23 General utilities library [utilities]

23.1 General [utilities.general]

This Clause describes utilities that are generally useful in C++ programs; some of these utilities are used by other elements of the C++ standard library.
These utilities are summarized in Table 34.
Table 34 — General utilities library summary
Subclause
Header(s)
Utility components
<utility>
Compile-time integer sequences
<utility>
Pairs
<utility>
Tuples
<tuple>
Optional objects
<optional>
Variants
<variant>
Storage for any type
<any>
Fixed-size sequences of bits
<bitset>
Memory
<memory>
<cstdlib>
Smart pointers
<memory>
Memory resources
<memory_­resource>
Scoped allocators
<scoped_­allocator>
Function objects
<functional>
Type traits
<type_­traits>
Compile-time rational arithmetic
<ratio>
Time utilities
<chrono>
<ctime>
Type indexes
<typeindex>
Execution policies
<execution>

23.2 Utility components [utility]

This subclause contains some basic function and class templates that are used throughout the rest of the library.

23.2.1 Header <utility> synopsis [utility.syn]

#include <initializer_list>     // see [initializer_list.syn]

namespace std {
  // [operators], operators
  namespace rel_ops {
    template<class T> bool operator!=(const T&, const T&);
    template<class T> bool operator> (const T&, const T&);
    template<class T> bool operator<=(const T&, const T&);
    template<class T> bool operator>=(const T&, const T&);
  }

  // [utility.swap], swap
  template <class T>
    void swap(T& a, T& b) noexcept(see below);
  template <class T, size_t N>
    void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable_v<T>);

  // [utility.exchange], exchange
  template <class T, class U = T>
    T exchange(T& obj, U&& new_val);

  // [forward], forward/move
  template <class T>
    constexpr T&& forward(remove_reference_t<T>& t) noexcept;
  template <class T>
    constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
  template <class T>
    constexpr remove_reference_t<T>&& move(T&&) noexcept;
  template <class T>
    constexpr conditional_t<
        !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&>
      move_if_noexcept(T& x) noexcept;

  // [utility.as_const], as_­const
  template <class T>
    constexpr add_const_t<T>& as_const(T& t) noexcept;
  template <class T>
    void as_const(const T&&) = delete;

  // [declval], declval
  template <class T>
    add_rvalue_reference_t<T> declval() noexcept;  // as unevaluated operand

  // [intseq], Compile-time integer sequences
  template<class T, T...>
    struct integer_sequence;
  template<size_t... I>
    using index_sequence = integer_sequence<size_t, I...>;

  template<class T, T N>
    using make_integer_sequence = integer_sequence<T, see below>;
  template<size_t N>
    using make_index_sequence = make_integer_sequence<size_t, N>;

  template<class... T>
    using index_sequence_for = make_index_sequence<sizeof...(T)>;

  // [pairs], class template pair
  template <class T1, class T2>
    struct pair;

  // [pairs.spec], pair specialized algorithms
  template <class T1, class T2>
    constexpr bool operator==(const pair<T1, T2>&, const pair<T1, T2>&);
  template <class T1, class T2>
    constexpr bool operator< (const pair<T1, T2>&, const pair<T1, T2>&);
  template <class T1, class T2>
    constexpr bool operator!=(const pair<T1, T2>&, const pair<T1, T2>&);
  template <class T1, class T2>
    constexpr bool operator> (const pair<T1, T2>&, const pair<T1, T2>&);
  template <class T1, class T2>
    constexpr bool operator>=(const pair<T1, T2>&, const pair<T1, T2>&);
  template <class T1, class T2>
    constexpr bool operator<=(const pair<T1, T2>&, const pair<T1, T2>&);

  template <class T1, class T2>
    void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));

  template <class T1, class T2>
    constexpr see below make_pair(T1&&, T2&&);

  // [pair.astuple], tuple-like access to pair
  template <class T> class tuple_size;
  template <size_t I, class T> class tuple_element;

  template <class T1, class T2> struct tuple_size<pair<T1, T2>>;
  template <size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;

  template<size_t I, class T1, class T2>
    constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>&) noexcept;
  template<size_t I, class T1, class T2>
    constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&&) noexcept;
  template<size_t I, class T1, class T2>
    constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>&) noexcept;
  template<size_t I, class T1, class T2>
    constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&&) noexcept;
  template <class T1, class T2>
    constexpr T1& get(pair<T1, T2>& p) noexcept;
  template <class T1, class T2>
    constexpr const T1& get(const pair<T1, T2>& p) noexcept;
  template <class T1, class T2>
    constexpr T1&& get(pair<T1, T2>&& p) noexcept;
  template <class T1, class T2>
    constexpr const T1&& get(const pair<T1, T2>&& p) noexcept;
  template <class T2, class T1>
    constexpr T2& get(pair<T1, T2>& p) noexcept;
  template <class T2, class T1>
    constexpr const T2& get(const pair<T1, T2>& p) noexcept;
  template <class T2, class T1>
    constexpr T2&& get(pair<T1, T2>&& p) noexcept;
  template <class T2, class T1>
    constexpr const T2&& get(const pair<T1, T2>&& p) noexcept;

  // [pair.piecewise], pair piecewise construction
  struct piecewise_construct_t {
    explicit piecewise_construct_t() = default;
  };
  inline constexpr piecewise_construct_t piecewise_construct{};
  template <class... Types> class tuple;        // defined in <tuple>

  // in-place construction
  struct in_place_t {
    explicit in_place_t() = default;
  };
  inline constexpr in_place_t in_place{};
  template <class T>
    struct in_place_type_t {
      explicit in_place_type_t() = default;
    };
  template <class T> inline constexpr in_place_type_t<T> in_place_type{};
  template <size_t I>
    struct in_place_index_t {
      explicit in_place_index_t() = default;
    };
  template <size_t I> inline constexpr in_place_index_t<I> in_place_index{};
}
The header <utility> defines several types and function templates that are described in this Clause.
It also defines the template pair and various function templates that operate on pair objects.
The type chars_­format is a bitmask type with elements scientific, fixed, and hex.

23.2.2 Operators [operators]

To avoid redundant definitions of operator!= out of operator== and operators >, <=, and >= out of operator<, the library provides the following:
template <class T> bool operator!=(const T& x, const T& y);
Requires: Type T is EqualityComparable.
Returns: !(x == y).
template <class T> bool operator>(const T& x, const T& y);
Requires: Type T is LessThanComparable.
Returns: y < x.
template <class T> bool operator<=(const T& x, const T& y);
Requires: Type T is LessThanComparable.
Returns: !(y < x).
template <class T> bool operator>=(const T& x, const T& y);
Requires: Type T is LessThanComparable.
Returns: !(x < y).
In this library, whenever a declaration is provided for an operator!=, operator>, operator>=, or operator<=, and requirements and semantics are not explicitly provided, the requirements and semantics are as specified in this Clause.

23.2.3 swap [utility.swap]

template <class T> void swap(T& a, T& b) noexcept(see below);
Remarks: This function shall not participate in overload resolution unless is_­move_­constructible_­v<T> is true and is_­move_­assignable_­v<T> is true.
The expression inside noexcept is equivalent to:
is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>
Requires: Type T shall be MoveConstructible and MoveAssignable.
Effects: Exchanges values stored in two locations.
template <class T, size_t N> void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable_v<T>);
Remarks: This function shall not participate in overload resolution unless is_­swappable_­v<T> is true.
Requires: a[i] shall be swappable with b[i] for all i in the range [0, N).
Effects: As if by swap_­ranges(a, a + N, b).

