x) { return emplace(x); } pair<iterator, bool> insert(value_type&& x) { return emplace(std::move(x)); } template<class K> pair<iterator, bool> insert(K&& x); iterator insert(const_iterator position, const value_type& x) { return emplace_hint(position, x); } iterator insert(const_iterator position, value_type&& x) { return emplace_hint(position, std::move(x)); } template<class K> iterator insert(const_iterator hint, K&& x); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<class InputIterator> void insert(sorted_unique_t, InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); } void insert(sorted_unique_t s, initializer_list<value_type> il) { insert(s, il.begin(), il.end()); } container_type extract() &&; void replace(container_type&&); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(flat_set& y) noexcept; void clear() noexcept; // observers key_compare key_comp() const; value_compare value_comp() const; // set operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; friend bool operator==(const flat_set& x, const flat_set& y); friend synth-three-way-result<value_type> operator<=>(const flat_set& x, const flat_set& y); friend void swap(flat_set& x, flat_set& y) noexcept { x.swap(y); } private: container_type c; // exposition only key_compare compare; // exposition only }; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_set(KeyContainer, Compare = Compare()) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_set(KeyContainer, Allocator) -> flat_set<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_set(KeyContainer, Compare, Allocator) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_set(sorted_unique_t, KeyContainer, Compare = Compare()) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_set(sorted_unique_t, KeyContainer, Allocator) -> flat_set<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_set(sorted_unique_t, KeyContainer, Compare, Allocator) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; template<class InputIterator, class Compare = less<iter-value-type<InputIterator>>> flat_set(InputIterator, InputIterator, Compare = Compare()) -> flat_set<iter-value-type<InputIterator>, Compare>; template<class InputIterator, class Compare = less<iter-value-type<InputIterator>>> flat_set(sorted_unique_t, InputIterator, InputIterator, Compare = Compare()) -> flat_set<iter-value-type<InputIterator>, Compare>; template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_set<ranges::range_value_t<R>, Compare, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<ranges::input_range R, class Allocator> flat_set(from_range_t, R&&, Allocator) -> flat_set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<class Key, class Compare = less<Key>> flat_set(initializer_list<Key>, Compare = Compare()) -> flat_set<Key, Compare>; template<class Key, class Compare = less<Key>> flat_set(sorted_unique_t, initializer_list<Key>, Compare = Compare()) -> flat_set<Key, Compare>; template<class Key, class Compare, class KeyContainer, class Allocator> struct uses_allocator<flat_set<Key, Compare, KeyContainer>, Allocator> : bool_constant<uses_allocator_v<KeyContainer, Allocator>> { }; }

23.6.11.3 Constructors [flat.set.cons]

🔗

explicit flat_set(container_type cont, const key_compare& comp = key_compare());

Effects: Initializes c with std::move(cont) and compare with comp, sorts the range [begin(), end()) with respect to compare, and finally erases all but the first element from each group of consecutive equivalent elements.

Complexity: Linear in N if cont is already sorted with respect to compare and otherwise

N log N

, where N is the value of cont.size() before this call.

23.6.11.4 Constructors with allocators [flat.set.cons.alloc]

The constructors in this subclause shall not participate in overload resolution unless uses_allocator_v<container_type, Alloc> is true.

🔗

template<class Alloc>
  flat_set(const container_type& cont, const Alloc& a);
template<class Alloc>
  flat_set(const container_type& cont, const key_compare& comp, const Alloc& a);

Effects: Equivalent to flat_set(cont) and flat_set(cont, comp), respectively, except that c is constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Same as flat_set(cont) and flat_set(cont, comp), respectively.

🔗

template<class Alloc>
  flat_set(sorted_unique_t s, const container_type& cont, const Alloc& a);
template<class Alloc>
  flat_set(sorted_unique_t s, const container_type& cont,
           const key_compare& comp, const Alloc& a);

Effects: Equivalent to flat_set(s, cont) and flat_set(s, cont, comp), respectively, except that c is constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Linear.

🔗

template<class Alloc>
  explicit flat_set(const Alloc& a);
template<class Alloc>
  flat_set(const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_set(const flat_set&, const Alloc& a);
template<class Alloc>
  flat_set(flat_set&&, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_set(InputIterator first, InputIterator last, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_set(InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_set(sorted_unique_t, InputIterator first, InputIterator last, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_set(sorted_unique_t, InputIterator first, InputIterator last,
           const key_compare& comp, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_set(from_range_t, R&& rg, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_set(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_set(initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_set(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_set(sorted_unique_t, initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_set(sorted_unique_t, initializer_list<value_type> il,
           const key_compare& comp, const Alloc& a);

Effects: Equivalent to the corresponding non-allocator constructors except that c is constructed with uses-allocator construction ([allocator.uses.construction]).

23.6.11.5 Modifiers [flat.set.modifiers]

🔗

template<class K> pair<iterator, bool> insert(K&& x);
template<class K> iterator insert(const_iterator hint, K&& x);

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

is_constructible_v<value_type, K> is true.

Preconditions: The conversion from x into value_type constructs an object u, for which find(x) == find(u) is true.

Effects: If the set already contains an element equivalent to x, *this and x are unchanged.

Otherwise, inserts a new element as if by emplace(std::forward<K>(x)).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the element whose key is equivalent to x.

🔗

template<class InputIterator>
  void insert(InputIterator first, InputIterator last);

Effects: Adds elements to c as if by: c.insert(c.end(), first, last);

Then, sorts the range of newly inserted elements with respect to compare; merges the resulting sorted range and the sorted range of pre-existing elements into a single sorted range; and finally erases all but the first element from each group of consecutive equivalent elements.

Complexity: N +

M log M

, where N is size() before the operation and M is distance(first, last).

Remarks: Since this operation performs an in-place merge, it may allocate memory.

🔗

template<class InputIterator>
  void insert(sorted_unique_t, InputIterator first, InputIterator last);

Effects: Equivalent to insert(first, last).

Complexity: Linear.

🔗

template<container-compatible-range<value_type> R>
  void insert_range(R&& rg);

Effects: Adds elements to c as if by: for (const auto& e : rg) { c.insert(c.end(), e); }

Complexity: N +

M log M

, where N is size() before the operation and M is ranges::distance(rg).

Remarks: Since this operation performs an in-place merge, it may allocate memory.

🔗

void swap(flat_set& y) noexcept;

Effects: Equivalent to: ranges::swap(compare, y.compare); ranges::swap(c, y.c);

🔗

container_type extract() &&;

Postconditions: *this is emptied, even if the function exits via an exception.

Returns: std::move(c).

🔗

void replace(container_type&& cont);

Preconditions: The elements of cont are sorted with respect to compare, and cont contains no equal elements.

Effects: Equivalent to: c = std::move(cont);

23.6.11.6 Erasure [flat.set.erasure]

🔗

template<class Key, class Compare, class KeyContainer, class Predicate>
  typename flat_set<Key, Compare, KeyContainer>::size_type
    erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred);

Preconditions: Key meets the Cpp17MoveAssignable requirements.

Effects: Let E be bool(pred(as_const(e))).

Erases all elements e in c for which E holds.

Returns: The number of elements erased.

Complexity: Exactly c.size() applications of the predicate.

Remarks: Stable ([algorithm.stable]).

If an invocation of erase_if exits via an exception, c is in a valid but unspecified state ([defns.valid]).

[Note 1:

c still meets its invariants, but can be empty.

— end note]

23.6.12 Class template flat_multiset [flat.multiset]

23.6.12.1 Overview [flat.multiset.overview]

A flat_multiset is a container adaptor that provides an associative container interface that supports equivalent keys (i.e., possibly containing multiple copies of the same key value) and provides for fast retrieval of the keys themselves.

flat_multiset supports iterators that model the random_access_iterator concept ([iterator.concept.random.access]).

A flat_multiset meets all of the requirements for a container ([container.reqmts]) and for a reversible container ([container.rev.reqmts]), plus the optional container requirements ([container.opt.reqmts]).

flat_multiset meets the requirements of an associative container ([associative.reqmts]), except that:

(2.1)
it does not meet the requirements related to node handles ([container.node.overview]),
(2.2)
it does not meet the requirements related to iterator invalidation, and
(2.3)
the time complexity of the operations that insert or erase a single element from the set is linear, including the ones that take an insertion position iterator.

[Note 1:

A flat_multiset does not meet the additional requirements of an allocator-aware container, as described in [container.alloc.reqmts].

— end note]

A flat_multiset also provides most operations described in [associative.reqmts] for equal keys.

This means that a flat_multiset supports the a_eq operations in [associative.reqmts] but not the a_uniq operations.

For a flat_multiset<Key>, both the key_type and value_type are Key.

Descriptions are provided here only for operations on flat_multiset that are not described in one of the general sections or for operations where there is additional semantic information.

A flat_multiset maintains the invariant that the keys are sorted with respect to the comparison object.

If any member function in [flat.multiset.defn] exits via an exception, the invariant is restored.

[Note 2:

This can result in the flat_multiset's being emptied.

— end note]

Any sequence container ([sequence.reqmts]) supporting Cpp17RandomAccessIterator can be used to instantiate flat_multiset.

In particular, vector ([vector]) and deque ([deque]) can be used.

[Note 3:

vector<bool> is not a sequence container.

— end note]

The program is ill-formed if Key is not the same type as KeyContainer::value_type.

The effect of calling a constructor or member function that takes a sorted_equivalent_t argument with a range that is not sorted with respect to key_comp() is undefined.

23.6.12.2 Definition [flat.multiset.defn]

namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_multiset { public: // types using key_type = Key; using value_type = Key; using key_compare = Compare; using value_compare = Compare; using reference = value_type&; using const_reference = const value_type&; using size_type = typename KeyContainer::size_type; using difference_type = typename KeyContainer::difference_type; using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using container_type = KeyContainer; // [flat.multiset.cons], constructors flat_multiset() : flat_multiset(key_compare()) { } explicit flat_multiset(const key_compare& comp) : c(), compare(comp) { } explicit flat_multiset(container_type cont, const key_compare& comp = key_compare()); flat_multiset(sorted_equivalent_t, container_type cont, const key_compare& comp = key_compare()) : c(std::move(cont)), compare(comp) { } template<class InputIterator> flat_multiset(InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(), compare(comp) { insert(first, last); } template<class InputIterator> flat_multiset(sorted_equivalent_t, InputIterator first, InputIterator last, const key_compare& comp = key_compare()) : c(first, last), compare(comp) { } template<container-compatible-range<value_type> R> flat_multiset(from_range_t, R&& rg) : flat_multiset(from_range, std::forward<R>(rg), key_compare()) { } template<container-compatible-range<value_type> R> flat_multiset(from_range_t, R&& rg, const key_compare& comp) : flat_multiset(comp) { insert_range(std::forward<R>(rg)); } flat_multiset(initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_multiset(il.begin(), il.end(), comp) { } flat_multiset(sorted_equivalent_t s, initializer_list<value_type> il, const key_compare& comp = key_compare()) : flat_multiset(s, il.begin(), il.end(), comp) { } // [flat.multiset.cons.alloc], constructors with allocators template<class Alloc> explicit flat_multiset(const Alloc& a); template<class Alloc> flat_multiset(const key_compare& comp, const Alloc& a); template<class Alloc> flat_multiset(const container_type& cont, const Alloc& a); template<class Alloc> flat_multiset(const container_type& cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multiset(sorted_equivalent_t, const container_type& cont, const Alloc& a); template<class Alloc> flat_multiset(sorted_equivalent_t, const container_type& cont, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multiset(const flat_multiset&, const Alloc& a); template<class Alloc> flat_multiset(flat_multiset&&, const Alloc& a); template<class InputIterator, class Alloc> flat_multiset(InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_multiset(InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<class InputIterator, class Alloc> flat_multiset(sorted_equivalent_t, InputIterator first, InputIterator last, const Alloc& a); template<class InputIterator, class Alloc> flat_multiset(sorted_equivalent_t, InputIterator first, InputIterator last, const key_compare& comp, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_multiset(from_range_t, R&& rg, const Alloc& a); template<container-compatible-range<value_type> R, class Alloc> flat_multiset(from_range_t, R&& rg, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multiset(initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_multiset(initializer_list<value_type> il, const key_compare& comp, const Alloc& a); template<class Alloc> flat_multiset(sorted_equivalent_t, initializer_list<value_type> il, const Alloc& a); template<class Alloc> flat_multiset(sorted_equivalent_t, initializer_list<value_type> il, const key_compare& comp, const Alloc& a); flat_multiset& operator=(initializer_list<value_type>); // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; reverse_iterator rbegin() noexcept; const_reverse_iterator rbegin() const noexcept; reverse_iterator rend() noexcept; const_reverse_iterator rend() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; const_reverse_iterator crbegin() const noexcept; const_reverse_iterator crend() const noexcept; // capacity bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [flat.multiset.modifiers], modifiers template<class... Args> iterator emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); iterator insert(const value_type& x) { return emplace(x); } iterator insert(value_type&& x) { return emplace(std::move(x)); } iterator insert(const_iterator position, const value_type& x) { return emplace_hint(position, x); } iterator insert(const_iterator position, value_type&& x) { return emplace_hint(position, std::move(x)); } template<class InputIterator> void insert(InputIterator first, InputIterator last); template<class InputIterator> void insert(sorted_equivalent_t, InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); } void insert(sorted_equivalent_t s, initializer_list<value_type> il) { insert(s, il.begin(), il.end()); } container_type extract() &&; void replace(container_type&&); iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(flat_multiset& y) noexcept; void clear() noexcept; // observers key_compare key_comp() const; value_compare value_comp() const; // set operations iterator find(const key_type& x); const_iterator find(const key_type& x) const; template<class K> iterator find(const K& x); template<class K> const_iterator find(const K& x) const; size_type count(const key_type& x) const; template<class K> size_type count(const K& x) const; bool contains(const key_type& x) const; template<class K> bool contains(const K& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; template<class K> iterator lower_bound(const K& x); template<class K> const_iterator lower_bound(const K& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type& x) const; template<class K> iterator upper_bound(const K& x); template<class K> const_iterator upper_bound(const K& x) const; pair<iterator, iterator> equal_range(const key_type& x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K> pair<iterator, iterator> equal_range(const K& x); template<class K> pair<const_iterator, const_iterator> equal_range(const K& x) const; friend bool operator==(const flat_multiset& x, const flat_multiset& y); friend synth-three-way-result<value_type> operator<=>(const flat_multiset& x, const flat_multiset& y); friend void swap(flat_multiset& x, flat_multiset& y) noexcept { x.swap(y); } private: container_type c; // exposition only key_compare compare; // exposition only }; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_multiset(KeyContainer, Compare = Compare()) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_multiset(KeyContainer, Allocator) -> flat_multiset<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_multiset(KeyContainer, Compare, Allocator) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_multiset(sorted_equivalent_t, KeyContainer, Compare = Compare()) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_multiset(sorted_equivalent_t, KeyContainer, Allocator) -> flat_multiset<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_multiset(sorted_equivalent_t, KeyContainer, Compare, Allocator) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; template<class InputIterator, class Compare = less<iter-value-type<InputIterator>>> flat_multiset(InputIterator, InputIterator, Compare = Compare()) -> flat_multiset<iter-value-type<InputIterator>, Compare>; template<class InputIterator, class Compare = less<iter-value-type<InputIterator>>> flat_multiset(sorted_equivalent_t, InputIterator, InputIterator, Compare = Compare()) -> flat_multiset<iter-value-type<InputIterator>, Compare>; template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_multiset<ranges::range_value_t<R>, Compare, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<ranges::input_range R, class Allocator> flat_multiset(from_range_t, R&&, Allocator) -> flat_multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<class Key, class Compare = less<Key>> flat_multiset(initializer_list<Key>, Compare = Compare()) -> flat_multiset<Key, Compare>; template<class Key, class Compare = less<Key>> flat_multiset(sorted_equivalent_t, initializer_list<Key>, Compare = Compare()) -> flat_multiset<Key, Compare>; template<class Key, class Compare, class KeyContainer, class Allocator> struct uses_allocator<flat_multiset<Key, Compare, KeyContainer>, Allocator> : bool_constant<uses_allocator_v<KeyContainer, Allocator>> { }; }

23.6.12.3 Constructors [flat.multiset.cons]

🔗

explicit flat_multiset(container_type cont, const key_compare& comp = key_compare());

Effects: Initializes c with std::move(cont) and compare with comp, and sorts the range [begin(), end()) with respect to compare.

Complexity: Linear in N if cont is already sorted with respect to compare and otherwise

N log N

, where N is the value of cont.size() before this call.

23.6.12.4 Constructors with allocators [flat.multiset.cons.alloc]

The constructors in this subclause shall not participate in overload resolution unless uses_allocator_v<container_type, Alloc> is true.

🔗

template<class Alloc>
  flat_multiset(const container_type& cont, const Alloc& a);
template<class Alloc>
  flat_multiset(const container_type& cont, const key_compare& comp, const Alloc& a);

Effects: Equivalent to flat_multiset(cont) and flat_multiset(cont, comp), respectively, except that c is constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Same as flat_multiset(cont) and flat_multiset(cont, comp), respectively.

🔗

template<class Alloc>
  flat_multiset(sorted_equivalent_t s, const container_type& cont, const Alloc& a);
template<class Alloc>
  flat_multiset(sorted_equivalent_t s, const container_type& cont,
                const key_compare& comp, const Alloc& a);

Effects: Equivalent to flat_multiset(s, cont) and flat_multiset(s, cont, comp), respectively, except that c is constructed with uses-allocator construction ([allocator.uses.construction]).

Complexity: Linear.

