23 Containers library [containers]

23.3 Sequence containers [sequences]

23.3.11 Class template list [list]

23.3.11.1 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.196

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.

The types iterator and const_iterator meet the constexpr iterator requirements[iterator.requirements.general].

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 constexpr list() : list(Allocator()) { } constexpr explicit list(const Allocator&); constexpr explicit list(size_type n, const Allocator& = Allocator()); constexpr list(size_type n, const T& value, const Allocator& = Allocator()); template<class InputIterator> constexpr list(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range<T> R> constexpr list(from_range_t, R&& rg, const Allocator& = Allocator()); constexpr list(const list& x); constexpr list(list&& x); constexpr list(const list&, const type_identity_t<Allocator>&); constexpr list(list&&, const type_identity_t<Allocator>&); constexpr list(initializer_list<T>, const Allocator& = Allocator()); constexpr ~list(); constexpr list& operator=(const list& x); constexpr list& operator=(list&& x) noexcept(allocator_traits<Allocator>::is_always_equal::value); constexpr list& 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& t); 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; // [list.capacity], capacity constexpr bool empty() const noexcept; constexpr size_type size() const noexcept; constexpr size_type max_size() const noexcept; constexpr void resize(size_type sz); constexpr void resize(size_type sz, const T& c); // element access constexpr reference front(); constexpr const_reference front() const; constexpr reference back(); constexpr const_reference back() const; // [list.modifiers], modifiers template<class... Args> constexpr reference emplace_front(Args&&... args); template<class... Args> constexpr reference emplace_back(Args&&... args); constexpr void push_front(const T& x); constexpr void push_front(T&& x); template<container-compatible-range<T> R> constexpr void prepend_range(R&& rg); constexpr void pop_front(); 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 position, const_iterator last); constexpr void swap(list&) noexcept(allocator_traits<Allocator>::is_always_equal::value); constexpr void clear() noexcept; // [list.ops], list operations constexpr void splice(const_iterator position, list& x); constexpr void splice(const_iterator position, list&& x); constexpr void splice(const_iterator position, list& x, const_iterator i); constexpr void splice(const_iterator position, list&& x, const_iterator i); constexpr void splice(const_iterator position, list& x, const_iterator first, const_iterator last); constexpr void splice(const_iterator position, list&& x, const_iterator first, const_iterator last); constexpr size_type remove(const T& value); template<class Predicate> constexpr size_type remove_if(Predicate pred); constexpr size_type unique(); template<class BinaryPredicate> constexpr size_type unique(BinaryPredicate binary_pred); constexpr void merge(list& x); constexpr void merge(list&& x); template<class Compare> constexpr void merge(list& x, Compare comp); template<class Compare> constexpr void merge(list&& x, Compare comp); constexpr void sort(); template<class Compare> constexpr void sort(Compare comp); constexpr 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.

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

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

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constexpr explicit list(const Allocator&);

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

Complexity: Constant.

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constexpr 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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constexpr 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>
  constexpr 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>
  constexpr 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).

23.3.11.3 Capacity [list.capacity]

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constexpr 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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constexpr 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

23.3.11.4 Modifiers [list.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_front(Args&&... args);
template<class... Args> constexpr reference emplace_back(Args&&... args);
template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args);
constexpr void push_front(const T& x);
constexpr void push_front(T&& x);
template<container-compatible-range<T> R>
  constexpr void prepend_range(R&& rg);
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: 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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constexpr iterator erase(const_iterator position);
constexpr iterator erase(const_iterator first, const_iterator last);
constexpr void pop_front();
constexpr void pop_back();
constexpr 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.

23.3.11.5 Operations [list.ops]

Since lists allow fast insertion and erasing from the middle of a list, certain operations are provided specifically for them.197

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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constexpr void splice(const_iterator position, list& x);
constexpr 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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constexpr void splice(const_iterator position, list& x, const_iterator i);
constexpr 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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constexpr void splice(const_iterator position, list& x,
                      const_iterator first, const_iterator last);
constexpr 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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constexpr size_type remove(const T& value);
template<class Predicate> constexpr 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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constexpr size_type unique();
template<class BinaryPredicate> constexpr 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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constexpr void merge(list& x);
constexpr void merge(list&& x);
template<class Compare> constexpr void merge(list& x, Compare comp);
template<class Compare> constexpr 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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constexpr 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.

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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 .

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As specified in [allocator.requirements], the requirements in this Clause apply only to lists whose allocators compare equal.

23.3.11.6 Erasure [list.erasure]

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

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

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

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