23.2.4 exchange [utility.exchange]

template <class T, class U = T> T exchange(T& obj, U&& new_val);
Effects: Equivalent to:
T old_val = std::move(obj);
obj = std::forward<U>(new_val);
return old_val;

23.2.5 Forward/move helpers [forward]

The library provides templated helper functions to simplify applying move semantics to an lvalue and to simplify the implementation of forwarding functions.
All functions specified in this subclause are signal-safe.
template <class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept; template <class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
Returns: static_­cast<T&&>(t).
Remarks: If the second form is instantiated with an lvalue reference type, the program is ill-formed.
[Example
:
template <class T, class A1, class A2>
shared_ptr<T> factory(A1&& a1, A2&& a2) {
  return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
}

struct A {
  A(int&, const double&);
};

void g() {
  shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int&
  int i = 2;
  shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
}
In the first call to factory, A1 is deduced as int, so 2 is forwarded to A's constructor as an rvalue.
In the second call to factory, A1 is deduced as int&, so i is forwarded to A's constructor as an lvalue.
In both cases, A2 is deduced as double, so 1.414 is forwarded to A's constructor as an rvalue.
end example
]
template <class T> constexpr remove_reference_t<T>&& move(T&& t) noexcept;
Returns: static_­cast<remove_­reference_­t<T>&&>(t).
[Example
:
template <class T, class A1>
shared_ptr<T> factory(A1&& a1) {
  return shared_ptr<T>(new T(std::forward<A1>(a1)));
}

struct A {
  A();
  A(const A&);      // copies from lvalues
  A(A&&);           // moves from rvalues
};

void g() {
  A a;
  shared_ptr<A> sp1 = factory<A>(a);                // “a” binds to A(const A&)
  shared_ptr<A> sp1 = factory<A>(std::move(a));     // “a” binds to A(A&&)
}
In the first call to factory, A1 is deduced as A&, so a is forwarded as a non-const lvalue.
This binds to the constructor A(const A&), which copies the value from a.
In the second call to factory, because of the call std​::​move(a), A1 is deduced as A, so a is forwarded as an rvalue.
This binds to the constructor A(A&&), which moves the value from a.
end example
]
template <class T> constexpr conditional_t< !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&> move_if_noexcept(T& x) noexcept;
Returns: std​::​move(x).

23.2.6 Function template as_­const [utility.as_const]

template <class T> constexpr add_const_t<T>& as_const(T& t) noexcept;
Returns: t.

23.2.7 Function template declval [declval]

The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands.
template <class T> add_rvalue_reference_t<T> declval() noexcept; // as unevaluated operand
Remarks: If this function is odr-used, the program is ill-formed.
Remarks: The template parameter T of declval may be an incomplete type.
[Example
:
template <class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&);
declares a function template convert which only participates in overloading if the type From can be explicitly converted to type To.
For another example see class template common_­type ([meta.trans.other]).
end example
]

23.3 Compile-time integer sequences [intseq]

23.3.1 In general [intseq.general]

The library provides a class template that can represent an integer sequence.
When used as an argument to a function template the parameter pack defining the sequence can be deduced and used in a pack expansion.
[Note
:
The index_­sequence alias template is provided for the common case of an integer sequence of type size_­t; see also [tuple.apply].
end note
]

23.3.2 Class template integer_­sequence [intseq.intseq]

namespace std {
  template<class T, T... I>
    struct integer_sequence {
      using value_type = T;
      static constexpr size_t size() noexcept { return sizeof...(I); }
    };
}
T shall be an integer type.

23.3.3 Alias template make_­integer_­sequence [intseq.make]

template<class T, T N> using make_integer_sequence = integer_sequence<T, see below>;
If N is negative the program is ill-formed.
The alias template make_­integer_­sequence denotes a specialization of integer_­sequence with N template non-type arguments.
The type make_­integer_­sequence<T, N> denotes the type integer_­sequence<T, 0, 1, ..., N-1>.
[Note
:
make_­integer_­sequence<int, 0> denotes the type integer_­sequence<int>
end note
]

23.4 Pairs [pairs]

23.4.1 In general [pairs.general]

The library provides a template for heterogeneous pairs of values.
The library also provides a matching function template to simplify their construction and several templates that provide access to pair objects as if they were tuple objects (see [tuple.helper] and [tuple.elem]).

23.4.2 Class template pair [pairs.pair]

namespace std {
  template <class T1, class T2>
    struct pair {
      using first_type  = T1;
      using second_type = T2;

      T1 first;
      T2 second;

      pair(const pair&) = default;
      pair(pair&&) = default;
      EXPLICIT constexpr pair();
      EXPLICIT constexpr pair(const T1& x, const T2& y);
      template<class U1, class U2> EXPLICIT constexpr pair(U1&& x, U2&& y);
      template<class U1, class U2> EXPLICIT constexpr pair(const pair<U1, U2>& p);
      template<class U1, class U2> EXPLICIT constexpr pair(pair<U1, U2>&& p);
      template <class... Args1, class... Args2>
        pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args);

      pair& operator=(const pair& p);
      template<class U1, class U2> pair& operator=(const pair<U1, U2>& p);
      pair& operator=(pair&& p) noexcept(see below);
      template<class U1, class U2> pair& operator=(pair<U1, U2>&& p);

      void swap(pair& p) noexcept(see below);
    };

  template<class T1, class T2>
    pair(T1, T2) -> pair<T1, T2>;
}
Constructors and member functions of pair shall not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.
The defaulted move and copy constructor, respectively, of pair shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.
The destructor of pair shall be a trivial destructor if (is_­trivially_­destructible_­v<T1> && is_­trivially_­destructible_­v<T2>) is true.
EXPLICIT constexpr pair();
Effects: Value-initializes first and second.
Remarks: This constructor shall not participate in overload resolution unless is_­default_­constructible_­v<first_­type> is true and is_­default_­constructible_­v<second_­type> is true.
[Note
:
This behavior can be implemented by a constructor template with default template arguments.
end note
]
The constructor is explicit if and only if either first_­type or second_­type is not implicitly default-constructible.
[Note
:
This behavior can be implemented with a trait that checks whether a const first_­type& or a const second_­type& can be initialized with {}.
end note
]
EXPLICIT constexpr pair(const T1& x, const T2& y);
Effects: Initializes first with x and second with y.
Remarks: This constructor shall not participate in overload resolution unless is_­copy_­constructible_­v<first_­type> is true and is_­copy_­constructible_­v<second_­type> is true.
The constructor is explicit if and only if is_­convertible_­v<const first_­type&, first_­type> is false or is_­convertible_­v<const second_­type&, second_­type> is false.
template<class U1, class U2> EXPLICIT constexpr pair(U1&& x, U2&& y);
Effects: Initializes first with std​::​forward<U1>(x) and second with std​::​forward<U2>(y).
Remarks: This constructor shall not participate in overload resolution unless is_­constructible_­v<first_­type, U1&&> is true and is_­constructible_­v<second_­type, U2&&> is true.
The constructor is explicit if and only if is_­convertible_­v<U1&&, first_­type> is false or is_­convertible_­v<U2&&, second_­type> is false.
template<class U1, class U2> EXPLICIT constexpr pair(const pair<U1, U2>& p);
Effects: Initializes members from the corresponding members of the argument.
Remarks: This constructor shall not participate in overload resolution unless is_­constructible_­v<first_­type, const U1&> is true and is_­constructible_­v<second_­type, const U2&> is true.
The constructor is explicit if and only if is_­convertible_­v<const U1&, first_­type> is false or is_­convertible_­v<const U2&, second_­type> is false.
template<class U1, class U2> EXPLICIT constexpr pair(pair<U1, U2>&& p);
Effects: Initializes first with std​::​forward<U1>(p.first) and second with std​::​forward<U2>(​p.second).
Remarks: This constructor shall not participate in overload resolution unless is_­constructible_­v<first_­type, U1&&> is true and is_­constructible_­v<second_­type, U2&&> is true.
The constructor is explicit if and only if is_­convertible_­v<U1&&, first_­type> is false or is_­convertible_­v<U2&&, second_­type> is false.
template<class... Args1, class... Args2> pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args);
Requires: is_­constructible_­v<first_­type, Args1&&...> is true and is_­constructible_­v<second_­type, Args2&&...> is true.
Effects: Initializes first with arguments of types Args1... obtained by forwarding the elements of first_­args and initializes second with arguments of types Args2... obtained by forwarding the elements of second_­args.
(Here, forwarding an element x of type U within a tuple object means calling std​::​forward<U>(x).)
This form of construction, whereby constructor arguments for first and second are each provided in a separate tuple object, is called piecewise construction.
pair& operator=(const pair& p);
Effects: Assigns p.first to first and p.second to second.
Remarks: This operator shall be defined as deleted unless is_­copy_­assignable_­v<first_­type> is true and is_­copy_­assignable_­v<second_­type> is true.
Returns: *this.
template<class U1, class U2> pair& operator=(const pair<U1, U2>& p);
Effects: Assigns p.first to first and p.second to second.
Remarks: This operator shall not participate in overload resolution unless is_­assignable_­v<first_­type&, const U1&> is true and is_­assignable_­v<second_­type&, const U2&> is true.
Returns: *this.
pair& operator=(pair&& p) noexcept(see below);
Effects: Assigns to first with std​::​forward<first_­type>(p.first) and to second with
std​::​forward<second_­type>(p.second).
Remarks: This operator shall be defined as deleted unless is_­move_­assignable_­v<first_­type> is true and is_­move_­assignable_­v<second_­type> is true.
Remarks: The expression inside noexcept is equivalent to:
is_nothrow_move_assignable_v<T1> && is_nothrow_move_assignable_v<T2>
Returns: *this.
template<class U1, class U2> pair& operator=(pair<U1, U2>&& p);
Effects: Assigns to first with std​::​forward<U>(p.first) and to second with
std​::​forward<V>(p.second).
Remarks: This operator shall not participate in overload resolution unless is_­assignable_­v<first_­type&, U1&&> is true and is_­assignable_­v<second_­type&, U2&&> is true.
Returns: *this.
void swap(pair& p) noexcept(see below);
Requires: first shall be swappable with ([swappable.requirements]) p.first and second shall be swappable with p.second.
Effects: Swaps first with p.first and second with p.second.
Remarks: The expression inside noexcept is equivalent to:
is_nothrow_swappable_v<first_type> && is_nothrow_swappable_v<second_type>