🔗

template<class Alloc>
  explicit flat_multiset(const Alloc& a);
template<class Alloc>
  flat_multiset(const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_multiset(const flat_multiset&, const Alloc& a);
template<class Alloc>
  flat_multiset(flat_multiset&&, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multiset(InputIterator first, InputIterator last, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multiset(InputIterator first, InputIterator last,
                const key_compare& comp, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multiset(sorted_equivalent_t, InputIterator first, InputIterator last, const Alloc& a);
template<class InputIterator, class Alloc>
  flat_multiset(sorted_equivalent_t, InputIterator first, InputIterator last,
                const key_compare& comp, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_multiset(from_range_t, R&& rg, const Alloc& a);
template<container-compatible-range<value_type> R, class Alloc>
  flat_multiset(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_multiset(initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_multiset(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
template<class Alloc>
  flat_multiset(sorted_equivalent_t, initializer_list<value_type> il, const Alloc& a);
template<class Alloc>
  flat_multiset(sorted_equivalent_t, initializer_list<value_type> il,
                const key_compare& comp, const Alloc& a);

Effects: Equivalent to the corresponding non-allocator constructors except that c is constructed with uses-allocator construction ([allocator.uses.construction]).

23.6.12.5 Modifiers [flat.multiset.modifiers]

🔗

template<class... Args> iterator emplace(Args&&... args);

Constraints: is_constructible_v<value_type, Args...> is true.

Effects: First, initializes an object t of type value_type with std::forward<Args>(args)..., then inserts t as if by: auto it = ranges::upper_bound(c, t, compare); c.insert(it, std::move(t));

Returns: An iterator that points to the inserted element.

🔗

template<class InputIterator>
  void insert(InputIterator first, InputIterator last);

Effects: Adds elements to c as if by: c.insert(c.end(), first, last);

Then, sorts the range of newly inserted elements with respect to compare, and merges the resulting sorted range and the sorted range of pre-existing elements into a single sorted range.

Complexity: N +

M log M

, where N is size() before the operation and M is distance(first, last).

Remarks: Since this operation performs an in-place merge, it may allocate memory.

🔗

template<class InputIterator>
  void insert(sorted_equivalent_t, InputIterator first, InputIterator last);

Effects: Equivalent to insert(first, last).

Complexity: Linear.

🔗

void swap(flat_multiset& y) noexcept;

Effects: Equivalent to: ranges::swap(compare, y.compare); ranges::swap(c, y.c);

🔗

container_type extract() &&;

Postconditions: *this is emptied, even if the function exits via an exception.

Returns: std::move(c).

🔗

void replace(container_type&& cont);

Preconditions: The elements of cont are sorted with respect to compare.

Effects: Equivalent to: c = std::move(cont);

23.6.12.6 Erasure [flat.multiset.erasure]

🔗

template<class Key, class Compare, class KeyContainer, class Predicate>
  typename flat_multiset<Key, Compare, KeyContainer>::size_type
    erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred);

Preconditions: Key meets the Cpp17MoveAssignable requirements.

Effects: Let E be bool(pred(as_const(e))).

Erases all elements e in c for which E holds.

Returns: The number of elements erased.

Complexity: Exactly c.size() applications of the predicate.

Remarks: Stable ([algorithm.stable]).

If an invocation of erase_if exits via an exception, c is in a valid but unspecified state ([defns.valid]).

[Note 1:

c still meets its invariants, but can be empty.

— end note]

23.6.13 Container adaptors formatting [container.adaptors.format]

For each of queue, priority_queue, and stack, the library provides the following formatter specialization where adaptor-type is the name of the template:

🔗

namespace std { template<class charT, class T, formattable<charT> Container, class... U> struct formatter<adaptor-type<T, Container, U...>, charT> { private: using maybe-const-container = // exposition only fmt-maybe-const<Container, charT>; using maybe-const-adaptor = // exposition only maybe-const<is_const_v<maybe-const-container>, // see [ranges.syn] adaptor-type<T, Container, U...>>; formatter<ranges::ref_view<maybe-const-container>, charT> underlying_; // exposition only public: template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(maybe-const-adaptor& r, FormatContext& ctx) const; }; }

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Equivalent to: return underlying_.parse(ctx);

🔗

template<class FormatContext>
  typename FormatContext::iterator
    format(maybe-const-adaptor& r, FormatContext& ctx) const;

Effects: Equivalent to: return underlying_.format(r.c, ctx);

23.7 Views [views]

23.7.1 General [views.general]

The header defines the view span.

The header <mdspan> defines the class template mdspan and other facilities for interacting with these multidimensional views.

23.7.2 Contiguous access [views.contiguous]

23.7.2.1 Header synopsis [span.syn]

🔗

#include <initializer_list> // see [initializer.list.syn] // mostly freestanding namespace std { // constants inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max(); template<class T> concept integral-constant-like = // exposition only is_integral_v<decltype(T::value)> && !is_same_v<bool, remove_const_t<decltype(T::value)>> && convertible_to<T, decltype(T::value)> && equality_comparable_with<T, decltype(T::value)> && bool_constant<T() == T::value>::value && bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value; template<class T> constexpr size_t maybe-static-ext = dynamic_extent; // exposition only template<integral-constant-like T> constexpr size_t maybe-static-ext<T> = {T::value}; // [views.span], class template span template<class ElementType, size_t Extent = dynamic_extent> class span; // partially freestanding template<class ElementType, size_t Extent> constexpr bool ranges::enable_view<span<ElementType, Extent>> = true; template<class ElementType, size_t Extent> constexpr bool ranges::enable_borrowed_range<span<ElementType, Extent>> = true; // [span.objectrep], views of object representation template<class ElementType, size_t Extent> span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent> as_bytes(span<ElementType, Extent> s) noexcept; template<class ElementType, size_t Extent> span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent> as_writable_bytes(span<ElementType, Extent> s) noexcept; }

23.7.2.2 Class template span [views.span]

23.7.2.2.1 Overview [span.overview]

A span is a view over a contiguous sequence of objects, the storage of which is owned by some other object.

All member functions of span have constant time complexity.

🔗

namespace std { template<class ElementType, size_t Extent = dynamic_extent> class span { public: // constants and types using element_type = ElementType; using value_type = remove_cv_t<ElementType>; using size_type = size_t; using difference_type = ptrdiff_t; using pointer = element_type*; using const_pointer = const element_type*; using reference = element_type&; using const_reference = const element_type&; using iterator = implementation-defined; // see [span.iterators] using const_iterator = std::const_iterator<iterator>; using reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::const_iterator<reverse_iterator>; static constexpr size_type extent = Extent; // [span.cons], constructors, copy, and assignment constexpr span() noexcept; template<class It> constexpr explicit(extent != dynamic_extent) span(It first, size_type count); template<class It, class End> constexpr explicit(extent != dynamic_extent) span(It first, End last); template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept; template<class T, size_t N> constexpr span(array<T, N>& arr) noexcept; template<class T, size_t N> constexpr span(const array<T, N>& arr) noexcept; template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r); constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il); constexpr span(const span& other) noexcept = default; template<class OtherElementType, size_t OtherExtent> constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept; constexpr span& operator=(const span& other) noexcept = default; // [span.sub], subviews template<size_t Count> constexpr span<element_type, Count> first() const; template<size_t Count> constexpr span<element_type, Count> last() const; template<size_t Offset, size_t Count = dynamic_extent> constexpr span<element_type, see below> subspan() const; constexpr span<element_type, dynamic_extent> first(size_type count) const; constexpr span<element_type, dynamic_extent> last(size_type count) const; constexpr span<element_type, dynamic_extent> subspan( size_type offset, size_type count = dynamic_extent) const; // [span.obs], observers constexpr size_type size() const noexcept; constexpr size_type size_bytes() const noexcept; constexpr bool empty() const noexcept; // [span.elem], element access constexpr reference operator[](size_type idx) const; constexpr reference at(size_type idx) const; // freestanding-deleted constexpr reference front() const; constexpr reference back() const; constexpr pointer data() const noexcept; // [span.iterators], iterator support constexpr iterator begin() const noexcept; constexpr iterator end() const noexcept; constexpr const_iterator cbegin() const noexcept { return begin(); } constexpr const_iterator cend() const noexcept { return end(); } constexpr reverse_iterator rbegin() const noexcept; constexpr reverse_iterator rend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); } constexpr const_reverse_iterator crend() const noexcept { return rend(); } private: pointer data_; // exposition only size_type size_; // exposition only }; template<class It, class EndOrSize> span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>, maybe-static-ext<EndOrSize>>; template<class T, size_t N> span(T (&)[N]) -> span<T, N>; template<class T, size_t N> span(array<T, N>&) -> span<T, N>; template<class T, size_t N> span(const array<T, N>&) -> span<const T, N>; template<class R> span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>; }

span<ElementType, Extent> is a trivially copyable type ([basic.types.general]).

ElementType is required to be a complete object type that is not an abstract class type.

For a span s, any operation that invalidates a pointer in the range [s.data(), s.data() + s.size()) invalidates pointers, iterators, and references to elements of s.

23.7.2.2.2 Constructors, copy, and assignment [span.cons]

🔗

constexpr span() noexcept;

Constraints: Extent == dynamic_extent || Extent == 0 is true.

Postconditions: size() == 0 && data() == nullptr.

🔗

template<class It>
  constexpr explicit(extent != dynamic_extent) span(It first, size_type count);

Constraints: Let U be remove_reference_t<iter_reference_t<It>>.

(3.1)
It satisfies contiguous_iterator .
(3.2)
is_convertible_v<U(*)[], element_type(*)[]> is true.

[Note 1:
The intent is to allow only qualification conversions of the iterator reference type to element_type.
— end note]

Preconditions:

(4.1)
[first, first + count) is a valid range.
(4.2)
It models contiguous_iterator .
(4.3)
If extent is not equal to dynamic_extent, then count is equal to extent.

Effects: Initializes data_ with to_address(first) and size_ with count.

Throws: Nothing.

🔗

template<class It, class End>
  constexpr explicit(extent != dynamic_extent) span(It first, End last);

Constraints: Let U be remove_reference_t<iter_reference_t<It>>.

(7.1)
is_convertible_v<U(*)[], element_type(*)[]> is true.

[Note 2:
The intent is to allow only qualification conversions of the iterator reference type to element_type.
— end note]
(7.2)
It satisfies contiguous_iterator .
(7.3)
End satisfies sized_sentinel_for<It>.
(7.4)
is_convertible_v<End, size_t> is false.

Preconditions:

(8.1)
If extent is not equal to dynamic_extent, then last - first is equal to extent.
(8.2)
[first, last) is a valid range.
(8.3)
It models contiguous_iterator .
(8.4)
End models sized_sentinel_for<It>.

Effects: Initializes data_ with to_address(first) and size_ with last - first.

Throws: When and what last - first throws.

🔗

template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
template<class T, size_t N> constexpr span(array<T, N>& arr) noexcept;
template<class T, size_t N> constexpr span(const array<T, N>& arr) noexcept;

Constraints: Let U be remove_pointer_t<decltype(std::data(arr))>.

(11.1)
extent == dynamic_extent || N == extent is true, and
(11.2)
is_convertible_v<U(*)[], element_type(*)[]> is true.
[Note 3:
The intent is to allow only qualification conversions of the array element type to element_type.
— end note]

Effects: Constructs a span that is a view over the supplied array.

[Note 4:

type_identity_t affects class template argument deduction.

— end note]

Postconditions: size() == N && data() == std::data(arr) is true.

🔗

template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);

Constraints: Let U be remove_reference_t<ranges::range_reference_t<R>>.

(14.1)
R satisfies ranges::contiguous_range and ranges::sized_range.
(14.2)
Either R satisfies ranges::borrowed_range or is_const_v<element_type> is true.
(14.3)
remove_cvref_t<R> is not a specialization of span.
(14.4)
remove_cvref_t<R> is not a specialization of array.
(14.5)
is_array_v<remove_cvref_t<R>> is false.
(14.6)
is_convertible_v<U(*)[], element_type(*)[]> is true.

[Note 5:
The intent is to allow only qualification conversions of the range reference type to element_type.
— end note]

Preconditions:

(15.1)
If extent is not equal to dynamic_extent, then ranges::size(r) is equal to extent.
(15.2)
R models ranges::contiguous_range and ranges::sized_range.
(15.3)
If is_const_v<element_type> is false, R models ranges::borrowed_range.

Effects: Initializes data_ with ranges::data(r) and size_ with ranges::size(r).

Throws: What and when ranges::data(r) and ranges::size(r) throw.

🔗

constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il);

Constraints: is_const_v<element_type> is true.

Preconditions: If extent is not equal to dynamic_extent, then il.size() is equal to extent.

Effects: Initializes data_ with il.begin() and size_ with il.size().

🔗

constexpr span(const span& other) noexcept = default;

Postconditions: other.size() == size() && other.data() == data().

🔗

template<class OtherElementType, size_t OtherExtent>
  constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;

Constraints:

(22.1)
extent == dynamic_extent || OtherExtent == dynamic_extent || extent == OtherExtent is true, and
(22.2)
is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true.
[Note 6:
The intent is to allow only qualification conversions of the OtherElementType to element_type.
— end note]

Preconditions: If extent is not equal to dynamic_extent, then s.size() is equal to extent.

Effects: Constructs a span that is a view over the range [s.data(), s.data() + s.size()).

Postconditions: size() == s.size() && data() == s.data().

Remarks: The expression inside explicit is equivalent to: extent != dynamic_extent && OtherExtent == dynamic_extent

🔗

constexpr span& operator=(const span& other) noexcept = default;

Postconditions: size() == other.size() && data() == other.data().

23.7.2.2.3 Deduction guides [span.deduct]

🔗

template<class It, class EndOrSize>
  span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>,
                              maybe-static-ext<EndOrSize>>;

Constraints: It satisfies contiguous_iterator .

🔗

template<class R>
  span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;

Constraints: R satisfies ranges::contiguous_range.

23.7.2.2.4 Subviews [span.sub]

🔗

template<size_t Count> constexpr span<element_type, Count> first() const;

Mandates: Count <= Extent is true.

Preconditions: Count <= size() is true.

Effects: Equivalent to: return R{data(), Count}; where R is the return type.

🔗

template<size_t Count> constexpr span<element_type, Count> last() const;

Mandates: Count <= Extent is true.

Preconditions: Count <= size() is true.

Effects: Equivalent to: return R{data() + (size() - Count), Count}; where R is the return type.

🔗

template<size_t Offset, size_t Count = dynamic_extent>
  constexpr span<element_type, see below> subspan() const;

Mandates: Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset) is true.

Preconditions: Offset <= size() && (Count == dynamic_extent || Count <= size() - Offset) is true.

Effects: Equivalent to: return span<ElementType, see below>( data() + Offset, Count != dynamic_extent ? Count : size() - Offset);

Remarks: The second template argument of the returned span type is: Count != dynamic_extent ? Count : (Extent != dynamic_extent ? Extent - Offset : dynamic_extent)

🔗

constexpr span<element_type, dynamic_extent> first(size_type count) const;

Preconditions: count <= size() is true.

Effects: Equivalent to: return {data(), count};

🔗

constexpr span<element_type, dynamic_extent> last(size_type count) const;

Preconditions: count <= size() is true.

Effects: Equivalent to: return {data() + (size() - count), count};

🔗

constexpr span<element_type, dynamic_extent> subspan(
  size_type offset, size_type count = dynamic_extent) const;

Preconditions: offset <= size() && (count == dynamic_extent || count <= size() - offset) is true.

Effects: Equivalent to: return {data() + offset, count == dynamic_extent ? size() - offset : count};

23.7.2.2.5 Observers [span.obs]

🔗

constexpr size_type size() const noexcept;

Effects: Equivalent to: return size_;

🔗

constexpr size_type size_bytes() const noexcept;

Effects: Equivalent to: return size() * sizeof(element_type);

🔗

constexpr bool empty() const noexcept;

Effects: Equivalent to: return size() == 0;

23.7.2.2.6 Element access [span.elem]

🔗

constexpr reference operator[](size_type idx) const;

Preconditions: idx < size() is true.

Returns: *(data() + idx).

Throws: Nothing.

🔗

constexpr reference at(size_type idx) const;

Returns: *(data() + idx).

Throws: out_of_range if idx >= size() is true.

🔗

constexpr reference front() const;

Preconditions: empty() is false.

Returns: *data().

Throws: Nothing.

🔗

constexpr reference back() const;

Preconditions: empty() is false.

Returns: *(data() + (size() - 1)).

Throws: Nothing.

🔗

constexpr pointer data() const noexcept;

Returns: data_.

23.7.2.2.7 Iterator support [span.iterators]

🔗

using iterator = implementation-defined;

The type models contiguous_iterator ([iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements ([random.access.iterators]), and meets the requirements for constexpr iterators ([iterator.requirements.general]), whose value type is value_type and whose reference type is reference.

All requirements on container iterators ([container.reqmts]) apply to span::iterator as well.

🔗

constexpr iterator begin() const noexcept;

Returns: An iterator referring to the first element in the span.

If empty() is true, then it returns the same value as end().

🔗

constexpr iterator end() const noexcept;

Returns: An iterator which is the past-the-end value.

🔗

constexpr reverse_iterator rbegin() const noexcept;

Effects: Equivalent to: return reverse_iterator(end());

🔗

constexpr reverse_iterator rend() const noexcept;

Effects: Equivalent to: return reverse_iterator(begin());

23.7.2.3 Views of object representation [span.objectrep]

🔗

template<class ElementType, size_t Extent>
  span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
    as_bytes(span<ElementType, Extent> s) noexcept;

Effects: Equivalent to: return R{reinterpret_cast<const byte*>(s.data()), s.size_bytes()}; where R is the return type.

🔗

template<class ElementType, size_t Extent>
  span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
    as_writable_bytes(span<ElementType, Extent> s) noexcept;

Constraints: is_const_v<ElementType> is false.

Effects: Equivalent to: return R{reinterpret_cast<byte*>(s.data()), s.size_bytes()}; where R is the return type.

23.7.3 Multidimensional access [views.multidim]

23.7.3.1 Overview [mdspan.overview]

A multidimensional index space is a Cartesian product of integer intervals.

Each interval can be represented by a half-open range

[L_{i}, U_{i})

, where

L_{i}

and

U_{i}

are the lower and upper bounds of the

i^{th}

dimension.

The rank of a multidimensional index space is the number of intervals it represents.

The size of a multidimensional index space is the product of

U_{i} - L_{i}

for each dimension i if its rank is greater than 0, and 1 otherwise.

An integer r is a rank index of an index space S if r is in the range

[0, rank of S)

A pack of integers idx is a multidimensional index in a multidimensional index space S (or representation thereof) if both of the following are true:

(3.1)
sizeof...(idx) is equal to the rank of S, and
(3.2)
for every rank index i of S, the $i^{th}$ value of idx is an integer in the interval $[L_{i}, U_{i})$ of S.

23.7.3.2 Header <mdspan> synopsis [mdspan.syn]

🔗

// all freestanding namespace std { // [mdspan.extents], class template extents template<class IndexType, size_t... Extents> class extents; // [mdspan.extents.dextents], alias template dextents template<class IndexType, size_t Rank> using dextents = see below; // [mdspan.extents.dims], alias template dims template<size_t Rank, class IndexType = size_t> using dims = see below; // [mdspan.layout], layout mapping struct layout_left; struct layout_right; struct layout_stride; template<size_t PaddingValue = dynamic_extent> struct layout_left_padded; template<size_t PaddingValue = dynamic_extent> struct layout_right_padded; // [mdspan.accessor.default], class template default_accessor template<class ElementType> class default_accessor; // [mdspan.accessor.aligned], class template aligned_accessor template<class ElementType, size_t ByteAlignment> class aligned_accessor; // [mdspan.mdspan], class template mdspan template<class ElementType, class Extents, class LayoutPolicy = layout_right, class AccessorPolicy = default_accessor<ElementType>> class mdspan; // [mdspan.sub], submdspan creation template<class OffsetType, class LengthType, class StrideType> struct strided_slice; template<class LayoutMapping> struct submdspan_mapping_result; struct full_extent_t { explicit full_extent_t() = default; }; inline constexpr full_extent_t full_extent{}; template<class IndexType, class... Extents, class... SliceSpecifiers> constexpr auto submdspan_extents(const extents<IndexType, Extents...>&, SliceSpecifiers...); // [mdspan.sub.sub], submdspan function template template<class ElementType, class Extents, class LayoutPolicy, class AccessorPolicy, class... SliceSpecifiers> constexpr auto submdspan( const mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& src, SliceSpecifiers... slices) -> see below; template<class T, class IndexType> concept index-pair-like = // exposition only pair-like<T> && convertible_to<tuple_element_t<0, T>, IndexType> && convertible_to<tuple_element_t<1, T>, IndexType>; }

23.7.3.3 Class template extents [mdspan.extents]

23.7.3.3.1 Overview [mdspan.extents.overview]

The class template extents represents a multidimensional index space of rank equal to sizeof...(Extents).

In ([views]), extents is used synonymously with multidimensional index space.

namespace std { template<class IndexType, size_t... Extents> class extents { public: using index_type = IndexType; using size_type = make_unsigned_t<index_type>; using rank_type = size_t; // [mdspan.extents.obs], observers of the multidimensional index space static constexpr rank_type rank() noexcept { return sizeof...(Extents); } static constexpr rank_type rank_dynamic() noexcept { return dynamic-index(rank()); } static constexpr size_t static_extent(rank_type) noexcept; constexpr index_type extent(rank_type) const noexcept; // [mdspan.extents.cons], constructors constexpr extents() noexcept = default; template<class OtherIndexType, size_t... OtherExtents> constexpr explicit(see below) extents(const extents<OtherIndexType, OtherExtents...>&) noexcept; template<class... OtherIndexTypes> constexpr explicit extents(OtherIndexTypes...) noexcept; template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) extents(span<OtherIndexType, N>) noexcept; template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) extents(const array<OtherIndexType, N>&) noexcept; // [mdspan.extents.cmp], comparison operators template<class OtherIndexType, size_t... OtherExtents> friend constexpr bool operator==(const extents&, const extents<OtherIndexType, OtherExtents...>&) noexcept; // [mdspan.extents.expo], exposition-only helpers constexpr size_t fwd-prod-of-extents(rank_type) const noexcept; // exposition only constexpr size_t rev-prod-of-extents(rank_type) const noexcept; // exposition only template<class OtherIndexType> static constexpr auto index-cast(OtherIndexType&&) noexcept; // exposition only private: static constexpr rank_type dynamic-index(rank_type) noexcept; // exposition only static constexpr rank_type dynamic-index-inv(rank_type) noexcept; // exposition only array<index_type, rank_dynamic()> dynamic-extents{}; // exposition only }; template<class... Integrals> explicit extents(Integrals...) -> see below; }

Mandates:

(1.1)
IndexType is a signed or unsigned integer type, and
(1.2)
each element of Extents is either equal to dynamic_extent, or is representable as a value of type IndexType.

Each specialization of extents models regular and is trivially copyable.

Let

E_{r}

be the

r^{th}

element of Extents.

E_{r}

is a dynamic extent if it is equal to dynamic_extent, otherwise

E_{r}

is a static extent.

Let

D_{r}

be the value of dynamic-extents[dynamic-index(r)] if

E_{r}

is a dynamic extent, otherwise

E_{r}

The

r^{th}

interval of the multidimensional index space represented by an extents object is

[0, D_{r})

23.7.3.3.2 Exposition-only helpers [mdspan.extents.expo]

🔗

static constexpr rank_type dynamic-index(rank_type i) noexcept;

Preconditions: i <= rank() is true.

Returns: The number of

E_{r}

with

r < i

for which

E_{r}

is a dynamic extent.

🔗

static constexpr rank_type dynamic-index-inv(rank_type i) noexcept;

Preconditions: i < rank_dynamic() is true.

Returns: The minimum value of r such that dynamic-index(r + 1) == i + 1 is true.

🔗

constexpr size_t fwd-prod-of-extents(rank_type i) const noexcept;

Preconditions: i <= rank() is true.

Returns: If i > 0 is true, the product of extent(k) for all k in the range [0, i), otherwise 1.

🔗

constexpr size_t rev-prod-of-extents(rank_type i) const noexcept;

Preconditions: i < rank() is true.

Returns: If i + 1 < rank() is true, the product of extent(k) for all k in the range [i + 1, rank()), otherwise 1.

🔗

template<class OtherIndexType>
  static constexpr auto index-cast(OtherIndexType&& i) noexcept;

Effects:

(9.1)
If OtherIndexType is an integral type other than bool, then equivalent to return i;,
(9.2)
otherwise, equivalent to return static_cast<index_type>(i);.

[Note 1:

This function will always return an integral type other than bool.

Since this function's call sites are constrained on convertibility of OtherIndexType to index_type, integer-class types can use the static_cast branch without loss of precision.

— end note]

23.7.3.3.3 Constructors [mdspan.extents.cons]

🔗

template<class OtherIndexType, size_t... OtherExtents>
  constexpr explicit(see below)
    extents(const extents<OtherIndexType, OtherExtents...>& other) noexcept;

Constraints:

(1.1)
sizeof...(OtherExtents) == rank() is true.
(1.2)
((OtherExtents == dynamic_extent || Extents == dynamic_extent || OtherExtents ==
Extents) && ...) is true.

Preconditions:

(2.1)
other.extent(r) equals $E_{r}$ for each r for which $E_{r}$ is a static extent, and
(2.2)
either
- (2.2.1)
  sizeof...(OtherExtents) is zero, or
- (2.2.2)
  other.extent(r) is representable as a value of type index_type for every rank index r of other.

Postconditions: *this == other is true.

Remarks: The expression inside explicit is equivalent to: (((Extents != dynamic_extent) && (OtherExtents == dynamic_extent)) || ... ) || (numeric_limits<index_type>::max() < numeric_limits<OtherIndexType>::max())

🔗

template<class... OtherIndexTypes>
  constexpr explicit extents(OtherIndexTypes... exts) noexcept;

Let N be sizeof...(OtherIndexTypes), and let exts_arr be array<index_type, N>{static_cast<
index_type>(std::move(exts))...}.

Constraints:

(6.1)
(is_convertible_v<OtherIndexTypes, index_type> && ...) is true,
(6.2)
(is_nothrow_constructible_v<index_type, OtherIndexTypes> && ...) is true, and
(6.3)
N == rank_dynamic() || N == rank() is true.
[Note 1:
One can construct extents from just dynamic extents, which are all the values getting stored, or from all the extents with a precondition.
— end note]

Preconditions:

(7.1)
If N != rank_dynamic() is true, exts_arr[r] equals $E_{r}$ for each r for which $E_{r}$ is a static extent, and
(7.2)
either
- (7.2.1)
  sizeof...(exts) == 0 is true, or
- (7.2.2)
  each element of exts is representable as a nonnegative value of type index_type.

Postconditions: *this == extents(exts_arr) is true.

🔗

template<class OtherIndexType, size_t N>
  constexpr explicit(N != rank_dynamic())
    extents(span<OtherIndexType, N> exts) noexcept;
template<class OtherIndexType, size_t N>
  constexpr explicit(N != rank_dynamic())
    extents(const array<OtherIndexType, N>& exts) noexcept;

Constraints:

(9.1)
is_convertible_v<const OtherIndexType&, index_type> is true,
(9.2)
is_nothrow_constructible_v<index_type, const OtherIndexType&> is true, and
(9.3)
N == rank_dynamic() || N == rank() is true.

Preconditions:

(10.1)
If N != rank_dynamic() is true, exts[r] equals $E_{r}$ for each r for which $E_{r}$ is a static extent, and
(10.2)
either
- (10.2.1)
  N is zero, or
- (10.2.2)
  exts[r] is representable as a nonnegative value of type index_type for every rank index r.

Effects:

(11.1)
If N equals rank_dynamic(), for all d in the range [0, rank_dynamic()), direct-non-list-initializes dynamic-extents[d] with as_const(exts[d]).
(11.2)
Otherwise, for all d in the range [0, rank_dynamic()), direct-non-list-initializes dynamic-extents[d] with as_const(exts[dynamic-index-inv(d)]).

🔗

template<class... Integrals>
  explicit extents(Integrals...) -> see below;

Constraints: (is_convertible_v<Integrals, size_t> && ...) is true.

Remarks: The deduced type is extents<size_t, maybe-static-ext<Integrals>...>.

23.7.3.3.4 Observers of the multidimensional index space [mdspan.extents.obs]

🔗

static constexpr size_t static_extent(rank_type i) noexcept;

Preconditions: i < rank() is true.

Returns:

E_{i}

🔗

constexpr index_type extent(rank_type i) const noexcept;

Preconditions: i < rank() is true.

Returns:

D_{i}

23.7.3.3.5 Comparison operators [mdspan.extents.cmp]

🔗

template<class OtherIndexType, size_t... OtherExtents>
  friend constexpr bool operator==(const extents& lhs,
                                   const extents<OtherIndexType, OtherExtents...>& rhs) noexcept;

Returns: true if lhs.rank() equals rhs.rank() and if lhs.extent(r) equals rhs.extent(r) for every rank index r of rhs, otherwise false.

23.7.3.3.6 Alias template dextents [mdspan.extents.dextents]

🔗

template<class IndexType, size_t Rank>
  using dextents = see below;

Result: A type E that is a specialization of extents such that E::rank() == Rank && E::rank() == E::rank_dynamic() is true, and E::index_type denotes IndexType.

23.7.3.3.7 Alias template dims [mdspan.extents.dims]

🔗

template<size_t Rank, class IndexType = size_t>
  using dims = see below;

Result: A type E that is a specialization of extents such that E::rank() == Rank && E::rank() == E::rank_dynamic() is true, and E::index_type denotes IndexType.

23.7.3.4 Layout mapping [mdspan.layout]

23.7.3.4.1 General [mdspan.layout.general]

In [mdspan.layout.reqmts] and [mdspan.layout.policy.reqmts]:

(1.1)
M denotes a layout mapping class.
(1.2)
m denotes a (possibly const) value of type M.
(1.3)
i and j are packs of (possibly const) integers that are multidimensional indices in m.extents() ([mdspan.overview]).

[Note 1:
The type of each element of the packs can be a different integer type.
— end note]
(1.4)
r is a (possibly const) rank index of typename M::extents_type.
(1.5)
$d_{r}$ is a pack of (possibly const) integers for which sizeof...( $d_{r}$ ) == M::extents_type::rank() is true, the $r^{th}$ element is equal to 1, and all other elements are equal to 0.

In [mdspan.layout.reqmts] through [mdspan.layout.stride]:

(2.1)
Let is-mapping-of be the exposition-only variable template defined as follows: template<class Layout, class Mapping> constexpr bool is-mapping-of = // exposition only is_same_v<typename Layout::template mapping<typename Mapping::extents_type>, Mapping>;
(2.2)
Let is-layout-left-padded-mapping-of be the exposition-only variable template defined as follows: template<class Mapping> constexpr bool is-layout-left-padded-mapping-of = see below; // exposition only where is-layout-left-padded-mapping-of<Mapping> is true if and only if Mapping denotes a specialization of layout_left_padded<S>::mapping for some value S of type size_t.
(2.3)
Let is-layout-right-padded-mapping-of be the exposition-only variable template defined as follows: template<class Mapping> constexpr bool is-layout-right-padded-mapping-of = see below; // exposition only where is-layout-right-padded-mapping-of<Mapping> is true if and only if Mapping denotes a specialization of layout_right_padded<S>::mapping for some value S of type size_t.
(2.4)
For nonnegative integers x and y, let $LEAST-MULTIPLE-AT-LEAST (x, y)$ denote
- (2.4.1)
  y if x is zero,
- (2.4.2)
  otherwise, the least multiple of x that is greater than or equal to y.

23.7.3.4.2 Requirements [mdspan.layout.reqmts]

A type M meets the layout mapping requirements if

(1.1)
M models copyable and equality_comparable,
(1.2)
is_nothrow_move_constructible_v<M> is true,
(1.3)
is_nothrow_move_assignable_v<M> is true,
(1.4)
is_nothrow_swappable_v<M> is true, and
(1.5)
the following types and expressions are well-formed and have the specified semantics.

🔗

typename M::extents_type

Result: A type that is a specialization of extents.

🔗

typename M::index_type

Result: typename M::extents_type::index_type.

🔗

typename M::rank_type

Result: typename M::extents_type::rank_type.

🔗

typename M::layout_type

Result: A type MP that meets the layout mapping policy requirements ([mdspan.layout.policy.reqmts]) and for which is-mapping-of<MP, M> is true.

🔗

m.extents()

Result: const typename M::extents_type&

🔗

m(i...)

Result: typename M::index_type

Returns: A nonnegative integer less than numeric_limits<typename M::index_type>::max() and less than or equal to numeric_limits<size_t>::max().

🔗

m(i...) == m(static_cast<typename M::index_type>(i)...)

Result: bool

Returns: true

🔗

m.required_span_size()

Result: typename M::index_type

Returns: If the size of the multidimensional index space m.extents() is 0, then 0, else 1 plus the maximum value of m(i...) for all i.

🔗

m.is_unique()

Result: bool

Returns: true only if for every i and j where (i != j || ...) is true, m(i...) != m(j...) is true.

[Note 1:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is unique.

— end note]

🔗

m.is_exhaustive()

Result: bool

Returns: true only if for all k in the range [0, m.required_span_size()) there exists an i such that m(i...) equals k.

[Note 2:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is exhaustive.

— end note]

🔗

m.is_strided()

Result: bool

Returns: true only if for every rank index r of m.extents() there exists an integer

s_{r}

such that, for all i where

(i + d_{r})

is a multidimensional index in m.extents() ([mdspan.overview]), m((i +

d_{r}

)...) - m(i...) equals

s_{r}

[Note 3:

This implies that for a strided layout

m (i_{0}, \dots, i_{k}) = m (0, \dots, 0) + i_{0} \times s_{0} + \dots + i_{k} \times s_{k}

— end note]

[Note 4:

A mapping can return false even if the condition is met.

For certain layouts, it is possibly not feasible to determine efficiently whether the layout is strided.

— end note]

🔗

m.stride(r)

Preconditions: m.is_strided() is true.

Result: typename M::index_type

Returns:

s_{r}

as defined in m.is_strided() above.

🔗

M::is_always_unique()

Result: A constant expression ([expr.const]) of type bool.

Returns: true only if m.is_unique() is true for all possible objects m of type M.

[Note 5:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is unique.

— end note]

🔗

M::is_always_exhaustive()

Result: A constant expression ([expr.const]) of type bool.

Returns: true only if m.is_exhaustive() is true for all possible objects m of type M.

[Note 6:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is exhaustive.

— end note]

🔗

M::is_always_strided()

Result: A constant expression ([expr.const]) of type bool.

Returns: true only if m.is_strided() is true for all possible objects m of type M.

[Note 7:

A mapping can return false even if the above condition is met.

For certain layout mappings, it is possibly not feasible to determine whether every instance is strided.