23.4.3 Specialized algorithms [pairs.spec]

template <class T1, class T2> constexpr bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y);
Returns: x.first == y.first && x.second == y.second.
template <class T1, class T2> constexpr bool operator<(const pair<T1, T2>& x, const pair<T1, T2>& y);
Returns: x.first < y.first || (!(y.first < x.first) && x.second < y.second).
template <class T1, class T2> constexpr bool operator!=(const pair<T1, T2>& x, const pair<T1, T2>& y);
Returns: !(x == y).
template <class T1, class T2> constexpr bool operator>(const pair<T1, T2>& x, const pair<T1, T2>& y);
Returns: y < x.
template <class T1, class T2> constexpr bool operator>=(const pair<T1, T2>& x, const pair<T1, T2>& y);
Returns: !(x < y).
template <class T1, class T2> constexpr bool operator<=(const pair<T1, T2>& x, const pair<T1, T2>& y);
Returns: !(y < x).
template<class T1, class T2> void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));
Effects: As if by x.swap(y).
Remarks: This function shall not participate in overload resolution unless is_­swappable_­v<T1> is true and is_­swappable_­v<T2> is true.
template <class T1, class T2> constexpr pair<V1, V2> make_pair(T1&& x, T2&& y);
Returns: pair<V1, V2>(std​::​forward<T1>(x), std​::​forward<T2>(y)), where V1 and V2 are determined as follows: Let Ui be decay_­t<Ti> for each Ti.
If Ui is a specialization of reference_­wrapper, then Vi is Ui​::​type&, otherwise Vi is Ui.
[Example
:
In place of:
  return pair<int, double>(5, 3.1415926);   // explicit types
a C++ program may contain:
  return make_pair(5, 3.1415926);           // types are deduced
end example
]

23.4.4 Tuple-like access to pair [pair.astuple]

template <class T1, class T2> struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> { }; tuple_element<I, pair<T1, T2>>::type
Requires: I < 2.
The program is ill-formed if I is out of bounds.
Value: The type T1 if I == 0, otherwise the type T2.
template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>& p) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>& p) noexcept; template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&& p) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&& p) noexcept;
Returns: If I == 0 returns a reference to p.first; if I == 1 returns a reference to p.second; otherwise the program is ill-formed.
template <class T1, class T2> constexpr T1& get(pair<T1, T2>& p) noexcept; template <class T1, class T2> constexpr const T1& get(const pair<T1, T2>& p) noexcept; template <class T1, class T2> constexpr T1&& get(pair<T1, T2>&& p) noexcept; template <class T1, class T2> constexpr const T1&& get(const pair<T1, T2>&& p) noexcept;
Requires: T1 and T2 are distinct types.
Otherwise, the program is ill-formed.
Returns: A reference to p.first.
template <class T2, class T1> constexpr T2& get(pair<T1, T2>& p) noexcept; template <class T2, class T1> constexpr const T2& get(const pair<T1, T2>& p) noexcept; template <class T2, class T1> constexpr T2&& get(pair<T1, T2>&& p) noexcept; template <class T2, class T1> constexpr const T2&& get(const pair<T1, T2>&& p) noexcept;
Requires: T1 and T2 are distinct types.
Otherwise, the program is ill-formed.
Returns: A reference to p.second.

23.4.5 Piecewise construction [pair.piecewise]

struct piecewise_construct_t { explicit piecewise_construct_t() = default; }; inline constexpr piecewise_construct_t piecewise_construct{};
The struct piecewise_­construct_­t is an empty structure type used as a unique type to disambiguate constructor and function overloading.
Specifically, pair has a constructor with piecewise_­construct_­t as the first argument, immediately followed by two tuple arguments used for piecewise construction of the elements of the pair object.

23.5 Tuples [tuple]

23.5.1 In general [tuple.general]

This subclause describes the tuple library that provides a tuple type as the class template tuple that can be instantiated with any number of arguments.
Each template argument specifies the type of an element in the tuple.
Consequently, tuples are heterogeneous, fixed-size collections of values.
An instantiation of tuple with two arguments is similar to an instantiation of pair with the same two arguments.

23.5.2 Header <tuple> synopsis [tuple.syn]

namespace std {
  // [tuple.tuple], class template tuple
  template <class... Types>
    class tuple;

  // [tuple.creation], tuple creation functions
  inline constexpr unspecified ignore;

  template <class... TTypes>
    constexpr tuple<VTypes...> make_tuple(TTypes&&...);

  template <class... TTypes>
    constexpr tuple<TTypes&&...> forward_as_tuple(TTypes&&...) noexcept;

  template<class... TTypes>
    constexpr tuple<TTypes&...> tie(TTypes&...) noexcept;

  template <class... Tuples>
    constexpr tuple<CTypes...> tuple_cat(Tuples&&...);

  // [tuple.apply], calling a function with a tuple of arguments
  template <class F, class Tuple>
    constexpr decltype(auto) apply(F&& f, Tuple&& t);

  template <class T, class Tuple>
    constexpr T make_from_tuple(Tuple&& t);

  // [tuple.helper], tuple helper classes
  template <class T> class tuple_size;                  // not defined
  template <class T> class tuple_size<const T>;
  template <class T> class tuple_size<volatile T>;
  template <class T> class tuple_size<const volatile T>;

  template <class... Types> class tuple_size<tuple<Types...>>;

  template <size_t I, class T> class tuple_element;     // not defined
  template <size_t I, class T> class tuple_element<I, const T>;
  template <size_t I, class T> class tuple_element<I, volatile T>;
  template <size_t I, class T> class tuple_element<I, const volatile T>;

  template <size_t I, class... Types>
    class tuple_element<I, tuple<Types...>>;

  template <size_t I, class T>
    using tuple_element_t = typename tuple_element<I, T>::type;

  // [tuple.elem], element access
  template <size_t I, class... Types>
    constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>&) noexcept;
  template <size_t I, class... Types>
    constexpr tuple_element_t<I, tuple<Types...>>&& get(tuple<Types...>&&) noexcept;
  template <size_t I, class... Types>
    constexpr const tuple_element_t<I, tuple<Types...>>& get(const tuple<Types...>&) noexcept;
  template <size_t I, class... Types>
    constexpr const tuple_element_t<I, tuple<Types...>>&& get(const tuple<Types...>&&) noexcept;
  template <class T, class... Types>
    constexpr T& get(tuple<Types...>& t) noexcept;
  template <class T, class... Types>
    constexpr T&& get(tuple<Types...>&& t) noexcept;
  template <class T, class... Types>
    constexpr const T& get(const tuple<Types...>& t) noexcept;
  template <class T, class... Types>
    constexpr const T&& get(const tuple<Types...>&& t) noexcept;

  // [tuple.rel], relational operators
  template<class... TTypes, class... UTypes>
    constexpr bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);
  template<class... TTypes, class... UTypes>
    constexpr bool operator<(const tuple<TTypes...>&, const tuple<UTypes...>&);
  template<class... TTypes, class... UTypes>
    constexpr bool operator!=(const tuple<TTypes...>&, const tuple<UTypes...>&);
  template<class... TTypes, class... UTypes>
    constexpr bool operator>(const tuple<TTypes...>&, const tuple<UTypes...>&);
  template<class... TTypes, class... UTypes>
    constexpr bool operator<=(const tuple<TTypes...>&, const tuple<UTypes...>&);
  template<class... TTypes, class... UTypes>
    constexpr bool operator>=(const tuple<TTypes...>&, const tuple<UTypes...>&);

  // [tuple.traits], allocator-related traits
  template <class... Types, class Alloc>
    struct uses_allocator<tuple<Types...>, Alloc>;

  // [tuple.special], specialized algorithms
  template <class... Types>
    void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

  // [tuple.helper], tuple helper classes
  template <class T>
    inline constexpr size_t tuple_size_v = tuple_size<T>::value;
}

23.5.3 Class template tuple [tuple.tuple]

namespace std {
  template <class... Types>
    class tuple  {
    public:
      // [tuple.cnstr], tuple construction
      EXPLICIT constexpr tuple();
      EXPLICIT constexpr tuple(const Types&...);         // only if sizeof...(Types) >= 1
      template <class... UTypes>
        EXPLICIT constexpr tuple(UTypes&&...);           // only if sizeof...(Types) >= 1

      tuple(const tuple&) = default;
      tuple(tuple&&) = default;

      template <class... UTypes>
        EXPLICIT constexpr tuple(const tuple<UTypes...>&);
      template <class... UTypes>
        EXPLICIT constexpr tuple(tuple<UTypes...>&&);

      template <class U1, class U2>
        EXPLICIT constexpr tuple(const pair<U1, U2>&);   // only if sizeof...(Types) == 2
      template <class U1, class U2>
        EXPLICIT constexpr tuple(pair<U1, U2>&&);        // only if sizeof...(Types) == 2

      // allocator-extended constructors
      template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a);
      template <class Alloc>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, const Types&...);
      template <class Alloc, class... UTypes>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, UTypes&&...);
      template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a, const tuple&);
      template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a, tuple&&);
      template <class Alloc, class... UTypes>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, const tuple<UTypes...>&);
      template <class Alloc, class... UTypes>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, tuple<UTypes...>&&);
      template <class Alloc, class U1, class U2>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&);
      template <class Alloc, class U1, class U2>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);

      // [tuple.assign], tuple assignment
      tuple& operator=(const tuple&);
      tuple& operator=(tuple&&) noexcept(see below);

      template <class... UTypes>
        tuple& operator=(const tuple<UTypes...>&);
      template <class... UTypes>
        tuple& operator=(tuple<UTypes...>&&);

      template <class U1, class U2>
        tuple& operator=(const pair<U1, U2>&);              // only if sizeof...(Types) == 2
      template <class U1, class U2>
        tuple& operator=(pair<U1, U2>&&);                   // only if sizeof...(Types) == 2

      // [tuple.swap], tuple swap
      void swap(tuple&) noexcept(see below);
    };

  template<class... UTypes>
    tuple(UTypes...) -> tuple<UTypes...>;
  template<class T1, class T2>
    tuple(pair<T1, T2>) -> tuple<T1, T2>;
  template<class Alloc, class... UTypes>
    tuple(allocator_arg_t, Alloc, UTypes...) -> tuple<UTypes...>;
  template<class Alloc, class T1, class T2>
    tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>;
  template<class Alloc, class... UTypes>
    tuple(allocator_arg_t, Alloc, tuple<UTypes...>) -> tuple<UTypes...>;
}

23.5.3.1 Construction [tuple.cnstr]

For each tuple constructor, an exception is thrown only if the construction of one of the types in Types throws an exception.
The defaulted move and copy constructor, respectively, of tuple shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.
The defaulted move and copy constructor of tuple<> shall be constexpr functions.
The destructor of tuple shall be a trivial destructor if (is_­trivially_­destructible_­v<Types> && ...) is true.
In the constructor descriptions that follow, let i be in the range [0, sizeof...(Types)) in order, be the type in Types, and be the type in a template parameter pack named UTypes, where indexing is zero-based.
EXPLICIT constexpr tuple();
Effects: Value-initializes each element.
Remarks: This constructor shall not participate in overload resolution unless is_­default_­constructible_­v<> is true for all i.
[Note
:
This behavior can be implemented by a constructor template with default template arguments.
end note
]
The constructor is explicit if and only if is not implicitly default-constructible for at least one i.
[Note
:
This behavior can be implemented with a trait that checks whether a const & can be initialized with {}.
end note
]
EXPLICIT constexpr tuple(const Types&...);
Effects: Initializes each element with the value of the corresponding parameter.
Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) >= 1 and is_­copy_­constructible_­v<> is true for all i.
The constructor is explicit if and only if is_­convertible_­v<const &, > is false for at least one i.
template <class... UTypes> EXPLICIT constexpr tuple(UTypes&&... u);
Effects: Initializes the elements in the tuple with the corresponding value in std​::​forward<UTypes>(u).
Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == sizeof...(UTypes) and sizeof...(Types) >= 1 and is_­constructible_­v<, &&> is true for all i.
The constructor is explicit if and only if is_­convertible_­v<&&, > is false for at least one i.
tuple(const tuple& u) = default;
Requires: is_­copy_­constructible_­v<> is true for all i.
Effects: Initializes each element of *this with the corresponding element of u.
tuple(tuple&& u) = default;
Requires: is_­move_­constructible_­v<> is true for all i.
Effects: For all i, initializes the element of *this with std​::​forward<>(get<i>(u)).
template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);
Effects: Initializes each element of *this with the corresponding element of u.
Remarks: This constructor shall not participate in overload resolution unless
  • sizeof...(Types) == sizeof...(UTypes) and
  • is_­constructible_­v<, const &> is true for all i, and
  • sizeof...(Types) != 1, or (when Types... expands to T and UTypes... expands to U)
    !is_­convertible_­v<const tuple<U>&, T> && !is_­constructible_­v<T, const tuple<U>&>
    && !is_­same_­v<T, U>
    is true.
The constructor is explicit if and only if is_­convertible_­v<const &, > is false for at least one i.
template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);
Effects: For all i, initializes the element of *this with std​::​forward<>(get<i>(u)).
Remarks: This constructor shall not participate in overload resolution unless
  • sizeof...(Types) == sizeof...(UTypes), and
  • is_­constructible_­v<, &&> is true for all i, and
  • sizeof...(Types) != 1, or (when Types... expands to T and UTypes... expands to U)
    !is_­convertible_­v<tuple<U>, T> && !is_­constructible_­v<T, tuple<U>> &&
    !is_­same_­v<T, U>
    is true.
The constructor is explicit if and only if is_­convertible_­v<&&, > is false for at least one i.
template <class U1, class U2> EXPLICIT constexpr tuple(const pair<U1, U2>& u);
Effects: Initializes the first element with u.first and the second element with u.second.
Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == 2, is_­constructible_­v<, const U1&> is true and is_­constructible_­v<, const U2&> is true.
The constructor is explicit if and only if is_­convertible_­v<const U1&, > is false or is_­convertible_­v<const U2&, > is false.
template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&& u);
Effects: Initializes the first element with std​::​forward<U1>(u.first) and the second element with std​::​forward<U2>(u.second).
Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == 2, is_­constructible_­v<, U1&&> is true and is_­constructible_­v<, U2&&> is true.
The constructor is explicit if and only if is_­convertible_­v<U1&&, > is false or is_­convertible_­v<U2&&, > is false.
template <class Alloc> tuple(allocator_arg_t, const Alloc& a); template <class Alloc> EXPLICIT tuple(allocator_arg_t, const Alloc& a, const Types&...); template <class Alloc, class... UTypes> EXPLICIT tuple(allocator_arg_t, const Alloc& a, UTypes&&...); template <class Alloc> tuple(allocator_arg_t, const Alloc& a, const tuple&); template <class Alloc> tuple(allocator_arg_t, const Alloc& a, tuple&&); template <class Alloc, class... UTypes> EXPLICIT tuple(allocator_arg_t, const Alloc& a, const tuple<UTypes...>&); template <class Alloc, class... UTypes> EXPLICIT tuple(allocator_arg_t, const Alloc& a, tuple<UTypes...>&&); template <class Alloc, class U1, class U2> EXPLICIT tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&); template <class Alloc, class U1, class U2> EXPLICIT tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);
Requires: Alloc shall meet the requirements for an Allocator.
Effects: Equivalent to the preceding constructors except that each element is constructed with uses-allocator construction.