— end note]

23.7.3.4.3 Layout mapping policy requirements [mdspan.layout.policy.reqmts]

A type MP meets the layout mapping policy requirements if for a type E that is a specialization of extents, MP::mapping<E> is valid and denotes a type X that meets the layout mapping requirements ([mdspan.layout.reqmts]), and for which the qualified-id X::layout_type is valid and denotes the type MP and the qualified-id X::extents_type denotes E.

23.7.3.4.4 Layout mapping policies [mdspan.layout.policy.overview]

namespace std { struct layout_left { template<class Extents> class mapping; }; struct layout_right { template<class Extents> class mapping; }; struct layout_stride { template<class Extents> class mapping; }; template<size_t PaddingValue> struct layout_left_padded { template<class Extents> class mapping; }; template<size_t PaddingValue> struct layout_right_padded { template<class Extents> class mapping; }; }

Each of layout_left, layout_right, and layout_stride, as well as each specialization of layout_left_padded and layout_right_padded, meets the layout mapping policy requirements and is a trivially copyable type.

Furthermore, is_trivially_default_constructible_v<T> is true for any such type T.

23.7.3.4.5 Class template layout_left::mapping [mdspan.layout.left]

23.7.3.4.5.1 Overview [mdspan.layout.left.overview]

layout_left provides a layout mapping where the leftmost extent has stride 1, and strides increase left-to-right as the product of extents.

namespace std { template<class Extents> class layout_left::mapping { public: using extents_type = Extents; using index_type = typename extents_type::index_type; using size_type = typename extents_type::size_type; using rank_type = typename extents_type::rank_type; using layout_type = layout_left; // [mdspan.layout.left.cons], constructors constexpr mapping() noexcept = default; constexpr mapping(const mapping&) noexcept = default; constexpr mapping(const extents_type&) noexcept; template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const mapping<OtherExtents>&) noexcept; template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_right::mapping<OtherExtents>&) noexcept; template<class LayoutLeftPaddedMapping> constexpr explicit(!is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type>) mapping(const LayoutLeftPaddedMapping&) noexcept; template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>&); constexpr mapping& operator=(const mapping&) noexcept = default; // [mdspan.layout.left.obs], observers constexpr const extents_type& extents() const noexcept { return extents_; } constexpr index_type required_span_size() const noexcept; template<class... Indices> constexpr index_type operator()(Indices...) const noexcept; static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept { return true; } static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } static constexpr bool is_exhaustive() noexcept { return true; } static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept; template<class OtherExtents> friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept; private: extents_type extents_{}; // exposition only // [mdspan.sub.map], submdspan mapping specialization template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const // exposition only -> see below; template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping( const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...); } }; }

If Extents is not a specialization of extents, then the program is ill-formed.

layout_left::mapping<E> is a trivially copyable type that models regular for each E.

Mandates: If Extents::rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type typename Extents::index_type.

23.7.3.4.5.2 Constructors [mdspan.layout.left.cons]

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constexpr mapping(const extents_type& e) noexcept;

Preconditions: The size of the multidimensional index space e is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with e.

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template<class OtherExtents>
  constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
    mapping(const mapping<OtherExtents>& other) noexcept;

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

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template<class OtherExtents>
  constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
    mapping(const layout_right::mapping<OtherExtents>& other) noexcept;

Constraints:

(6.1)
extents_type::rank() <= 1 is true, and
(6.2)
is_constructible_v<extents_type, OtherExtents> is true.

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

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template<class LayoutLeftPaddedMapping>
  constexpr explicit(!is_convertible_v<typename LayoutLeftPaddedMapping::extents_type,
                                       extents_type>)
    mapping(const LayoutLeftPaddedMapping&) noexcept;

Constraints:

(9.1)
is-layout-left-padded-mapping-of<LayoutLeftPaddedMapping> is true.
(9.2)
is_constructible_v<extents_type, typename LayoutLeftPaddedMapping::extents_type>
is true.

Mandates: If

(10.1)
Extents::rank() is greater than one,
(10.2)
Extents::static_extent(0) does not equal dynamic_extent, and
(10.3)
LayoutLeftPaddedMapping::static-padding-stride does not equal dynamic_extent,

then Extents::static_extent(0) equals LayoutLeftPaddedMapping::static-padding-stride.

Preconditions:

(11.1)
If extents_type::rank() > 1 is true, then other.stride(1) equals other.extents(0).
(11.2)
other.required_span_size() is representable as a value of type index_type.

Effects: Direct-non-list-initializes extents_ with other.extents().

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template<class OtherExtents>
  constexpr explicit(extents_type::rank() > 0)
    mapping(const layout_stride::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

(14.1)
If extents_type::rank() > 0 is true, then for all r in the range [0, extents_type::rank()), other.stride(r) equals other.extents().fwd-prod-of-extents(r), and
(14.2)
other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

23.7.3.4.5.3 Observers [mdspan.layout.left.obs]

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constexpr index_type required_span_size() const noexcept;

Returns: extents().fwd-prod-of-extents(extents_type::rank()).

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template<class... Indices>
  constexpr index_type operator()(Indices... i) const noexcept;

Constraints:

(2.1)
sizeof...(Indices) == extents_type::rank() is true,
(2.2)
(is_convertible_v<Indices, index_type> && ...) is true, and
(2.3)
(is_nothrow_constructible_v<index_type, Indices> && ...) is true.

Preconditions: extents_type::index-cast(i) is a multidimensional index in extents_ ([mdspan.overview]).

Effects: Let P be a parameter pack such that is_same_v<index_sequence_for<Indices...>, index_sequence<P...>> is true.

Equivalent to: return ((static_cast<index_type>(i) * stride(P)) + ... + 0);

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constexpr index_type stride(rank_type i) const;

Constraints: extents_type::rank() > 0 is true.

Preconditions: i < extents_type::rank() is true.

Returns: extents().fwd-prod-of-extents(i).

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template<class OtherExtents>
  friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;

Constraints: extents_type::rank() == OtherExtents::rank() is true.

Effects: Equivalent to: return x.extents() == y.extents();

23.7.3.4.6 Class template layout_right::mapping [mdspan.layout.right]

23.7.3.4.6.1 Overview [mdspan.layout.right.overview]

layout_right provides a layout mapping where the rightmost extent is stride 1, and strides increase right-to-left as the product of extents.

namespace std { template<class Extents> class layout_right::mapping { public: using extents_type = Extents; using index_type = typename extents_type::index_type; using size_type = typename extents_type::size_type; using rank_type = typename extents_type::rank_type; using layout_type = layout_right; // [mdspan.layout.right.cons], constructors constexpr mapping() noexcept = default; constexpr mapping(const mapping&) noexcept = default; constexpr mapping(const extents_type&) noexcept; template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const mapping<OtherExtents>&) noexcept; template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_left::mapping<OtherExtents>&) noexcept; template<class LayoutRightPaddedMapping> constexpr explicit(!is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type>) mapping(const LayoutRightPaddedMapping&) noexcept; template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>&) noexcept; constexpr mapping& operator=(const mapping&) noexcept = default; // [mdspan.layout.right.obs], observers constexpr const extents_type& extents() const noexcept { return extents_; } constexpr index_type required_span_size() const noexcept; template<class... Indices> constexpr index_type operator()(Indices...) const noexcept; static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept { return true; } static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } static constexpr bool is_exhaustive() noexcept { return true; } static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept; template<class OtherExtents> friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept; private: extents_type extents_{}; // exposition only // [mdspan.sub.map], submdspan mapping specialization template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const // exposition only -> see below; template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping( const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...); } }; }

If Extents is not a specialization of extents, then the program is ill-formed.

layout_right::mapping<E> is a trivially copyable type that models regular for each E.

Mandates: If Extents::rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type typename Extents::index_type.

23.7.3.4.6.2 Constructors [mdspan.layout.right.cons]

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constexpr mapping(const extents_type& e) noexcept;

Preconditions: The size of the multidimensional index space e is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with e.

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template<class OtherExtents>
  constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
    mapping(const mapping<OtherExtents>& other) noexcept;

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

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template<class OtherExtents>
  constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
    mapping(const layout_left::mapping<OtherExtents>& other) noexcept;

Constraints:

(6.1)
extents_type::rank() <= 1 is true, and
(6.2)
is_constructible_v<extents_type, OtherExtents> is true.

Preconditions: other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

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template<class LayoutRightPaddedMapping>
  constexpr explicit(!is_convertible_v<typename LayoutRightPaddedMapping::extents_type,
                                       extents_type>)
    mapping(const LayoutRightPaddedMapping&) noexcept;

Constraints:

(9.1)
is-layout-right-padded-mapping-of<LayoutRightPaddedMapping> is true.
(9.2)
is_constructible_v<extents_type, typename LayoutRightPaddedMapping::extents_-
type> is true.

Mandates: If

(10.1)
Extents::rank() is greater than one,
(10.2)
Extents::static_extent(Extents::rank() - 1) does not equal dynamic_extent, and
(10.3)
LayoutRightPaddedMapping::static-padding-stride does not equal dynamic_extent,

then Extents::static_extent(Extents::rank() - 1) equals LayoutRightPaddedMapping::static-padding-stride.

Preconditions:

(11.1)
If extents_type::rank() > 1 is true, then other.stride(extents_type::rank() - 2)
equals other.extents().extent(extents_type::rank() - 1).
(11.2)
other.required_span_size() is representable as a value of type index_type.

Effects: Direct-non-list-initializes extents_ with other.extents().

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template<class OtherExtents>
 constexpr explicit(extents_type::rank() > 0)
    mapping(const layout_stride::mapping<OtherExtents>& other) noexcept;

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

(14.1)
If extents_type::rank() > 0 is true, then for all r in the range [0, extents_type::rank()), other.stride(r) equals other.extents().rev-prod-of-extents(r).
(14.2)
other.required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with other.extents().

23.7.3.4.6.3 Observers [mdspan.layout.right.obs]

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index_type required_span_size() const noexcept;

Returns: extents().fwd-prod-of-extents(extents_type::rank()).

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template<class... Indices>
  constexpr index_type operator()(Indices... i) const noexcept;

Constraints:

(2.1)
sizeof...(Indices) == extents_type::rank() is true,
(2.2)
(is_convertible_v<Indices, index_type> && ...) is true, and
(2.3)
(is_nothrow_constructible_v<index_type, Indices> && ...) is true.

Preconditions: extents_type::index-cast(i) is a multidimensional index in extents_ ([mdspan.overview]).

Effects: Let P be a parameter pack such that is_same_v<index_sequence_for<Indices...>, index_sequence<P...>> is true.

Equivalent to: return ((static_cast<index_type>(i) * stride(P)) + ... + 0);

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constexpr index_type stride(rank_type i) const noexcept;

Constraints: extents_type::rank() > 0 is true.

Preconditions: i < extents_type::rank() is true.

Returns: extents().rev-prod-of-extents(i).

🔗

template<class OtherExtents>
  friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;

Constraints: extents_type::rank() == OtherExtents::rank() is true.

Effects: Equivalent to: return x.extents() == y.extents();

23.7.3.4.7 Class template layout_stride::mapping [mdspan.layout.stride]

23.7.3.4.7.1 Overview [mdspan.layout.stride.overview]

layout_stride provides a layout mapping where the strides are user-defined.

namespace std { template<class Extents> class layout_stride::mapping { public: using extents_type = Extents; using index_type = typename extents_type::index_type; using size_type = typename extents_type::size_type; using rank_type = typename extents_type::rank_type; using layout_type = layout_stride; private: static constexpr rank_type rank_ = extents_type::rank(); // exposition only public: // [mdspan.layout.stride.cons], constructors constexpr mapping() noexcept; constexpr mapping(const mapping&) noexcept = default; template<class OtherIndexType> constexpr mapping(const extents_type&, span<OtherIndexType, rank_>) noexcept; template<class OtherIndexType> constexpr mapping(const extents_type&, const array<OtherIndexType, rank_>&) noexcept; template<class StridedLayoutMapping> constexpr explicit(see below) mapping(const StridedLayoutMapping&) noexcept; constexpr mapping& operator=(const mapping&) noexcept = default; // [mdspan.layout.stride.obs], observers constexpr const extents_type& extents() const noexcept { return extents_; } constexpr array<index_type, rank_> strides() const noexcept { return strides_; } constexpr index_type required_span_size() const noexcept; template<class... Indices> constexpr index_type operator()(Indices...) const noexcept; static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept { return false; } static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } constexpr bool is_exhaustive() const noexcept; static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type i) const noexcept { return strides_[i]; } template<class OtherMapping> friend constexpr bool operator==(const mapping&, const OtherMapping&) noexcept; private: extents_type extents_{}; // exposition only array<index_type, rank_> strides_{}; // exposition only // [mdspan.sub.map], submdspan mapping specialization template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const // exposition only -> see below; template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping( const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...); } }; }

If Extents is not a specialization of extents, then the program is ill-formed.

layout_stride::mapping<E> is a trivially copyable type that models regular for each E.

Mandates: If Extents::rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type typename Extents::index_type.

23.7.3.4.7.2 Exposition-only helpers [mdspan.layout.stride.expo]

Let REQUIRED-SPAN-SIZE(e, strides) be:

(1.1)
1, if e.rank() == 0 is true,
(1.2)
otherwise 0, if the size of the multidimensional index space e is 0,
(1.3)
otherwise 1 plus the sum of products of (e.extent(r) - 1) and extents_type::index-cast(strides[r]) for all r in the range [0, e.rank()).

Let OFFSET(m) be:

(2.1)
m(), if e.rank() == 0 is true,
(2.2)
otherwise 0, if the size of the multidimensional index space e is 0,
(2.3)
otherwise m(z...) for a pack of integers z that is a multidimensional index in m.extents() and each element of z equals 0.

Let is-extents be the exposition-only variable template defined as follows: template<class T> constexpr bool is-extents = false; // exposition only template<class IndexType, size_t... Args> constexpr bool is-extents<extents<IndexType, Args...>> = true; // exposition only

Let layout-mapping-alike be the exposition-only concept defined as follows: template<class M> concept layout-mapping-alike = requires { // exposition only requires is-extents<typename M::extents_type>; { M::is_always_strided() } -> same_as<bool>; { M::is_always_exhaustive() } -> same_as<bool>; { M::is_always_unique() } -> same_as<bool>; bool_constant<M::is_always_strided()>::value; bool_constant<M::is_always_exhaustive()>::value; bool_constant<M::is_always_unique()>::value; };

[Note 1:

This concept checks that the functions M::is_always_strided(), M::is_always_exhaustive(), and M::is_always_unique() exist, are constant expressions, and have a return type of bool.

— end note]

23.7.3.4.7.3 Constructors [mdspan.layout.stride.cons]

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constexpr mapping() noexcept;

Preconditions: layout_right::mapping<extents_type>().required_span_size() is representable as a value of type index_type ([basic.fundamental]).

Effects: Direct-non-list-initializes extents_ with extents_type(), and for all d in the range [0, rank_), direct-non-list-initializes strides_[d] with layout_right::mapping<extents_type>().stride(d).

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template<class OtherIndexType>
  constexpr mapping(const extents_type& e, span<OtherIndexType, rank_> s) noexcept;
template<class OtherIndexType>
  constexpr mapping(const extents_type& e, const array<OtherIndexType, rank_>& s) noexcept;

Constraints:

(3.1)
is_convertible_v<const OtherIndexType&, index_type> is true, and
(3.2)
is_nothrow_constructible_v<index_type, const OtherIndexType&> is true.

Preconditions:

(4.1)
The result of converting s[i] to index_type is greater than 0 for all i in the range [0, rank_).
(4.2)
REQUIRED-SPAN-SIZE(e, s) is representable as a value of type index_type ([basic.fundamental]).
(4.3)
If rank_ is greater than 0, then there exists a permutation P of the integers in the range [0, rank_), such that s[ $p_{i}$ ] >= s[ $p_{i - 1}$ ] * e.extent(p $_{i - 1}$ ) is true for all i in the range [1, rank_), where $p_{i}$ is the $i^{th}$ element of P.

[Note 1:
For layout_stride, this condition is necessary and sufficient for is_unique() to be true.
— end note]

Effects: Direct-non-list-initializes extents_ with e, and for all d in the range [0, rank_), direct-non-list-initializes strides_[d] with as_const(s[d]).

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template<class StridedLayoutMapping>
  constexpr explicit(see below)
    mapping(const StridedLayoutMapping& other) noexcept;

Constraints:

(6.1)
layout-mapping-alike<StridedLayoutMapping> is satisfied.
(6.2)
is_constructible_v<extents_type, typename StridedLayoutMapping::extents_type> is

true.
(6.3)
StridedLayoutMapping::is_always_unique() is true.
(6.4)
StridedLayoutMapping::is_always_strided() is true.

Preconditions:

(7.1)
StridedLayoutMapping meets the layout mapping requirements ([mdspan.layout.reqmts]),
(7.2)
other.stride(r) > 0 is true for every rank index r of extents(),
(7.3)
other.required_span_size() is representable as a value of type index_type ([basic.fundamental]), and
(7.4)
OFFSET(other) == 0 is true.

Effects: Direct-non-list-initializes extents_ with other.extents(), and for all d in the range [0, rank_), direct-non-list-initializes strides_[d] with other.stride(d).

Remarks: The expression inside explicit is equivalent to: !(is_convertible_v<typename StridedLayoutMapping::extents_type, extents_type> && (is-mapping-of<layout_left, StridedLayoutMapping> || is-mapping-of<layout_right, StridedLayoutMapping> || is-layout-left-padded-mapping-of<StridedLayoutMapping> || is-layout-right-padded-mapping-of<StridedLayoutMapping> || is-mapping-of<layout_stride, StridedLayoutMapping>))

23.7.3.4.7.4 Observers [mdspan.layout.stride.obs]

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constexpr index_type required_span_size() const noexcept;

Returns: REQUIRED-SPAN-SIZE(extents(), strides_).