23.5.3.2 Assignment [tuple.assign]

For each tuple assignment operator, an exception is thrown only if the assignment of one of the types in Types throws an exception.
In the function descriptions that follow, let i be in the range [0, sizeof...​(Types)) in order, be the type in Types, and be the type in a template parameter pack named UTypes, where indexing is zero-based.
tuple& operator=(const tuple& u);
Effects: Assigns each element of u to the corresponding element of *this.
Remarks: This operator shall be defined as deleted unless is_­copy_­assignable_­v<> is true for all i.
Returns: *this.
tuple& operator=(tuple&& u) noexcept(see below);
Effects: For all i, assigns std​::​forward<>(get<i>(u)) to get<i>(*this).
Remarks: This operator shall be defined as deleted unless is_­move_­assignable_­v<> is true for all i.
Remarks: The expression inside noexcept is equivalent to the logical and of the following expressions:
is_nothrow_move_assignable_v<>
where is the type in Types.
Returns: *this.
template <class... UTypes> tuple& operator=(const tuple<UTypes...>& u);
Effects: Assigns each element of u to the corresponding element of *this.
Remarks: This operator shall not participate in overload resolution unless sizeof...(Types) == sizeof...(UTypes) and is_­assignable_­v<&, const &> is true for all i.
Returns: *this.
template <class... UTypes> tuple& operator=(tuple<UTypes...>&& u);
Effects: For all i, assigns std​::​forward<>(get<i>(u)) to get<i>(*this).
Remarks: This operator shall not participate in overload resolution unless is_­assignable_­v<&, &&> == true for all i and sizeof...(Types) == sizeof...(UTypes).
Returns: *this.
template <class U1, class U2> tuple& operator=(const pair<U1, U2>& u);
Effects: Assigns u.first to the first element of *this and u.second to the second element of *this.
Remarks: This operator shall not participate in overload resolution unless sizeof...(Types) == 2 and is_­assignable_­v<&, const U1&> is true for the first type in Types and is_­assignable_­v<&, const U2&> is true for the second type in Types.
Returns: *this.
template <class U1, class U2> tuple& operator=(pair<U1, U2>&& u);
Effects: Assigns std​::​forward<U1>(u.first) to the first element of *this and
std​::​forward<U2>(u.second) to the second element of *this.
Remarks: This operator shall not participate in overload resolution unless sizeof...(Types) == 2 and is_­assignable_­v<&, U1&&> is true for the first type in Types and is_­assignable_­v<&, U2&&> is true for the second type in Types.
Returns: *this.

23.5.3.3 swap [tuple.swap]

void swap(tuple& rhs) noexcept(see below);
Requires: Each element in *this shall be swappable with ([swappable.requirements]) the corresponding element in rhs.
Effects: Calls swap for each element in *this and its corresponding element in rhs.
Remarks: The expression inside noexcept is equivalent to the logical and of the following expressions:
is_nothrow_swappable_v<>
where is the type in Types.
Throws: Nothing unless one of the element-wise swap calls throws an exception.

23.5.3.4 Tuple creation functions [tuple.creation]

In the function descriptions that follow, the members of a parameter pack XTypes are denoted by X for i in [0, sizeof...(XTypes)) in order, where indexing is zero-based.
template<class... TTypes> constexpr tuple<VTypes...> make_tuple(TTypes&&... t);
The pack VTypes is defined as follows.
Let U be decay_­t<T> for each T in TTypes.
If U is a specialization of reference_­wrapper, then V in VTypes is U​::​type&, otherwise V is U.
Returns: tuple<VTypes...>(std​::​forward<TTypes>(t)...).
[Example
:
int i; float j;
make_tuple(1, ref(i), cref(j))
creates a tuple of type tuple<int, int&, const float&>.
end example
]
template<class... TTypes> constexpr tuple<TTypes&&...> forward_as_tuple(TTypes&&... t) noexcept;
Effects: Constructs a tuple of references to the arguments in t suitable for forwarding as arguments to a function.
Because the result may contain references to temporary variables, a program shall ensure that the return value of this function does not outlive any of its arguments (e.g., the program should typically not store the result in a named variable).
Returns: tuple<TTypes&&...>(std​::​forward<TTypes>(t)...).
template<class... TTypes> constexpr tuple<TTypes&...> tie(TTypes&... t) noexcept;
Returns: tuple<TTypes&...>(t...).
When an argument in t is ignore, assigning any value to the corresponding tuple element has no effect.
[Example
:
tie functions allow one to create tuples that unpack tuples into variables.
ignore can be used for elements that are not needed:
int i; std::string s;
tie(i, ignore, s) = make_tuple(42, 3.14, "C++");
// i == 42, s == "C++"
end example
]
template <class... Tuples> constexpr tuple<CTypes...> tuple_cat(Tuples&&... tpls);
In the following paragraphs, let be the type in Tuples, be remove_­reference_­t<T>, and be the parameter in the function parameter pack tpls, where all indexing is zero-based.
Requires: For all i, shall be the type tuple<...>, where is the (possibly empty) cv-qualifier-seq and is the parameter pack representing the element types in .
Let be the type in .
For all the following requirements shall be satisfied:
  • If is deduced as an lvalue reference type, then is_­constructible_­v<, &> == true, otherwise
  • is_­constructible_­v<, &&> == true.
Remarks: The types in CTypes shall be equal to the ordered sequence of the extended types ..., ..., , ..., where n is equal to sizeof...(Tuples).
Let ... be the ordered sequence of tuple elements of the resulting tuple object corresponding to the type sequence .
Returns: A tuple object constructed by initializing the type element in ... with
get<>(std::forward<>())
for each valid and each group in order.
[Note
:
An implementation may support additional types in the parameter pack Tuples that support the tuple-like protocol, such as pair and array.
end note
]

23.5.3.5 Calling a function with a tuple of arguments [tuple.apply]

template <class F, class Tuple> constexpr decltype(auto) apply(F&& f, Tuple&& t);
Effects: Given the exposition-only function:
template <class F, class Tuple, size_t... I>
constexpr decltype(auto)
    apply_impl(F&& f, Tuple&& t, index_sequence<I...>) {                // exposition only
  return INVOKE(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...);  // see [func.require]
}
Equivalent to:
return apply_impl(std::forward<F>(f), std::forward<Tuple>(t),
                  make_index_sequence<tuple_size_v<decay_t<Tuple>>>{});
template <class T, class Tuple> constexpr T make_from_tuple(Tuple&& t);
Effects: Given the exposition-only function:
template <class T, class Tuple, size_t... I>
constexpr T make_from_tuple_impl(Tuple&& t, index_sequence<I...>) {     // exposition only
  return T(get<I>(std::forward<Tuple>(t))...);
}
Equivalent to:
return make_from_tuple_impl<T>(forward<Tuple>(t),
                               make_index_sequence<tuple_size_v<decay_t<Tuple>>>{});
[Note
:
The type of T must be supplied as an explicit template parameter, as it cannot be deduced from the argument list.
end note
]

23.5.3.6 Tuple helper classes [tuple.helper]

template <class T> struct tuple_size;
Remarks: All specializations of tuple_­size shall meet the UnaryTypeTrait requirements with a base characteristic of integral_­constant<size_­t, N> for some N.
template <class... Types> class tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> { }; template <size_t I, class... Types> class tuple_element<I, tuple<Types...>> { public: using type = TI; };
Requires: I < sizeof...(Types).
The program is ill-formed if I is out of bounds.
Type: TI is the type of the Ith element of Types, where indexing is zero-based.
template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>;
Let TS denote tuple_­size<T> of the cv-unqualified type T.
If the expression TS​::​value is well-formed when treated as an unevaluated operand, then each of the three templates shall meet the UnaryTypeTrait requirements with a base characteristic of
integral_constant<size_t, TS::value>
Otherwise, they shall have no member value.
Access checking is performed as if in a context unrelated to TS and T.
Only the validity of the immediate context of the expression is considered.
[Note
:
The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.
Such side effects are not in the “immediate context” and can result in the program being ill-formed.
end note
]
In addition to being available via inclusion of the <tuple> header, the three templates are available when either of the headers <array> or <utility> are included.
template <size_t I, class T> class tuple_element<I, const T>; template <size_t I, class T> class tuple_element<I, volatile T>; template <size_t I, class T> class tuple_element<I, const volatile T>;
Let TE denote tuple_­element_­t<I, T> of the cv-unqualified type T.
Then each of the three templates shall meet the TransformationTrait requirements with a member typedef type that names the following type:
  • for the first specialization, add_­const_­t<TE>,
  • for the second specialization, add_­volatile_­t<TE>, and
  • for the third specialization, add_­cv_­t<TE>.
In addition to being available via inclusion of the <tuple> header, the three templates are available when either of the headers <array> or <utility> are included.

23.5.3.7 Element access [tuple.elem]

template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>& t) noexcept; template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...>>&& get(tuple<Types...>&& t) noexcept; // Note A template <size_t I, class... Types> constexpr const tuple_element_t<I, tuple<Types...>>& get(const tuple<Types...>& t) noexcept; // Note B template <size_t I, class... Types> constexpr const tuple_element_t<I, tuple<Types...>>&& get(const tuple<Types...>&& t) noexcept;
Requires: I < sizeof...(Types).
The program is ill-formed if I is out of bounds.
Returns: A reference to the Ith element of t, where indexing is zero-based.
[Note
:
[Note A] If a T in Types is some reference type X&, the return type is X&, not X&&.
However, if the element type is a non-reference type T, the return type is T&&.
end note
]
[Note
:
[Note B] Constness is shallow.
If a T in Types is some reference type X&, the return type is X&, not const X&.
However, if the element type is a non-reference type T, the return type is const T&.
This is consistent with how constness is defined to work for member variables of reference type.
end note
]
template <class T, class... Types> constexpr T& get(tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr T&& get(tuple<Types...>&& t) noexcept; template <class T, class... Types> constexpr const T& get(const tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr const T&& get(const tuple<Types...>&& t) noexcept;
Requires: The type T occurs exactly once in Types....
Otherwise, the program is ill-formed.
Returns: A reference to the element of t corresponding to the type T in Types....
[Example
:
  const tuple<int, const int, double, double> t(1, 2, 3.4, 5.6);
  const int& i1 = get<int>(t);        // OK. Not ambiguous. i1 == 1
  const int& i2 = get<const int>(t);  // OK. Not ambiguous. i2 == 2
  const double& d = get<double>(t);   // ERROR. ill-formed
end example
]
[Note
:
The reason get is a non-member function is that if this functionality had been provided as a member function, code where the type depended on a template parameter would have required using the template keyword.
end note
]

23.5.3.8 Relational operators [tuple.rel]

template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
Requires: For all i, where 0 <= i and i < sizeof...(TTypes), get<i>(t) == get<i>(u) is a valid expression returning a type that is convertible to bool.
sizeof...(TTypes) == sizeof...(UTypes).
Returns: true if get<i>(t) == get<i>(u) for all i, otherwise false.
For any two zero-length tuples e and f, e == f returns true.
Effects: The elementary comparisons are performed in order from the zeroth index upwards.
No comparisons or element accesses are performed after the first equality comparison that evaluates to false.
template<class... TTypes, class... UTypes> constexpr bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
Requires: For all i, where 0 <= i and i < sizeof...(TTypes), both get<i>(t) < get<i>(u) and get<i>(u) < get<i>(t) are valid expressions returning types that are convertible to bool.
sizeof...(TTypes) == sizeof...(UTypes).
Returns: The result of a lexicographical comparison between t and u.
The result is defined as: (bool)(get<0>(t) < get<0>(u)) || (!(bool)(get<0>(u) < get<0>(t)) && t < u), where r for some tuple r is a tuple containing all but the first element of r.
For any two zero-length tuples e and f, e < f returns false.
template<class... TTypes, class... UTypes> constexpr bool operator!=(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
Returns: !(t == u).
template<class... TTypes, class... UTypes> constexpr bool operator>(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
Returns: u < t.
template<class... TTypes, class... UTypes> constexpr bool operator<=(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
Returns: !(u < t).
template<class... TTypes, class... UTypes> constexpr bool operator>=(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
Returns: !(t < u).
[Note
:
The above definitions for comparison functions do not require t (or u) to be constructed.
It may not even be possible, as t and u are not required to be copy constructible.
Also, all comparison functions are short circuited; they do not perform element accesses beyond what is required to determine the result of the comparison.
end note
]

23.5.3.9 Tuple traits [tuple.traits]

template <class... Types, class Alloc> struct uses_allocator<tuple<Types...>, Alloc> : true_type { };
Requires: Alloc shall be an Allocator.
[Note
:
Specialization of this trait informs other library components that tuple can be constructed with an allocator, even though it does not have a nested allocator_­type.
end note
]

23.5.3.10 Tuple specialized algorithms [tuple.special]

template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);
Remarks: This function shall not participate in overload resolution unless is_­swappable_­v<> is true for all i, where .
The expression inside noexcept is equivalent to:
noexcept(x.swap(y))
Effects: As if by x.swap(y).