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template<class... Indices>
  constexpr index_type operator()(Indices... i) const noexcept;

Constraints:

(2.1)
sizeof...(Indices) == rank_ is true,
(2.2)
(is_convertible_v<Indices, index_type> && ...) is true, and
(2.3)
(is_nothrow_constructible_v<index_type, Indices> && ...) is true.

Preconditions: extents_type::index-cast(i) is a multidimensional index in extents_ ([mdspan.overview]).

Effects: Let P be a parameter pack such that is_same_v<index_sequence_for<Indices...>, index_sequence<P...>> is true.

Equivalent to: return ((static_cast<index_type>(i) * stride(P)) + ... + 0);

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constexpr bool is_exhaustive() const noexcept;

Returns:

(5.1)
true if rank_ is 0.
(5.2)
Otherwise, true if there is a permutation P of the integers in the range [0, rank_) such that stride( $p_{0}$ ) equals 1, and stride( $p_{i}$ ) equals stride( $p_{i - 1}$ ) * extents().extent( $p_{i - 1}$ ) for i in the range [1, rank_), where $p_{i}$ is the $i^{th}$ element of P.
(5.3)
Otherwise, false.

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template<class OtherMapping>
  friend constexpr bool operator==(const mapping& x, const OtherMapping& y) noexcept;

Constraints:

(6.1)
layout-mapping-alike<OtherMapping> is satisfied.
(6.2)
rank_ == OtherMapping::extents_type::rank() is true.
(6.3)
OtherMapping::is_always_strided() is true.

Preconditions: OtherMapping meets the layout mapping requirements ([mdspan.layout.policy.reqmts]).

Returns: true if x.extents() == y.extents() is true, OFFSET(y) == 0 is true, and each of x.stride(r) == y.stride(r) is true for r in the range [0, x.extents().rank()).

Otherwise, false.

23.7.3.4.8 Class template layout_left_padded::mapping [mdspan.layout.leftpad]

23.7.3.4.8.1 Overview [mdspan.layout.leftpad.overview]

layout_left_padded provides a layout mapping that behaves like layout_left::mapping, except that the padding stride stride(1) can be greater than or equal to extent(0).

namespace std { template<size_t PaddingValue> template<class Extents> class layout_left_padded<PaddingValue>::mapping { public: static constexpr size_t padding_value = PaddingValue; using extents_type = Extents; using index_type = typename extents_type::index_type; using size_type = typename extents_type::size_type; using rank_type = typename extents_type::rank_type; using layout_type = layout_left_padded<PaddingValue>; private: static constexpr size_t rank_ = extents_type::rank(); // exposition only static constexpr size_t first-static-extent = // exposition only extents_type::static_extent(0); // [mdspan.layout.leftpad.expo], exposition-only members static constexpr size_t static-padding-stride = see below; // exposition only public: // [mdspan.layout.leftpad.cons], constructors constexpr mapping() noexcept : mapping(extents_type{}) {} constexpr mapping(const mapping&) noexcept = default; constexpr mapping(const extents_type&); template<class OtherIndexType> constexpr mapping(const extents_type&, OtherIndexType); template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_left::mapping<OtherExtents>&); template<class OtherExtents> constexpr explicit(extents_type::rank() > 0) mapping(const layout_stride::mapping<OtherExtents>&); template<class LayoutLeftPaddedMapping> constexpr explicit(see below) mapping(const LayoutLeftPaddedMapping&); template<class LayoutRightPaddedMapping> constexpr explicit(see below) mapping(const LayoutRightPaddedMapping&) noexcept; constexpr mapping& operator=(const mapping&) noexcept = default; // [mdspan.layout.leftpad.obs], observers constexpr const extents_type& extents() const noexcept { return extents_; } constexpr array<index_type, rank_> strides() const noexcept; constexpr index_type required_span_size() const noexcept; template<class... Indices> constexpr index_type operator()(Indices...) const noexcept; static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept; static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } constexpr bool is_exhaustive() const noexcept; static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept; template<class LayoutLeftPaddedMapping> friend constexpr bool operator==(const mapping&, const LayoutLeftPaddedMapping&) noexcept; private: // [mdspan.layout.leftpad.expo], exposition-only members index_type stride-1 = static-padding-stride; // exposition only extents_type extents_{}; // exposition only // [mdspan.sub.map], submdspan mapping specialization template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const // exposition only -> see below; template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping(const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...); } }; }

If Extents is not a specialization of extents, then the program is ill-formed.

layout_left_padded::mapping<E> is a trivially copyable type that models regular for each E.

Throughout [mdspan.layout.leftpad], let P_rank be the following size rank_ parameter pack of size_t values:

(4.1)
the empty parameter pack, if rank_ equals zero;
(4.2)
otherwise, 0zu, if rank_ equals one;
(4.3)
otherwise, the parameter pack 0zu, 1zu, …, rank_- 1.

Mandates:

(5.1)
If rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type index_type.
(5.2)
padding_value is representable as a value of type index_type.
(5.3)
If
- (5.3.1)
  rank_ is greater than one,
- (5.3.2)
  padding_value does not equal dynamic_extent, and
- (5.3.3)
  first-static-extent does not equal dynamic_extent,
then LEAST-MULTIPLE-AT-LEAST(padding_value, first-static-extent) is representable as a value of type size_t, and is representable as a value of type index_type.
(5.4)
If
- (5.4.1)
  rank_ is greater than one,
- (5.4.2)
  padding_value does not equal dynamic_extent, and
- (5.4.3)
  extents_type::static_extent(k) does not equal dynamic_extent for all k in the range [0, extents_type::rank()),
then the product of LEAST-MULTIPLE-AT-LEAST(padding_value, ext.static_extent(0)) and all values ext.static_extent(k) with k in the range of [1, rank_) is representable as a value of type size_t, and is representable as a value of type index_type.

23.7.3.4.8.2 Exposition-only members [mdspan.layout.leftpad.expo]

🔗

static constexpr size_t static-padding-stride = see below;

The value is

(1.1)
0, if rank_ equals zero or one;
(1.2)
otherwise, dynamic_extent, if padding_value or first-static-extent equals dynamic_extent;
(1.3)
otherwise, the size_t value which is LEAST-MULTIPLE-AT-LEAST(padding_value, first-static-extent).

🔗

index_type stride-1 = static-padding-stride;

Recommended practice: Implementations should not store this value if static-padding-stride is not dynamic_extent.

[Note 1:

Using extents<index_type, static-padding-stride> instead of index_type as the type of stride-1 would achieve this.

— end note]

23.7.3.4.8.3 Constructors [mdspan.layout.leftpad.cons]

🔗

constexpr mapping(const extents_type& ext);

Preconditions:

(1.1)
The size of the multidimensional index space ext is representable as a value of type index_type.
(1.2)
If rank_ is greater than one and padding_value does not equal dynamic_extent, then LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(0)) is representable as a value of type index_type.
(1.3)
If rank_ is greater than one and padding_value does not equal dynamic_extent, then the product of LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(0)) and all values ext.extent(k) with k in the range of [1, rank_) is representable as a value of type index_type.

Effects:

(2.1)
Direct-non-list-initializes extents_ with ext; and
(2.2)
if rank_ is greater than one, direct-non-list-initializes stride-1
- (2.2.1)
  with ext.extent(0) if padding_value is dynamic_extent,
- (2.2.2)
  otherwise with LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(0)).

🔗

template<class OtherIndexType>
constexpr mapping(const extents_type& ext, OtherIndexType pad);

Constraints:

(3.1)
is_convertible_v<OtherIndexType, index_type> is true.
(3.2)
is_nothrow_constructible_v<index_type, OtherIndexType> is true.

Preconditions:

(4.1)
pad is representable as a value of type index_type.
(4.2)
extents_type::index-cast(pad) is greater than zero.
(4.3)
If rank_ is greater than one, then LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(0)) is representable as a value of type index_type.
(4.4)
If rank_ is greater than one, then the product of LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(0)) and all values ext.extent(k) with k in the range of [1, rank_) is representable as a value of type index_type.
(4.5)
If padding_value is not equal to dynamic_extent, padding_value equals extents_type::index-cast(pad).

Effects: Direct-non-list-initializes extents_ with ext, and if rank_ is greater than one, direct-non-list-initializes stride-1 with LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(0)).

🔗

template<class OtherExtents>
  constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
    mapping(const layout_left::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Mandates: If OtherExtents::rank() is greater than 1, then (static-padding-stride == dynamic_extent) || (OtherExtents::static_extent(0) == dynamic_extent) || (static-padding-stride == OtherExtents::static_extent(0)) is true.

Preconditions:

(8.1)
If extents_type::rank() > 1 is true and padding_value == dynamic_extent is false, then other.stride(1) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extents().extent(0))) and
(8.2)
other.required_span_size() is representable as a value of type index_type.

Effects: Equivalent to mapping(other.extents()).

🔗

template<class OtherExtents>
  constexpr explicit(rank_ > 0)
    mapping(const layout_stride::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

(11.1)
If rank_ is greater than 1 and padding_value does not equal dynamic_extent, then other.stride(1) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extents().extent(0)))
(11.2)
If rank_ is greater than 0, then other.stride(0) equals 1.
(11.3)
If rank_ is greater than 2, then for all r in the range [2, rank_), other.stride(r) equals (other.extents().fwd-prod-of-extents(r) / other.extents().extent(0)) * other.stride(1)
(11.4)
other.required_span_size() is representable as a value of type index_type.

Effects:

(12.1)
Direct-non-list-initializes extents_ with other.extents() and
(12.2)
if rank_ is greater than one, direct-non-list-initializes stride-1 with other.stride(1).

🔗

template<class LayoutLeftPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutLeftPaddedMapping& other);

Constraints:

(13.1)
is-layout-left-padded-mapping-of<LayoutLeftPaddedMapping> is true.
(13.2)
is_constructible_v<extents_type, typename LayoutLeftPaddedMapping::extents_type>
is true.

Mandates: If rank_ is greater than 1, then padding_value == dynamic_extent || LayoutLeftPaddedMapping::padding_value == dynamic_extent || padding_value == LayoutLeftPaddedMapping::padding_value is true.

(15.1)
If rank_ is greater than 1 and padding_value does not equal dynamic_extent, then other.stride(1) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extent(0)))
(15.2)
other.required_span_size() is representable as a value of type index_type.

Effects:

(16.1)
Direct-non-list-initializes extents_ with other.extents() and
(16.2)
if rank_ is greater than one, direct-non-list-initializes stride-1 with other.stride(1).

Remarks: The expression inside explicit is equivalent to: rank_> 1 && (padding_value != dynamic_extent || LayoutLeftPaddedMapping::padding_value == dynamic_extent)

🔗

template<class LayoutRightPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutRightPaddedMapping& other) noexcept;

Constraints:

(18.1)
is-layout-right-padded-mapping-of<LayoutRightPaddedMapping> is true or
is-mapping-of<layout_right, LayoutRightPaddedMapping> is true.
(18.2)
rank_ equals zero or one.
(18.3)
is_constructible_v<extents_type, typename LayoutRightPaddedMapping::extents_-
type> is true.

Preconditions: other.required_span_size() is representable as a value of type index_type.

Effects: Direct-non-list-initializes extents_ with other.extents().

Remarks: The expression inside explicit is equivalent to: !is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type>

[Note 1:

Neither the input mapping nor the mapping to be constructed uses the padding stride in the rank-0 or rank-1 case, so the padding stride does not affect either the constraints or the preconditions.

— end note]

23.7.3.4.8.4 Observers [mdspan.layout.leftpad.obs]

🔗

constexpr array<index_type, rank_> strides() const noexcept;

Returns: array<index_type, rank_>({stride(P_rank)...}).

🔗

constexpr index_type required_span_size() const noexcept;

Returns:

(2.1)
0 if the multidimensional index space extents_ is empty,
(2.2)
otherwise, *this(((extents_(P_rank) - index_type(1))...)) + 1.

🔗

template<class... Indices>
constexpr size_t operator()(Indices... idxs) const noexcept;

Constraints:

(3.1)
sizeof...(Indices) == rank_ is true.
(3.2)
(is_convertible_v<Indices, index_type> && ...) is true.
(3.3)
(is_nothrow_constructible_v<index_type, Indices> && ...) is true.

Preconditions: extents_type::index-cast(idxs) is a multidimensional index in extents() ([mdspan.overview]).

Returns: ((static_cast<index_type>(idxs) * stride(P_rank)) + ... + 0).

🔗

static constexpr bool is_always_exhaustive() noexcept;

Returns:

(6.1)
If rank_ equals zero or one, then true;
(6.2)
otherwise, if neither static-padding-stride nor first-static-extent equal dynamic_extent, then static-padding-stride == first-static-extent;
(6.3)
otherwise, false.

🔗

constexpr bool is_exhaustive() const noexcept;

Returns: true if rank_ equals zero or one; otherwise, extents_.extent(0) == stride(1).

🔗

constexpr index_type stride(rank_type r) const noexcept;

Preconditions: r is smaller than rank_.

Returns:

(9.1)
If r equals zero: 1;
(9.2)
otherwise, if r equals one: stride-1;
(9.3)
otherwise, the product of stride-1 and all values extents_.extent(k) with k in the range [1, r).

🔗

template<class LayoutLeftPaddedMapping>
  friend constexpr bool operator==(const mapping& x, const LayoutLeftPaddedMapping& y) noexcept;

Constraints:

(10.1)
is-layout-left-padded-mapping-of<LayoutLeftPaddedMapping> is true.
(10.2)
LayoutLeftPaddedMapping::extents_type::rank() == rank_ is true.

Returns: true if x.extents() == y.extents() is true and rank_ < 2 || x.stride(1) == y.
stride(1) is true.

Otherwise, false.

23.7.3.4.9 Class template layout_right_padded::mapping [mdspan.layout.rightpad]

23.7.3.4.9.1 Overview [mdspan.layout.rightpad.overview]

layout_right_padded provides a layout mapping that behaves like layout_right::mapping, except that the padding stride stride(extents_type::rank() - 2) can be greater than or equal to extents_type::extent(extents_type::rank() - 1).

namespace std { template<size_t PaddingValue> template<class Extents> class layout_right_padded<PaddingValue>::mapping { public: static constexpr size_t padding_value = PaddingValue; using extents_type = Extents; using index_type = typename extents_type::index_type; using size_type = typename extents_type::size_type; using rank_type = typename extents_type::rank_type; using layout_type = layout_right_padded<PaddingValue>; private: static constexpr size_t rank_ = extents_type::rank(); // exposition only static constexpr size_t last-static-extent = // exposition only extents_type::static_extent(rank_ - 1); // [mdspan.layout.rightpad.expo], exposition-only members static constexpr size_t static-padding-stride = see below; // exposition only public: // [mdspan.layout.rightpad.cons], constructors constexpr mapping() noexcept : mapping(extents_type{}) {} constexpr mapping(const mapping&) noexcept = default; constexpr mapping(const extents_type&); template<class OtherIndexType> constexpr mapping(const extents_type&, OtherIndexType); template<class OtherExtents> constexpr explicit(!is_convertible_v<OtherExtents, extents_type>) mapping(const layout_right::mapping<OtherExtents>&); template<class OtherExtents> constexpr explicit(rank_ > 0) mapping(const layout_stride::mapping<OtherExtents>&); template<class LayoutRightPaddedMapping> constexpr explicit(see below) mapping(const LayoutRightPaddedMapping&); template<class LayoutLeftPaddedMapping> constexpr explicit(see below) mapping(const LayoutLeftPaddedMapping&) noexcept; constexpr mapping& operator=(const mapping&) noexcept = default; // [mdspan.layout.rightpad.obs], observers constexpr const extents_type& extents() const noexcept { return extents_; } constexpr array<index_type, rank_> strides() const noexcept; constexpr index_type required_span_size() const noexcept; template<class... Indices> constexpr index_type operator()(Indices...) const noexcept; static constexpr bool is_always_unique() noexcept { return true; } static constexpr bool is_always_exhaustive() noexcept; static constexpr bool is_always_strided() noexcept { return true; } static constexpr bool is_unique() noexcept { return true; } constexpr bool is_exhaustive() const noexcept; static constexpr bool is_strided() noexcept { return true; } constexpr index_type stride(rank_type) const noexcept; template<class LayoutRightPaddedMapping> friend constexpr bool operator==(const mapping&, const LayoutRightPaddedMapping&) noexcept; private: // [mdspan.layout.rightpad.expo], exposition-only members index_type stride-rm2 = static-padding-stride; // exposition only extents_type extents_{}; // exposition only // [mdspan.sub.map], submdspan mapping specialization template<class... SliceSpecifiers> constexpr auto submdspan-mapping-impl(SliceSpecifiers...) const // exposition only -> see below; template<class... SliceSpecifiers> friend constexpr auto submdspan_mapping(const mapping& src, SliceSpecifiers... slices) { return src.submdspan-mapping-impl(slices...); } }; }

If Extents is not a specialization of extents, then the program is ill-formed.

layout_right_padded::mapping<E> is a trivially copyable type that models regular for each E.

Throughout [mdspan.layout.rightpad], let P_rank be the following size rank_ parameter pack of size_t values:

(4.1)
the empty parameter pack, if rank_ equals zero;
(4.2)
otherwise, 0zu, if rank_ equals one;
(4.3)
otherwise, the parameter pack 0zu, 1zu, …, rank_- 1.

Mandates:

(5.1)
If rank_dynamic() == 0 is true, then the size of the multidimensional index space Extents() is representable as a value of type index_type.
(5.2)
padding_value is representable as a value of type index_type.
(5.3)
If
- (5.3.1)
  rank_ is greater than one,
- (5.3.2)
  padding_value does not equal dynamic_extent, and
- (5.3.3)
  last-static-extent does not equal dynamic_extent,
then LEAST-MULTIPLE-AT-LEAST(padding_value, last-static-extent) is representable as a value of type size_t, and is representable as a value of type index_type.
(5.4)
If
- (5.4.1)
  rank_ is greater than one,
- (5.4.2)
  padding_value does not equal dynamic_extent, and
- (5.4.3)
  extents_type::static_extent(k) does not equal dynamic_extent for all k in the range [0, rank_),
then the product of LEAST-MULTIPLE-AT-LEAST(padding_value, ext.static_extent(rank_ - 1)) and all values ext.static_extent(k) with k in the range of [0, rank_ - 1) is representable as a value of type size_t, and is representable as a value of type index_type.

23.7.3.4.9.2 Exposition-only members [mdspan.layout.rightpad.expo]

🔗

static constexpr size_t static-padding-stride = see below;

The value is

(1.1)
0, if rank_ equals zero or one;
(1.2)
otherwise, dynamic_extent, if padding_value or last-static-extent equals dynamic_extent;
(1.3)
otherwise, the size_t value which is LEAST-MULTIPLE-AT-LEAST(padding_value, last-static-extent).

🔗

index_type stride-rm2 = static-padding-stride;

Recommended practice: Implementations should not store this value if static-padding-stride is not dynamic_extent.

[Note 1:

Using extents<index_type, static-padding-stride> instead of index_type as the type of stride-rm2 would achieve this.

— end note]

23.7.3.4.9.3 Constructors [mdspan.layout.rightpad.cons]

🔗

constexpr mapping(const extents_type& ext);

Preconditions:

(1.1)
The size of the multidimensional index space ext is representable as a value of type index_type.
(1.2)
If rank_ is greater than one and padding_value does not equal dynamic_extent, then LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(rank_ - 1)) is representable as a value of type index_type.
(1.3)
If rank_ is greater than one and padding_value does not equal dynamic_extent, then the product of LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(rank_ - 1)) and all values ext.extent(k) with k in the range of [0, rank_ - 1) is representable as a value of type index_type.

Effects:

(2.1)
Direct-non-list-initializes extents_ with ext; and
(2.2)
if rank_ is greater than one, direct-non-list-initializes stride-rm2
- (2.2.1)
  with ext.extent(rank_ - 1) if padding_value is dynamic_extent,
- (2.2.2)
  otherwise with LEAST-MULTIPLE-AT-LEAST(padding_value, ext.extent(rank_ - 1)).

🔗

template<class OtherIndexType>
constexpr mapping(const extents_type& ext, OtherIndexType pad);

Constraints:

(3.1)
is_convertible_v<OtherIndexType, index_type> is true.
(3.2)
is_nothrow_constructible_v<index_type, OtherIndexType> is true.

Preconditions:

(4.1)
pad is representable as a value of type index_type.
(4.2)
extents_type::index-cast(pad) is greater than zero.
(4.3)
If rank_ is greater than one, then LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1)) is representable as a value of type index_type.
(4.4)
If rank_ is greater than one, then the product of LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1)) and all values ext.extent(k) with k in the range of [0, rank_ - 1) is representable as a value of type index_type.
(4.5)
If padding_value is not equal to dynamic_extent, padding_value equals extents_type::index-cast(pad).

Effects: Direct-non-list-initializes extents_ with ext, and if rank_ is greater than one, direct-non-list-initializes stride-rm2 with LEAST-MULTIPLE-AT-LEAST(pad, ext.extent(rank_ - 1)).

🔗

template<class OtherExtents>
  constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
    mapping(const layout_right::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Mandates: If OtherExtents::rank() is greater than 1, then (static-padding-stride == dynamic_extent) || (OtherExtents::static_extent(rank_ - 1) == dynamic_extent) || (static-padding-stride == OtherExtents::static_extent(rank_ - 1)) is true.

Preconditions:

(8.1)
If rank_ > 1 is true and padding_value == dynamic_extent is false, then other.stride(
rank_ - 2) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extents().extent(rank_ - 1))) and
(8.2)
other.required_span_size() is representable as a value of type index_type.

Effects: Equivalent to mapping(other.extents()).

🔗

template<class OtherExtents>
  constexpr explicit(rank_ > 0)
    mapping(const layout_stride::mapping<OtherExtents>& other);

Constraints: is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

(11.1)
If rank_ is greater than 1 and padding_value does not equal dynamic_extent, then other.stride(rank_ - 2) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extents().extent(rank_ - 1)))
(11.2)
If rank_ is greater than 0, then other.stride(rank_ - 1) equals 1.
(11.3)
If rank_ is greater than 2, then for all r in the range [0, rank_ - 2), other.stride(r) equals (other.extents().rev-prod-of-extents(r) / other.extents().extent(rank_ - 1)) * other.stride(rank_ - 2)
(11.4)
other.required_span_size() is representable as a value of type index_type.

Effects:

(12.1)
Direct-non-list-initializes extents_ with other.extents(); and
(12.2)
if rank_ is greater than one, direct-non-list-initializes stride-rm2 with other.stride(rank_ - 2).

🔗

template<class LayoutRightPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutRightPaddedMapping& other);

Constraints:

(13.1)
is-layout-right-padded-mapping-of<LayoutRightPaddedMapping> is true.
(13.2)
is_constructible_v<extents_type, typename LayoutRightPaddedMapping::extents_-
type> is true.

Mandates: If rank_ is greater than 1, then padding_value == dynamic_extent || LayoutRightPaddedMapping::padding_value == dynamic_extent || padding_value == LayoutRightPaddedMapping::padding_value is true.

Preconditions:

(15.1)
If rank_ is greater than 1 and padding_value does not equal dynamic_extent, then other.stride(rank_ - 2) equals LEAST-MULTIPLE-AT-LEAST(padding_value, extents_type::index-cast(other.extent(rank_ - 1)))
(15.2)
other.required_span_size() is representable as a value of type index_type.

Effects:

(16.1)
Direct-non-list-initializes extents_ with other.extents(); and
(16.2)
if rank_ is greater than one, direct-non-list-initializes stride-rm2 with other.stride(rank_ - 2).

Remarks: The expression inside explicit is equivalent to: rank_ > 1 && (padding_value != dynamic_extent || LayoutRightPaddedMapping::padding_value == dynamic_extent)

🔗

template<class LayoutLeftPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutLeftPaddedMapping& other) noexcept;

Constraints:

(18.1)
is-layout-left-padded-mapping-of<LayoutLeftPaddedMapping> is true or
is-mapping-of<layout_left, LayoutLeftPaddedMapping> is true.
(18.2)
rank_ equals zero or one.
(18.3)
is_constructible_v<extents_type, typename LayoutLeftPaddedMapping::extents_type>
is true.

Preconditions: other.required_span_size() is representable as a value of type index_type.

Effects: Direct-non-list-initializes extents_ with other.extents().

Remarks: The expression inside explicit is equivalent to: !is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type>

[Note 1:

Neither the input mapping nor the mapping to be constructed uses the padding stride in the rank-0 or rank-1 case, so the padding stride affects neither the constraints nor the preconditions.

— end note]

23.7.3.4.9.4 Observers [mdspan.layout.rightpad.obs]

🔗

constexpr array<index_type, rank_> strides() const noexcept;

Returns: array<index_type, rank_>(stride(P_rank)...).

🔗

constexpr index_type required_span_size() const noexcept;

Returns: 0 if the multidimensional index space extents_ is empty, otherwise *this(((extents_(P_rank) - index_type(1))...)) + 1.

🔗

template<class... Indices>
constexpr size_t operator()(Indices... idxs) const noexcept;

Constraints:

(3.1)
sizeof...(Indices) == rank_ is true.
(3.2)
(is_convertible_v<Indices, index_type> && ...) is true.
(3.3)
(is_nothrow_constructible_v<index_type, Indices> && ...) is true.

Preconditions: extents_type::index-cast(idxs) is a multidimensional index in extents() ([mdspan.overview]).

Returns: ((static_cast<index_type>(idxs) * stride(P_rank)) + ... + 0).

🔗

static constexpr bool is_always_exhaustive() noexcept;

Returns:

(6.1)
If rank_ equals zero or one, then true;
(6.2)
otherwise, if neither static-padding-stride nor last-static-extent equal dynamic_extent, then static-padding-stride == last-static-extent;
(6.3)
otherwise, false.

🔗

constexpr bool is_exhaustive() const noexcept;

Returns: true if rank_ equals zero or one; otherwise, extents_.extent(rank_ - 1) == stride(rank_ - 2)

🔗

constexpr index_type stride(rank_type r) const noexcept;

Preconditions: r is smaller than rank_.

Returns:

(9.1)
If r equals rank_ - 1: 1;
(9.2)
otherwise, if r equals rank_ - 2: stride-rm2;
(9.3)
otherwise, the product of stride-rm2 and all values extents_.extent(k) with k in the range of [r + 1, rank_ - 1).

🔗

template<class LayoutRightPaddedMapping>
  friend constexpr bool operator==(const mapping& x, const LayoutRightPaddedMapping& y) noexcept;

Constraints:

(10.1)
is-layout-right-padded-mapping-of<LayoutRightPaddedMapping> is true.
(10.2)
LayoutRightPaddedMapping::extents_type::rank() == rank_ is true.

Returns: true if x.extents() == y.extents() is true and rank_ < 2 || x.stride(rank_ - 2) == y.stride(rank_ - 2) is true.

Otherwise, false.

23.7.3.5 Accessor policy [mdspan.accessor]

23.7.3.5.1 General [mdspan.accessor.general]

An accessor policy defines types and operations by which a reference to a single object is created from an abstract data handle to a number of such objects and an index.

A range of indices

[0, N)

is an accessible range of a given data handle and an accessor if, for each i in the range, the accessor policy's access function produces a valid reference to an object.

In [mdspan.accessor.reqmts],

(3.1)
A denotes an accessor policy.
(3.2)
a denotes a value of type A or const A.
(3.3)
p denotes a value of type A::data_handle_type or const A::data_handle_type.

[Note 1:
The type A::data_handle_type need not be dereferenceable.
— end note]
(3.4)
n, i, and j each denote values of type size_t.

23.7.3.5.2 Requirements [mdspan.accessor.reqmts]

A type A meets the accessor policy requirements if

(1.1)
A models copyable,
(1.2)
is_nothrow_move_constructible_v<A> is true,
(1.3)
is_nothrow_move_assignable_v<A> is true,
(1.4)
is_nothrow_swappable_v<A> is true, and
(1.5)
the following types and expressions are well-formed and have the specified semantics.

🔗

typename A::element_type

Result: A complete object type that is not an abstract class type.

🔗

typename A::data_handle_type

Result: A type that models copyable, and for which is_nothrow_move_constructible_v<A::data_handle_type> is true, is_nothrow_move_assignable_v<A::data_handle_type> is true, and is_nothrow_swappable_v<A::data_handle_type> is true.

[Note 1:

The type of data_handle_type need not be element_type*.

— end note]

🔗

typename A::reference

Result: A type that models common_reference_with<A::reference&&, A::element_type&>.

[Note 2:

The type of reference need not be element_type&.

— end note]

🔗

typename A::offset_policy

Result: A type OP such that:

(5.1)
OP meets the accessor policy requirements,
(5.2)
constructible_from<OP, const A&> is modeled, and
(5.3)
is_same_v<typename OP::element_type, typename A::element_type> is true.

🔗

a.access(p, i)

Result: A::reference

Remarks: The expression is equality preserving.

[Note 3:

Concrete accessor policies can impose preconditions for their access function.

However, they might not.

For example, an accessor where p is span<A::element_type, dynamic_extent> and access(p, i) returns p[i % p.size()] does not need to impose a precondition on i.

— end note]

🔗

a.offset(p, i)

Result: A::offset_policy::data_handle_type

Returns: q such that for b being A::offset_policy(a), and any integer n for which [0, n) is an accessible range of p and a:

(10.1)
$[0, n - i)$ is an accessible range of q and b; and
(10.2)
b.access(q, j) provides access to the same element as a.access(p, i + j), for every j in the range $[0, n - i)$ .

Remarks: The expression is equality-preserving.

23.7.3.5.3 Class template default_accessor [mdspan.accessor.default]

23.7.3.5.3.1 Overview [mdspan.accessor.default.overview]

namespace std { template<class ElementType> struct default_accessor { using offset_policy = default_accessor; using element_type = ElementType; using reference = ElementType&; using data_handle_type = ElementType*; constexpr default_accessor() noexcept = default; template<class OtherElementType> constexpr default_accessor(default_accessor<OtherElementType>) noexcept; constexpr reference access(data_handle_type p, size_t i) const noexcept; constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept; }; }

default_accessor meets the accessor policy requirements.

ElementType is required to be a complete object type that is neither an abstract class type nor an array type.

Each specialization of default_accessor is a trivially copyable type that models semiregular .

[0, n)

is an accessible range for an object p of type data_handle_type and an object of type default_accessor if and only if [p, p + n) is a valid range.

23.7.3.5.3.2 Members [mdspan.accessor.default.members]

🔗

template<class OtherElementType>
  constexpr default_accessor(default_accessor<OtherElementType>) noexcept {}

Constraints: is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true.

🔗

constexpr reference access(data_handle_type p, size_t i) const noexcept;

Effects: Equivalent to: return p[i];

🔗

constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;

Effects: Equivalent to: return p + i;

23.7.3.5.4 Class template aligned_accessor [mdspan.accessor.aligned]

23.7.3.5.4.1 Overview [mdspan.accessor.aligned.overview]

namespace std { template<class ElementType, size_t ByteAlignment> struct aligned_accessor { using offset_policy = default_accessor<ElementType>; using element_type = ElementType; using reference = ElementType&; using data_handle_type = ElementType*; static constexpr size_t byte_alignment = ByteAlignment; constexpr aligned_accessor() noexcept = default; template<class OtherElementType, size_t OtherByteAlignment> constexpr aligned_accessor( aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept; template<class OtherElementType> constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept; template<class OtherElementType> constexpr operator default_accessor<OtherElementType>() const noexcept; constexpr reference access(data_handle_type p, size_t i) const noexcept; constexpr typename offset_policy::data_handle_type offset( data_handle_type p, size_t i) const noexcept; }; }

Mandates:

(1.1)
byte_alignment is a power of two, and
(1.2)
byte_alignment >= alignof(ElementType) is true.

aligned_accessor meets the accessor policy requirements.

ElementType is required to be a complete object type that is neither an abstract class type nor an array type.

Each specialization of aligned_accessor is a trivially copyable type that models semiregular .

[0, n) is an accessible range for an object p of type data_handle_type and an object of type aligned_accessor if and only if

(5.1)
[p, p + n) is a valid range, and,
(5.2)
if n is greater than zero, then is_sufficiently_aligned<byte_alignment>(p) is true.

[Example 1:

The following function compute uses is_sufficiently_aligned to check whether a given mdspan with default_accessor has a data handle with sufficient alignment to be used with aligned_accessor<float, 4 * sizeof(float)>.

If so, the function dispatches to a function compute_using_fourfold_overalignment that requires fourfold over-alignment of arrays, but can therefore use hardware-specific instructions, such as four-wide SIMD (Single Instruction Multiple Data) instructions.

Otherwise, compute dispatches to a possibly less optimized function compute_without_requiring_overalignment that has no over-alignment requirement.

void compute_using_fourfold_overalignment( std::mdspan<float, std::dims<1>, std::layout_right, std::aligned_accessor<float, 4 * alignof(float)>> x); void compute_without_requiring_overalignment( std::mdspan<float, std::dims<1>, std::layout_right> x); void compute(std::mdspan<float, std::dims<1>> x) { constexpr auto byte_alignment = 4 * sizeof(float); auto accessor = std::aligned_accessor<float, byte_alignment>{}; auto x_handle = x.data_handle(); if (std::is_sufficiently_aligned<byte_alignment>(x_handle)) { compute_using_fourfold_overalignment(std::mdspan{x_handle, x.mapping(), accessor}); } else { compute_without_requiring_overalignment(x); } } — end example]

23.7.3.5.4.2 Members [mdspan.accessor.aligned.members]

🔗

template<class OtherElementType, size_t OtherByteAlignment>
  constexpr aligned_accessor(aligned_accessor<OtherElementType, OtherByteAlignment>) noexcept;

Constraints:

(1.1)
is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true.
(1.2)
OtherByteAlignment >= byte_alignment is true.

Effects: None.

🔗

template<class OtherElementType>
  constexpr explicit aligned_accessor(default_accessor<OtherElementType>) noexcept;

Constraints: is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true.

Effects: None.

🔗

constexpr reference access(data_handle_type p, size_t i) const noexcept;

Preconditions: [0, i + 1) is an accessible range for p and *this.

Effects: Equivalent to: return assume_aligned<byte_alignment>(p)[i];

🔗

template<class OtherElementType>
  constexpr operator default_accessor<OtherElementType>() const noexcept;

Constraints: is_convertible_v<element_type(*)[], OtherElementType(*)[]> is true.

Effects: Equivalent to: return {};

🔗

constexpr typename offset_policy::data_handle_type
  offset(data_handle_type p, size_t i) const noexcept;

Preconditions: [0, i + 1) is an accessible range for p and *this.