23.6 Optional objects [optional]

23.6.1 In general [optional.general]

This subclause describes class template optional that represents optional objects.
An optional object is an object that contains the storage for another object and manages the lifetime of this contained object, if any.
The contained object may be initialized after the optional object has been initialized, and may be destroyed before the optional object has been destroyed.
The initialization state of the contained object is tracked by the optional object.

23.6.2 Header <optional> synopsis [optional.syn]

namespace std {
  // [optional.optional], class template optional
  template <class T>
    class optional;

  // [optional.nullopt], no-value state indicator
  struct nullopt_t{see below};
  inline constexpr nullopt_t nullopt(unspecified);

  // [optional.bad.access], class bad_­optional_­access
  class bad_optional_access;

  // [optional.relops], relational operators
  template <class T, class U>
  constexpr bool operator==(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator!=(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator<(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator>(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator<=(const optional<T>&, const optional<U>&);
  template <class T, class U>
  constexpr bool operator>=(const optional<T>&, const optional<U>&);

  // [optional.nullops], comparison with nullopt
  template <class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator==(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator!=(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator!=(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator<(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator<=(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator>(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator>=(nullopt_t, const optional<T>&) noexcept;

  // [optional.comp_with_t], comparison with T
  template <class T, class U> constexpr bool operator==(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator==(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator!=(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator<(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator<(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator<=(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator>(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator>(const U&, const optional<T>&);
  template <class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
  template <class T, class U> constexpr bool operator>=(const U&, const optional<T>&);

  // [optional.specalg], specialized algorithms
  template <class T>
    void swap(optional<T>&, optional<T>&) noexcept(see below);

  template <class T>
    constexpr optional<see below> make_optional(T&&);
  template <class T, class... Args>
    constexpr optional<T> make_optional(Args&&... args);
  template <class T, class U, class... Args>
    constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);

  // [optional.hash], hash support
  template <class T> struct hash;
  template <class T> struct hash<optional<T>>;
}
A program that necessitates the instantiation of template optional for a reference type, or for possibly cv-qualified types in_­place_­t or nullopt_­t is ill-formed.

23.6.3 Class template optional [optional.optional]

template <class T>
  class optional {
  public:
    using value_type = T;

    // [optional.ctor], constructors
    constexpr optional() noexcept;
    constexpr optional(nullopt_t) noexcept;
    constexpr optional(const optional&);
    constexpr optional(optional&&) noexcept(see below);
    template <class... Args>
      constexpr explicit optional(in_place_t, Args&&...);
    template <class U, class... Args>
      constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);
    template <class U = T>
      EXPLICIT constexpr optional(U&&);
    template <class U>
      EXPLICIT optional(const optional<U>&);
    template <class U>
      EXPLICIT optional(optional<U>&&);

    // [optional.dtor], destructor
    ~optional();

    // [optional.assign], assignment
    optional& operator=(nullopt_t) noexcept;
    optional& operator=(const optional&);
    optional& operator=(optional&&) noexcept(see below);
    template <class U = T> optional& operator=(U&&);
    template <class U> optional& operator=(const optional<U>&);
    template <class U> optional& operator=(optional<U>&&);
    template <class... Args> T& emplace(Args&&...);
    template <class U, class... Args> T& emplace(initializer_list<U>, Args&&...);

    // [optional.swap], swap
    void swap(optional&) noexcept(see below);

    // [optional.observe], observers
    constexpr const T* operator->() const;
    constexpr T* operator->();
    constexpr const T& operator*() const&;
    constexpr T& operator*() &;
    constexpr T&& operator*() &&;
    constexpr const T&& operator*() const&&;
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr const T& value() const&;
    constexpr T& value() &;
    constexpr T&& value() &&;
    constexpr const T&& value() const&&;
    template <class U> constexpr T value_or(U&&) const&;
    template <class U> constexpr T value_or(U&&) &&;

    // [optional.mod], modifiers
    void reset() noexcept;

  private:
    T *val; // exposition only
  };

template<class T> optional(T) -> optional<T>;
Any instance of optional<T> at any given time either contains a value or does not contain a value.
When an instance of optional<T> contains a value, it means that an object of type T, referred to as the optional object's contained value, is allocated within the storage of the optional object.
Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.
The contained value shall be allocated in a region of the optional<T> storage suitably aligned for the type T.
When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value; otherwise the conversion returns false.
Member val is provided for exposition only.
When an optional<T> object contains a value, val points to the contained value.
T shall be an object type and shall satisfy the requirements of Destructible.

23.6.3.1 Constructors [optional.ctor]

constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept;
Postconditions: *this does not contain a value.
Remarks: No contained value is initialized.
For every object type T these constructors shall be constexpr constructors.
constexpr optional(const optional& rhs);
Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks: This constructor shall be defined as deleted unless is_­copy_­constructible_­v<T> is true.
If is_­trivially_­copy_­constructible_­v<T> is true, this constructor shall be a constexpr constructor.
constexpr optional(optional&& rhs) noexcept(see below);
Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​move(*rhs).
bool(rhs) is unchanged.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks: The expression inside noexcept is equivalent to is_­nothrow_­move_­constructible_­v<T>.
This constructor shall not participate in overload resolution unless is_­move_­constructible_­v<T> is true.
If is_­trivially_­move_­constructible_­v<T> is true, this constructor shall be a constexpr constructor.
template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std​::​forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor.
This constructor shall not participate in overload resolution unless is_­constructible_­v<T, Args...> is true.
template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std​::​forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: This constructor shall not participate in overload resolution unless is_­constructible_­v<T, initializer_­list<U>&, Args&&...> is true.
If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor.
[Note
:
The following constructors are conditionally specified as explicit.
This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution.
end note
]
template <class U = T> EXPLICIT constexpr optional(U&& v);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​forward<U>(v).
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
This constructor shall not participate in overload resolution unless is_­constructible_­v<T, U&&> is true, is_­same_­v<decay_­t<U>, in_­place_­t> is false, and is_­same_­v<optional<T>, decay_­t<U>> is false.
The constructor is explicit if and only if is_­convertible_­v<U&&, T> is false.
template <class U> EXPLICIT optional(const optional<U>& rhs);
Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks: This constructor shall not participate in overload resolution unless
  • is_­constructible_­v<T, const U&> is true,
  • is_­constructible_­v<T, optional<U>&> is false,
  • is_­constructible_­v<T, optional<U>&&> is false,
  • is_­constructible_­v<T, const optional<U>&> is false,
  • is_­constructible_­v<T, const optional<U>&&> is false,
  • is_­convertible_­v<optional<U>&, T> is false,
  • is_­convertible_­v<optional<U>&&, T> is false,
  • is_­convertible_­v<const optional<U>&, T> is false, and
  • is_­convertible_­v<const optional<U>&&, T> is false.
The constructor is explicit if and only if is_­convertible_­v<const U&, T> is false.
template <class U> EXPLICIT optional(optional<U>&& rhs);
Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression std​::​move(*rhs).
bool(rhs) is unchanged.
Postconditions: bool(rhs) == bool(*this).
Throws: Any exception thrown by the selected constructor of T.
Remarks: This constructor shall not participate in overload resolution unless
  • is_­constructible_­v<T, U&&> is true,
  • is_­constructible_­v<T, optional<U>&> is false,
  • is_­constructible_­v<T, optional<U>&&> is false,
  • is_­constructible_­v<T, const optional<U>&> is false,
  • is_­constructible_­v<T, const optional<U>&&> is false,
  • is_­convertible_­v<optional<U>&, T> is false,
  • is_­convertible_­v<optional<U>&&, T> is false,
  • is_­convertible_­v<const optional<U>&, T> is false, and
  • is_­convertible_­v<const optional<U>&&, T> is false.
The constructor is explicit if and only if is_­convertible_­v<U&&, T> is false.