Effects: Equivalent to: return assume_aligned<byte_alignment>(p) + i;

23.7.3.6 Class template mdspan [mdspan.mdspan]

23.7.3.6.1 Overview [mdspan.mdspan.overview]

mdspan is a view of a multidimensional array of elements.

namespace std { template<class ElementType, class Extents, class LayoutPolicy = layout_right, class AccessorPolicy = default_accessor<ElementType>> class mdspan { public: using extents_type = Extents; using layout_type = LayoutPolicy; using accessor_type = AccessorPolicy; using mapping_type = typename layout_type::template mapping<extents_type>; using element_type = ElementType; using value_type = remove_cv_t<element_type>; using index_type = typename extents_type::index_type; using size_type = typename extents_type::size_type; using rank_type = typename extents_type::rank_type; using data_handle_type = typename accessor_type::data_handle_type; using reference = typename accessor_type::reference; static constexpr rank_type rank() noexcept { return extents_type::rank(); } static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); } static constexpr size_t static_extent(rank_type r) noexcept { return extents_type::static_extent(r); } constexpr index_type extent(rank_type r) const noexcept { return extents().extent(r); } // [mdspan.mdspan.cons], constructors constexpr mdspan(); constexpr mdspan(const mdspan& rhs) = default; constexpr mdspan(mdspan&& rhs) = default; template<class... OtherIndexTypes> constexpr explicit mdspan(data_handle_type ptr, OtherIndexTypes... exts); template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) mdspan(data_handle_type p, span<OtherIndexType, N> exts); template<class OtherIndexType, size_t N> constexpr explicit(N != rank_dynamic()) mdspan(data_handle_type p, const array<OtherIndexType, N>& exts); constexpr mdspan(data_handle_type p, const extents_type& ext); constexpr mdspan(data_handle_type p, const mapping_type& m); constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a); template<class OtherElementType, class OtherExtents, class OtherLayoutPolicy, class OtherAccessorPolicy> constexpr explicit(see below) mdspan(const mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessorPolicy>& other); constexpr mdspan& operator=(const mdspan& rhs) = default; constexpr mdspan& operator=(mdspan&& rhs) = default; // [mdspan.mdspan.members], members template<class... OtherIndexTypes> constexpr reference operator[](OtherIndexTypes... indices) const; template<class OtherIndexType> constexpr reference operator[](span<OtherIndexType, rank()> indices) const; template<class OtherIndexType> constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const; constexpr size_type size() const noexcept; constexpr bool empty() const noexcept; friend constexpr void swap(mdspan& x, mdspan& y) noexcept; constexpr const extents_type& extents() const noexcept { return map_.extents(); } constexpr const data_handle_type& data_handle() const noexcept { return ptr_; } constexpr const mapping_type& mapping() const noexcept { return map_; } constexpr const accessor_type& accessor() const noexcept { return acc_; } static constexpr bool is_always_unique() { return mapping_type::is_always_unique(); } static constexpr bool is_always_exhaustive() { return mapping_type::is_always_exhaustive(); } static constexpr bool is_always_strided() { return mapping_type::is_always_strided(); } constexpr bool is_unique() const { return map_.is_unique(); } constexpr bool is_exhaustive() const { return map_.is_exhaustive(); } constexpr bool is_strided() const { return map_.is_strided(); } constexpr index_type stride(rank_type r) const { return map_.stride(r); } private: accessor_type acc_; // exposition only mapping_type map_; // exposition only data_handle_type ptr_; // exposition only }; template<class CArray> requires (is_array_v<CArray> && rank_v<CArray> == 1) mdspan(CArray&) -> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>; template<class Pointer> requires (is_pointer_v<remove_reference_t<Pointer>>) mdspan(Pointer&&) -> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>; template<class ElementType, class... Integrals> requires ((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0) explicit mdspan(ElementType*, Integrals...) -> mdspan<ElementType, extents<size_t, maybe-static-ext<Integrals>...>>; template<class ElementType, class OtherIndexType, size_t N> mdspan(ElementType*, span<OtherIndexType, N>) -> mdspan<ElementType, dextents<size_t, N>>; template<class ElementType, class OtherIndexType, size_t N> mdspan(ElementType*, const array<OtherIndexType, N>&) -> mdspan<ElementType, dextents<size_t, N>>; template<class ElementType, class IndexType, size_t... ExtentsPack> mdspan(ElementType*, const extents<IndexType, ExtentsPack...>&) -> mdspan<ElementType, extents<IndexType, ExtentsPack...>>; template<class ElementType, class MappingType> mdspan(ElementType*, const MappingType&) -> mdspan<ElementType, typename MappingType::extents_type, typename MappingType::layout_type>; template<class MappingType, class AccessorType> mdspan(const typename AccessorType::data_handle_type&, const MappingType&, const AccessorType&) -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type, typename MappingType::layout_type, AccessorType>; }

Mandates:

(2.1)
ElementType is a complete object type that is neither an abstract class type nor an array type,
(2.2)
Extents is a specialization of extents, and
(2.3)
is_same_v<ElementType, typename AccessorPolicy::element_type> is true.

LayoutPolicy shall meet the layout mapping policy requirements ([mdspan.layout.policy.reqmts]), and AccessorPolicy shall meet the accessor policy requirements ([mdspan.accessor.reqmts]).

Each specialization MDS of mdspan models copyable and

(4.1)
is_nothrow_move_constructible_v<MDS> is true,
(4.2)
is_nothrow_move_assignable_v<MDS> is true, and
(4.3)
is_nothrow_swappable_v<MDS> is true.

A specialization of mdspan is a trivially copyable type if its accessor_type, mapping_type, and data_handle_type are trivially copyable types.

23.7.3.6.2 Constructors [mdspan.mdspan.cons]

🔗

constexpr mdspan();

Constraints:

(1.1)
rank_dynamic() > 0 is true.
(1.2)
is_default_constructible_v<data_handle_type> is true.
(1.3)
is_default_constructible_v<mapping_type> is true.
(1.4)
is_default_constructible_v<accessor_type> is true.

Preconditions: [0, map_.required_span_size()) is an accessible range of ptr_ and acc_ for the values of map_ and acc_ after the invocation of this constructor.

Effects: Value-initializes ptr_, map_, and acc_.

🔗

template<class... OtherIndexTypes>
  constexpr explicit mdspan(data_handle_type p, OtherIndexTypes... exts);

Let N be sizeof...(OtherIndexTypes).

Constraints:

(5.1)
(is_convertible_v<OtherIndexTypes, index_type> && ...) is true,
(5.2)
(is_nothrow_constructible<index_type, OtherIndexTypes> && ...) is true,
(5.3)
N == rank() || N == rank_dynamic() is true,
(5.4)
is_constructible_v<mapping_type, extents_type> is true, and
(5.5)
is_default_constructible_v<accessor_type> is true.

Preconditions: [0, map_.required_span_size()) is an accessible range of p and acc_ for the values of map_ and acc_ after the invocation of this constructor.

Effects:

(7.1)
Direct-non-list-initializes ptr_ with std::move(p),
(7.2)
direct-non-list-initializes map_ with extents_type(static_cast<index_type>(std::move(exts))...), and
(7.3)
value-initializes acc_.

🔗

template<class OtherIndexType, size_t N>
  constexpr explicit(N != rank_dynamic())
    mdspan(data_handle_type p, span<OtherIndexType, N> exts);
template<class OtherIndexType, size_t N>
  constexpr explicit(N != rank_dynamic())
    mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);

Constraints:

(8.1)
is_convertible_v<const OtherIndexType&, index_type> is true,
(8.2)
is_nothrow_constructible_v<index_type, const OtherIndexType&> is true,
(8.3)
N == rank() || N == rank_dynamic() is true,
(8.4)
is_constructible_v<mapping_type, extents_type> is true, and
(8.5)
is_default_constructible_v<accessor_type> is true.

Preconditions: [0, map_.required_span_size()) is an accessible range of p and acc_ for the values of map_ and acc_ after the invocation of this constructor.

Effects:

(10.1)
Direct-non-list-initializes ptr_ with std::move(p),
(10.2)
direct-non-list-initializes map_ with extents_type(exts), and
(10.3)
value-initializes acc_.

🔗

constexpr mdspan(data_handle_type p, const extents_type& ext);

Constraints:

(11.1)
is_constructible_v<mapping_type, const extents_type&> is true, and
(11.2)
is_default_constructible_v<accessor_type> is true.

Preconditions: [0, map_.required_span_size()) is an accessible range of p and acc_ for the values of map_ and acc_ after the invocation of this constructor.

Effects:

(13.1)
Direct-non-list-initializes ptr_ with std::move(p),
(13.2)
direct-non-list-initializes map_ with ext, and
(13.3)
value-initializes acc_.

🔗

constexpr mdspan(data_handle_type p, const mapping_type& m);

Constraints: is_default_constructible_v<accessor_type> is true.

Preconditions: [0, m.required_span_size()) is an accessible range of p and acc_ for the value of acc_ after the invocation of this constructor.

Effects:

(16.1)
Direct-non-list-initializes ptr_ with std::move(p),
(16.2)
direct-non-list-initializes map_ with m, and
(16.3)
value-initializes acc_.

🔗

constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);

Preconditions: [0, m.required_span_size()) is an accessible range of p and a.

Effects:

(18.1)
Direct-non-list-initializes ptr_ with std::move(p),
(18.2)
direct-non-list-initializes map_ with m, and
(18.3)
direct-non-list-initializes acc_ with a.

🔗

template<class OtherElementType, class OtherExtents,
         class OtherLayoutPolicy, class OtherAccessor>
  constexpr explicit(see below)
    mdspan(const mdspan<OtherElementType, OtherExtents,
                        OtherLayoutPolicy, OtherAccessor>& other);

Constraints:

(19.1)
is_constructible_v<mapping_type, const OtherLayoutPolicy::template mapping<Oth-
erExtents>&> is true, and
(19.2)
is_constructible_v<accessor_type, const OtherAccessor&> is true.

Mandates:

(20.1)
is_constructible_v<data_handle_type, const OtherAccessor::data_handle_type&> is
true, and
(20.2)
is_constructible_v<extents_type, OtherExtents> is true.

Preconditions:

(21.1)
For each rank index r of extents_type, static_extent(r) == dynamic_extent || static_extent(r) == other.extent(r) is true.
(21.2)
[0, map_.required_span_size()) is an accessible range of ptr_ and acc_ for values of ptr_, map_, and acc_ after the invocation of this constructor.

Effects:

(22.1)
Direct-non-list-initializes ptr_ with other.ptr_,
(22.2)
direct-non-list-initializes map_ with other.map_, and
(22.3)
direct-non-list-initializes acc_ with other.acc_.

Remarks: The expression inside explicit is equivalent to: !is_convertible_v<const OtherLayoutPolicy::template mapping<OtherExtents>&, mapping_type> || !is_convertible_v<const OtherAccessor&, accessor_type>

23.7.3.6.3 Members [mdspan.mdspan.members]

🔗

template<class... OtherIndexTypes>
  constexpr reference operator[](OtherIndexTypes... indices) const;

Constraints:

(1.1)
(is_convertible_v<OtherIndexTypes, index_type> && ...) is true,
(1.2)
(is_nothrow_constructible_v<index_type, OtherIndexTypes> && ...) is true, and
(1.3)
sizeof...(OtherIndexTypes) == rank() is true.

Let I be extents_type::index-cast(std::move(indices)).

Preconditions: I is a multidimensional index in extents().

[Note 1:

This implies that map_(I) < map_.required_span_size() is true.

— end note]

Effects: Equivalent to: return acc_.access(ptr_, map_(static_cast<index_type>(std::move(indices))...));

🔗

template<class OtherIndexType>
  constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
template<class OtherIndexType>
  constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;

Constraints:

(5.1)
is_convertible_v<const OtherIndexType&, index_type> is true, and
(5.2)
is_nothrow_constructible_v<index_type, const OtherIndexType&> is true.

Effects: Let P be a parameter pack such that is_same_v<make_index_sequence<rank()>, index_sequence<P...>> is true.

Equivalent to: return operator[](extents_type::index-cast(as_const(indices[P]))...);

🔗

constexpr size_type size() const noexcept;

Preconditions: The size of the multidimensional index space extents() is representable as a value of type size_type ([basic.fundamental]).

Returns: extents().fwd-prod-of-extents(rank()).

🔗

constexpr bool empty() const noexcept;

Returns: true if the size of the multidimensional index space extents() is 0, otherwise false.

🔗

friend constexpr void swap(mdspan& x, mdspan& y) noexcept;

Effects: Equivalent to: swap(x.ptr_, y.ptr_); swap(x.map_, y.map_); swap(x.acc_, y.acc_);

23.7.3.7 submdspan [mdspan.sub]

23.7.3.7.1 Overview [mdspan.sub.overview]

The submdspan facilities create a new mdspan viewing a subset of elements of an existing input mdspan.

The subset viewed by the created mdspan is determined by the SliceSpecifier arguments.

For each function defined in [mdspan.sub] that takes a parameter pack named slices as an argument:

(2.1)
let index_type be
- (2.1.1)
 M::index_type if the function is a member of a class M,
- (2.1.2)
 otherwise, remove_reference_t<decltype(src)>::index_type if the function has a parameter named src,
- (2.1.3)
 otherwise, the same type as the function's template argument IndexType;
(2.2)
let rank be the number of elements in slices;
(2.3)
let $s_{k}$ be the $k^{th}$ element of slices;
(2.4)
let $S_{k}$ be the type of $s_{k}$ ; and
(2.5)
let map-rank be an array<size_t, rank> such that for each k in the range [0, rank), map-rank[k] equals:
- (2.5.1)
 dynamic_extent if $S_{k}$ models convertible_to<index_type>,
- (2.5.2)
 otherwise, the number of types $S_{j}$ with $j < k$ that do not model convertible_to<index_type>.

23.7.3.7.2 strided_slice [mdspan.sub.strided.slice]

strided_slice represents a set of extent regularly spaced integer indices.

The indices start at offset, and increase by increments of stride.

🔗

namespace std { template<class OffsetType, class ExtentType, class StrideType> struct strided_slice { using offset_type = OffsetType; using extent_type = ExtentType; using stride_type = StrideType; [[no_unique_address]] offset_type offset{}; [[no_unique_address]] extent_type extent{}; [[no_unique_address]] stride_type stride{}; }; }

strided_slice has the data members and special members specified above.

It has no base classes or members other than those specified.

Mandates: OffsetType, ExtentType, and StrideType are signed or unsigned integer types, or model integral-constant-like.

[Note 1:

strided_slice{.offset = 1, .extent = 10, .stride = 3} indicates the indices 1, 4, 7, and 10.

Indices are selected from the half-open interval [1, 1 + 10).

— end note]

23.7.3.7.3 submdspan_mapping_result [mdspan.sub.map.result]

Specializations of submdspan_mapping_result are returned by overloads of submdspan_mapping.

🔗

namespace std { template<class LayoutMapping> struct submdspan_mapping_result { [[no_unique_address]] LayoutMapping mapping = LayoutMapping(); size_t offset{}; }; }

submdspan_mapping_result has the data members and special members specified above.

It has no base classes or members other than those specified.

LayoutMapping shall meet the layout mapping requirements ([mdspan.layout.policy.reqmts]).

23.7.3.7.4 Exposition-only helpers [mdspan.sub.helpers]

🔗

template<class T>
  constexpr T de-ice(T val) { return val; }
template<integral-constant-like T>
  constexpr auto de-ice(T) { return T::value; }

template<class IndexType, size_t k, class... SliceSpecifiers>
  constexpr IndexType first_(SliceSpecifiers... slices);

Mandates: IndexType is a signed or unsigned integer type.

Let

ϕ_{k}

denote the following value:

(2.1)
$s_{k}$ if $S_{k}$ models convertible_to<IndexType>;
(2.2)
otherwise, get<0>( $s_{k}$ ) if $S_{k}$ models index-pair-like<IndexType>;
(2.3)
otherwise, de-ice( $s_{k}$ .offset) if $S_{k}$ is a specialization of strided_slice;
(2.4)
otherwise, 0.

Preconditions:

ϕ_{k}

is representable as a value of type IndexType.

Returns: extents<IndexType>::index-cast(

ϕ_{k}

🔗

template<size_t k, class Extents, class... SliceSpecifiers>
  constexpr auto last_(const Extents& src, SliceSpecifiers... slices);

Mandates: Extents is a specialization of extents.

Let index_type be typename Extents::index_type.

Let

λ_{k}

denote the following value:

(7.1)
de-ice( $s_{k}$ ) + 1 if $S_{k}$ models convertible_to<index_type>; otherwise
(7.2)
get<1>( $s_{k}$ ) if $S_{k}$ models index-pair-like<index_type>; otherwise
(7.3)
de-ice( $s_{k}$ .offset) + de-ice( $s_{k}$ .extent) if $S_{k}$ is a specialization of strided_slice; otherwise
(7.4)
src.extent(k).

Preconditions:

λ_{k}

is representable as a value of type index_type.

Returns: Extents::index-cast(

λ_{k}

🔗

template<class IndexType, size_t N, class... SliceSpecifiers>
  constexpr array<IndexType, sizeof...(SliceSpecifiers)>
    src-indices(const array<IndexType, N>& indices, SliceSpecifiers... slices);

Mandates: IndexType is a signed or unsigned integer type.

Returns: An array<IndexType, sizeof...(SliceSpecifiers)> src_idx such that for each k in the range [0, sizeof...(SliceSpecifiers)), src_idx[k] equals

(11.1)
first_<IndexType, k>(slices...) for each k where map-rank[k] equals dynamic_extent,
(11.2)
otherwise, first_<IndexType, k>(slices...) + indices[map-rank[k]].

23.7.3.7.5 submdspan_extents function [mdspan.sub.extents]

🔗

template<class IndexType, class... Extents, class... SliceSpecifiers>
  constexpr auto submdspan_extents(const extents<IndexType, Extents...>& src,
                                   SliceSpecifiers... slices);

Constraints: sizeof...(slices) equals Extents::rank().

Mandates: For each rank index k of src.extents(), exactly one of the following is true:

(2.1)
$S_{k}$ models convertible_to<IndexType>,
(2.2)
$S_{k}$ models index-pair-like<IndexType>,
(2.3)
is_convertible_v< $S_{k}$ , full_extent_t> is true, or
(2.4)
$S_{k}$ is a specialization of strided_slice.

Preconditions: For each rank index k of src.extents(), all of the following are true:

(3.1)
if $S_{k}$ is a specialization of strided_slice
- (3.1.1)
  $s_{k} .extent = 0$ , or
- (3.1.2)
  $s_{k} .stride > 0$
(3.2)
0 ≤ first_<IndexType, k>(slices...) ≤ last_<k>(src, slices...) ≤ src.extent(k)

Let SubExtents be a specialization of extents such that:

(4.1)
SubExtents::rank() equals the number of k such that $S_{k}$ does not model convertible_to<IndexType>; and
(4.2)
for each rank index k of Extents such that map-rank[k] != dynamic_extent is true, SubExtents::static_extent(map-rank[k]) equals:
- (4.2.1)
 Extents::static_extent(k) if is_convertible_v< $S_{k}$ , full_extent_t> is true; otherwise
- (4.2.2)
 de-ice(tuple_element_t<1, $S_{k}$ >()) - de-ice(tuple_element_t<0, $S_{k}$ >()) if $S_{k}$ models index-pair-like<IndexType>, and both tuple_element_t<0, $S_{k}$ > and tuple_element_t<1, $S_{k}$ > model integral-constant-like; otherwise
- (4.2.3)
 0, if $S_{k}$ is a specialization of strided_slice, whose extent_type models integral-constant-like, for which extent_type() equals zero; otherwise
- (4.2.4)
 1 + (de-ice( $S_{k}$ ::extent_type()) - 1) / de-ice( $S_{k}$ ::stride_type()), if $S_{k}$ is a specialization of strided_slice whose extent_type and stride_type model integral-constant-like;
- (4.2.5)
 otherwise, dynamic_extent.

Returns: A value ext of type SubExtents such that for each k for which map-rank[k] != dynamic_extent is true, ext.extent(map-rank[k]) equals:

(5.1)
$s_{k}$ .extent == 0 ? 0 : 1 + (de-ice( $s_{k}$ .extent) - 1) / de-ice( $s_{k}$ .stride) if $S_{k}$ is a specialization of strided_slice,
(5.2)
otherwise, last_<k>(src, slices...) - first_<IndexType, k>(slices...).

23.7.3.7.6 Specializations of submdspan_mapping [mdspan.sub.map]

23.7.3.7.6.1 Common [mdspan.sub.map.common]

The following elements apply to all functions in [mdspan.sub.map].

Constraints: sizeof...(slices) equals extents_type::rank().

Mandates: For each rank index k of extents(), exactly one of the following is true:

(3.1)
$S_{k}$ models convertible_to<index_type>,
(3.2)
$S_{k}$ models index-pair-like<index_type>,
(3.3)
is_convertible_v< $S_{k}$ , full_extent_t> is true, or
(3.4)
$S_{k}$ is a specialization of strided_slice.

Preconditions: For each rank index k of extents(), all of the following are true:

(4.1)
if $S_{k}$ is a specialization of strided_slice, $s_{k}$ .extent is equal to zero or $s_{k}$ .stride is greater than zero; and
(4.2)
0 ≤ first_<index_type, k>(slices...)
0 ≤ last_<k>(extents(), slices...)
0 ≤ extents().extent(k)

Let sub_ext be the result of submdspan_extents(extents(), slices...) and let SubExtents be decltype(sub_ext).

Let sub_strides be an array<SubExtents::index_type, SubExtents::rank()> such that for each rank index k of extents() for which map-rank[k] is not dynamic_extent, sub_strides[map-rank[k]] equals:

(6.1)
stride(k) * de-ice( $s_{k}$ .stride) if $S_{k}$ is a specialization of strided_slice and $s_{k}$ .stride < $s_{k}$ .extent is true;
(6.2)
otherwise, stride(k).

Let P be a parameter pack such that is_same_v<make_index_sequence<rank()>, index_sequence<P...>> is true.

If first_<index_type, k>(slices...) equals extents().extent(k) for any rank index k of extents(), then let offset be a value of type size_t equal to (*this).required_span_size().

Otherwise, let offset be a value of type size_t equal to (*this)(first_<index_type, P>(slices...)...).

Given a layout mapping type M, a type S is a unit-stride slice for M if

(9.1)
S is a specialization of strided_slice where S::stride_type models integral-constant-like and S::stride_type::value equals 1,
(9.2)
S models index-pair-like<M::index_type>, or
(9.3)
is_convertible_v<S, full_extent_t> is true.

23.7.3.7.6.2 layout_left specialization of submdspan_mapping [mdspan.sub.map.left]

🔗

template<class Extents>
template<class... SliceSpecifiers>
constexpr auto layout_left::mapping<Extents>::submdspan-mapping-impl(
      SliceSpecifiers... slices) const -> see below;

Returns:

(1.1)
submdspan_mapping_result{*this, 0}, if Extents::rank() == 0 is true;
(1.2)
otherwise, submdspan_mapping_result{layout_left::mapping(sub_ext), offset}, if SubExtents::rank() == 0 is true;
(1.3)
otherwise, submdspan_mapping_result{layout_left::mapping(sub_ext), offset}, if
- (1.3.1)
 for each k in the range [0, SubExtents::rank() - 1)), is_convertible_v< $S_{k}$ , full_extent_t> is true; and
- (1.3.2)
 for k equal to SubExtents::rank() - 1, $S_{k}$ is a unit-stride slice for mapping;
[Note 1:
If the above conditions are true, all $S_{k}$ with k larger than SubExtents::rank() - 1 are convertible to index_type.
— end note]
(1.4)
otherwise, submdspan_mapping_result{layout_left_padded<S_static>::mapping(sub_ext, stride(u + 1)), offset} if for a value u for which $u + 1$ is the smallest value p larger than zero for which $S_{p}$ is a unit-stride slice for mapping, the following conditions are met:
- (1.4.1)
 $S_{0}$ is a unit-stride slice for mapping; and
- (1.4.2)
 for each k in the range [u + 1, u + SubExtents::rank() - 1), is_convertible_v< $S_{k}$ , full_extent_t> is true; and
- (1.4.3)
 for k equal to u + SubExtents::rank() - 1, $S_{k}$ is a unit-stride slice for mapping;
and where S_static is:
- (1.4.4)
 dynamic_extent, if static_extent(k) is dynamic_extent for any k in the range [0, u + 1),
- (1.4.5)
 otherwise, the product of all values static_extent(k) for k in the range [0, u + 1);
(1.5)
otherwise, submdspan_mapping_result{layout_stride::mapping(sub_ext, sub_strides), offset}

23.7.3.7.6.3 layout_right specialization of submdspan_mapping [mdspan.sub.map.right]

🔗

template<class Extents>
template<class... SliceSpecifiers>
constexpr auto layout_right::mapping<Extents>::submdspan-mapping-impl(
      SliceSpecifiers... slices) const -> see below;

Returns:

(1.1)
submdspan_mapping_result{*this, 0}, if Extents::rank() == 0 is true;
(1.2)
otherwise, submdspan_mapping_result{layout_right::mapping(sub_ext), offset}, if SubExtents::rank() == 0 is true;
(1.3)
otherwise, submdspan_mapping_result{layout_left::mapping(sub_ext), offset}, if
- (1.3.1)
 for each k in the range [rank_ - SubExtents::rank() + 1, rank_), is_convertible_v< $S_{k}$ , full_extent_t> is true; and
- (1.3.2)
 for k equal to _rank - SubExtents::rank(), $S_{k}$ is a unit-stride slice for mapping;
[Note 1:
If the above conditions are true, all $S_{k}$ with $k<\class{textit}{\texttt{_rank}} - SubExtents::rank()$ are convertible to index_type.
— end note]
(1.4)
otherwise, submdspan_mapping_result{layout_right_padded<S_static>::mapping(sub_ext, stride(rank_ - u - 2)), offset} if for a value u for which $rank_- u - 2$ is the largest value p smaller than rank_ - 1 for which $S_{p}$ is a unit-stride slice for mapping, the following conditions are met:
- (1.4.1)
 for k equal to rank_ - 1, $S_{k}$ is a unit-stride slice for mapping; and
- (1.4.2)
 for each k in the range [rank_ - SubExtents::rank() - u + 1, rank_ - u - 1), is_convertible_v< $S_{k}$ , full_extent_t> is true; and
- (1.4.3)
 for k equal to rank_ - SubExtents::rank() - u,
 $S_{k}$ is a unit-stride slice for mapping;
and where S_static is:
- (1.4.4)
 dynamic_extent, if static_extent(k) is dynamic_extent for any k in the range [rank_ - u - 1, rank_),
- (1.4.5)
 otherwise, the product of all values static_extent(k) for k in the range [rank_ - u - 1, rank_);
(1.5)
otherwise, submdspan_mapping_result{layout_stride::mapping(sub_ext, sub_strides), offset}

23.7.3.7.6.4 layout_stride specialization of submdspan_mapping [mdspan.sub.map.stride]

🔗

template<class Extents>
template<class... SliceSpecifiers>
constexpr auto layout_stride::mapping<Extents>::submdspan-mapping-impl(
      SliceSpecifiers... slices) const -> see below;

Returns:

(1.1)
submdspan_mapping_result{*this, 0}, if Extents::rank() == 0 is true;
(1.2)
otherwise, submdspan_mapping_result{layout_stride::mapping(sub_ext, sub_strides), offset}

23.7.3.7.6.5 layout_left_padded specialization of submdspan_mapping [mdspan.sub.map.leftpad]

🔗

template<class Extents>
template<class... SliceSpecifiers>
constexpr auto layout_left_padded::mapping<Extents>::submdspan-mapping-impl(
    SliceSpecifiers... slices) const -> see below;

Returns:

(1.1)
submdspan_mapping_result{*this, 0}, if Extents::rank() == 0 is true;
(1.2)
otherwise, submdspan_mapping_result{layout_left::mapping(sub_ext), offset}, if rank_ == 1 is true or SubExtents::rank() == 0 is true;
(1.3)
otherwise, submdspan_mapping_result{layout_left::mapping(sub_ext), offset}, if
- (1.3.1)
  SubExtents::rank() == 1 is true and
- (1.3.2)
  $S_{0}$ is a unit-stride slice for mapping;
(1.4)
otherwise, submdspan_mapping_result{layout_left_padded<S_static>::mapping(sub_ext, stride(u + 1)), offset} if for a value u for which u + 1 is the smallest value p larger than zero for which $S_{p}$ is a unit-stride slice for mapping, the following conditions are met:
- (1.4.1)
 $S_{0}$ is a unit-stride slice for mapping; and
- (1.4.2)
 for each k in the range [u + 1, u + SubExtents::rank() - 1), is_convertible_v< $S_{k}$ , full_extent_t> is true; and
- (1.4.3)
 for k equal to u + SubExtents::rank() - 1, $S_{k}$ is a unit-stride slice for mapping;
where S_static is:
- (1.4.4)
 dynamic_extent, if static-padding-stride is dynamic_extent or static_extent(k) is dynamic_extent for any k in the range [1, u + 1),
- (1.4.5)
 otherwise, the product of static-padding-stride and all values static_extent(k) for k in the range [1, u + 1);
(1.5)
otherwise, submdspan_mapping_result{layout_stride::mapping(sub_ext, sub_strides), offset}

23.7.3.7.6.6 layout_right_padded specialization of submdspan_mapping [mdspan.sub.map.rightpad]

🔗

template<class Extents>
template<class... SliceSpecifiers>
constexpr auto layout_right_padded::mapping<Extents>::submdspan-mapping-impl(
    SliceSpecifiers... slices) const -> see below;

Returns:

(1.1)
submdspan_mapping_result{*this, 0}, if rank_ == 0 is true;
(1.2)
otherwise, submdspan_mapping_result{layout_right::mapping(sub_ext), offset},
if rank_ == 1 is true or SubExtents::rank() == 0 is true;
(1.3)
otherwise, submdspan_mapping_result{layout_right::mapping(sub_ext), offset}, if
- (1.3.1)
  SubExtents::rank() == 1 is true and
- (1.3.2)
  for k equal to rank_ - 1, $S_{k}$ is a unit-stride slice for mapping;
(1.4)
otherwise, submdspan_mapping_result{layout_right_padded<S_static>::mapping(sub_ext, stride(rank_ - u - 2)), offset} if for a value u for which rank_ - u - 2 is the largest value p smaller than rank_ - 1 for which $S_{p}$ is a unit-stride slice for mapping, the following conditions are met:
- (1.4.1)
 for k equal to rank_ - 1, $S_{k}$ is a unit-stride slice for mapping; and
- (1.4.2)
 for each k in the range [rank_ - SubExtents::rank() - u + 1, rank_ - u - 1)), is_convertible_v< $S_{k}$ , full_extent_t> is true; and
- (1.4.3)
 for k equal to rank_ - SubExtents::rank() - u,
 $S_{k}$ is a unit-stride slice for mapping;
and where S_static is:
- (1.4.4)
 dynamic_extent if static-padding-stride is dynamic_extent or for any k in the range [rank_ - u - 1, rank_ - 1) static_extent(k) is dynamic_extent,
- (1.4.5)
 otherwise, the product of static-padding-stride and all values static_extent(k) with k in the range [rank_ - u - 1, rank_ - 1);
(1.5)
otherwise, submdspan_mapping_result{layout_stride::mapping(sub_ext, sub_strides), offset}

23.7.3.7.7 submdspan function template [mdspan.sub.sub]

🔗

template<class ElementType, class Extents, class LayoutPolicy,
         class AccessorPolicy, class... SliceSpecifiers>
  constexpr auto submdspan(
    const mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& src,
    SliceSpecifiers... slices) -> see below;

Let index_type be typename Extents::index_type.

Let sub_map_offset be the result of submdspan_mapping(src.mapping(), slices...).

[Note 1:

This invocation of submdspan_mapping selects a function call via overload resolution on a candidate set that includes the lookup set found by argument-dependent lookup ([basic.lookup.argdep]).

— end note]

Constraints:

(3.1)
sizeof...(slices) equals Extents::rank(), and
(3.2)
the expression submdspan_mapping(src.mapping(), slices...) is well-formed when treated as an unevaluated operand.

Mandates:

(4.1)
decltype(submdspan_mapping(src.mapping(), slices...)) is a specialization of submdspan_mapping_result.
(4.2)
is_same_v<remove_cvref_t<decltype(sub_map_offset.mapping.extents())>, decltype(submdspan_extents(src.mapping(), slices...))> is true.
(4.3)
For each rank index k of src.extents(), exactly one of the following is true:
- (4.3.1)
 $S_{k}$ models convertible_to<index_type>,
- (4.3.2)
 $S_{k}$ models index-pair-like<index_type>,
- (4.3.3)
 is_convertible_v< $S_{k}$ , full_extent_t> is true, or
- (4.3.4)
 $S_{k}$ is a specialization of strided_slice.

Preconditions:

(5.1)
For each rank index k of src.extents(), all of the following are true:
- (5.1.1)
 if $S_{k}$ is a specialization of strided_slice
 - (5.1.1.1)
 $s_{k} .extent = 0$ , or
 - (5.1.1.2)
 $s_{k} .stride > 0$
- (5.1.2)
 0 ≤ first_<index_type, k>(slices...) ≤ last_<k>(src.extents(), slices...) ≤ src.extent(k)
(5.2)
sub_map_offset.mapping.extents() == submdspan_extents(src.mapping(), slices...) is true; and
(5.3)
for each integer pack I which is a multidimensional index in sub_map_offset.mapping.extents(), sub_map_offset.mapping(I...) + sub_map_offset.offset == src.mapping()(src-indices(array{I...}, slices...)) is true.

[Note 2:

These conditions ensure that the mapping returned by submdspan_mapping matches the algorithmically expected index-mapping given the slice specifiers.

— end note]

Effects: Equivalent to: auto sub_map_result = submdspan_mapping(src.mapping(), slices...); return mdspan(src.accessor().offset(src.data(), sub_map_result.offset), sub_map_result.mapping, AccessorPolicy::offset_policy(src.accessor()));

[Example 1:

Given a rank-3 mdspan grid3d representing a three-dimensional grid of regularly spaced points in a rectangular prism, the function zero_surface sets all elements on the surface of the 3-dimensional shape to zero.

It does so by reusing a function zero_2d that takes a rank-2 mdspan.

// zero out all elements in an mdspan template<class T, class E, class L, class A> void zero_2d(mdspan<T, E, L, A> a) { static_assert(a.rank() == 2); for (int i = 0; i < a.extent(0); i++) for (int j = 0; j < a.extent(1); j++) a[i, j] = 0; } // zero out just the surface template<class T, class E, class L, class A> void zero_surface(mdspan<T, E, L, A> grid3d) { static_assert(grid3d.rank() == 3); zero_2d(submdspan(grid3d, 0, full_extent, full_extent)); zero_2d(submdspan(grid3d, full_extent, 0, full_extent)); zero_2d(submdspan(grid3d, full_extent, full_extent, 0)); zero_2d(submdspan(grid3d, grid3d.extent(0) - 1, full_extent, full_extent)); zero_2d(submdspan(grid3d, full_extent, grid3d.extent(1) - 1, full_extent)); zero_2d(submdspan(grid3d, full_extent, full_extent, grid3d.extent(2) - 1)); } — end example]