23.6.3.2 Destructor [optional.dtor]

~optional();
Effects: If is_­trivially_­destructible_­v<T> != true and *this contains a value, calls
val->T::~T()
Remarks: If is_­trivially_­destructible_­v<T> == true then this destructor shall be a trivial destructor.

23.6.3.3 Assignment [optional.assign]

optional<T>& operator=(nullopt_t) noexcept;
Effects: If *this contains a value, calls val->T​::​~T() to destroy the contained value; otherwise no effect.
Returns: *this.
Postconditions: *this does not contain a value.
optional<T>& operator=(const optional& rhs);
Effects: See Table 35.
Table 35optional​::​operator=(const optional&) effects
*this contains a value
*this does not contain a value
rhs contains a value
assigns *rhs to the contained value
initializes the contained value as if direct-non-list-initializing an object of type T with *rhs
rhs does not contain a value
destroys the contained value by calling val->T​::​~T()
no effect
Returns: *this.
Postconditions: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.
If an exception is thrown during the call to T's copy constructor, no effect.
If an exception is thrown during the call to T's copy assignment, the state of its contained value is as defined by the exception safety guarantee of T's copy assignment.
This operator shall be defined as deleted unless is_­copy_­constructible_­v<T> is true and is_­copy_­assignable_­v<T> is true.
optional<T>& operator=(optional&& rhs) noexcept(see below);
Effects: See Table 36.
The result of the expression bool(rhs) remains unchanged.
Table 36optional​::​operator=(optional&&) effects
*this contains a value
*this does not contain a value
rhs contains a value
assigns std​::​move(*rhs) to the contained value
initializes the contained value as if direct-non-list-initializing an object of type T with std​::​move(*rhs)
rhs does not contain a value
destroys the contained value by calling val->T​::​~T()
no effect
Returns: *this.
Postconditions: bool(rhs) == bool(*this).
Remarks: The expression inside noexcept is equivalent to:
is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>
If any exception is thrown, the result of the expression bool(*this) remains unchanged.
If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor.
If an exception is thrown during the call to T's move assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment.
This operator shall not participate in overload resolution unless is_­move_­constructible_­v<T> is true and is_­move_­assignable_­v<T> is true.
template <class U = T> optional<T>& operator=(U&& v);
Effects: If *this contains a value, assigns std​::​forward<U>(v) to the contained value; otherwise initializes the contained value as if direct-non-list-initializing object of type T with std​::​forward<U>(v).
Returns: *this.
Postconditions: *this contains a value.
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.
If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor.
If an exception is thrown during the call to T's assignment, the state of *val and v is determined by the exception safety guarantee of T's assignment.
This function shall not participate in overload resolution unless is_­same_­v<optional<T>, decay_­t<U>> is false, conjunction_­v<is_­scalar<T>, is_­same<T, decay_­t<U>>> is false, is_­constructible_­v<T, U> is true, and is_­assignable_­v<T&, U> is true.
template <class U> optional<T>& operator=(const optional<U>& rhs);
Effects: See Table 37.
Table 37optional​::​operator=(const optional<U>&) effects
*this contains a value
*this does not contain a value
rhs contains a value
assigns *rhs to the contained value
initializes the contained value as if direct-non-list-initializing an object of type T with *rhs
rhs does not contain a value
destroys the contained value by calling val->T​::​~T()
no effect
Returns: *this.
Postconditions: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.
If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor.
If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
This function shall not participate in overload resolution unless
  • is_­constructible_­v<T, const U&> is true,
  • is_­assignable_­v<T&, const U&> is true,
  • is_­constructible_­v<T, optional<U>&> is false,
  • is_­constructible_­v<T, optional<U>&&> is false,
  • is_­constructible_­v<T, const optional<U>&> is false,
  • is_­constructible_­v<T, const optional<U>&&> is false,
  • is_­convertible_­v<optional<U>&, T> is false,
  • is_­convertible_­v<optional<U>&&, T> is false,
  • is_­convertible_­v<const optional<U>&, T> is false,
  • is_­convertible_­v<const optional<U>&&, T> is false,
  • is_­assignable_­v<T&, optional<U>&> is false,
  • is_­assignable_­v<T&, optional<U>&&> is false,
  • is_­assignable_­v<T&, const optional<U>&> is false, and
  • is_­assignable_­v<T&, const optional<U>&&> is false.
template <class U> optional<T>& operator=(optional<U>&& rhs);
Effects: See Table 38.
The result of the expression bool(rhs) remains unchanged.
Table 38optional​::​operator=(optional<U>&&) effects
*this contains a value
*this does not contain a value
rhs contains a value
assigns std​::​move(*rhs) to the contained value
initializes the contained value as if direct-non-list-initializing an object of type T with std​::​move(*rhs)
rhs does not contain a value
destroys the contained value by calling val->T​::​~T()
no effect
Returns: *this.
Postconditions: bool(rhs) == bool(*this).
Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.
If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor.
If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.
This function shall not participate in overload resolution unless
  • is_­constructible_­v<T, U> is true,
  • is_­assignable_­v<T&, U> is true,
  • is_­constructible_­v<T, optional<U>&> is false,
  • is_­constructible_­v<T, optional<U>&&> is false,
  • is_­constructible_­v<T, const optional<U>&> is false,
  • is_­constructible_­v<T, const optional<U>&&> is false,
  • is_­convertible_­v<optional<U>&, T> is false,
  • is_­convertible_­v<optional<U>&&, T> is false,
  • is_­convertible_­v<const optional<U>&, T> is false,
  • is_­convertible_­v<const optional<U>&&, T> is false,
  • is_­assignable_­v<T&, optional<U>&> is false,
  • is_­assignable_­v<T&, optional<U>&&> is false,
  • is_­assignable_­v<T&, const optional<U>&> is false, and
  • is_­assignable_­v<T&, const optional<U>&&> is false.
template <class... Args> T& emplace(Args&&... args);
Requires: is_­constructible_­v<T, Args&&...> is true.
Effects: Calls *this = nullopt.
Then initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std​::​forward<Args>(args)....
Postconditions: *this contains a value.
Returns: A reference to the new contained value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.
template <class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);
Effects: Calls *this = nullopt.
Then initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std​::​forward<Args>(args)....
Postconditions: *this contains a value.
Returns: A reference to the new contained value.
Throws: Any exception thrown by the selected constructor of T.
Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.
This function shall not participate in overload resolution unless is_­constructible_­v<T, initializer_­list<U>&, Args&&...> is true.

23.6.3.4 Swap [optional.swap]

void swap(optional& rhs) noexcept(see below);
Requires: Lvalues of type T shall be swappable and is_­move_­constructible_­v<T> is true.
Effects: See Table 39.
Table 39optional​::​swap(optional&) effects
*this contains a value
*this does not contain a value
rhs contains a value
calls swap(*(*this), *rhs)
initializes the contained value of *this as if direct-non-list-initializing an object of type T with the expression std​::​move(*rhs), followed by rhs.val->T​::​~T(); postcondition is that *this contains a value and rhs does not contain a value
rhs does not contain a value
initializes the contained value of rhs as if direct-non-list-initializing an object of type T with the expression std​::​move(*(*this)), followed by val->T​::​~T(); postcondition is that *this does not contain a value and rhs contains a value
no effect
Throws: Any exceptions thrown by the operations in the relevant part of Table 39.
Remarks: The expression inside noexcept is equivalent to:
is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>
If any exception is thrown, the results of the expressions bool(*this) and bool(rhs) remain unchanged.
If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T.
If an exception is thrown during the call to T's move constructor, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move constructor.

23.6.3.5 Observers [optional.observe]

constexpr const T* operator->() const; constexpr T* operator->();
Requires: *this contains a value.
Returns: val.
Throws: Nothing.
Remarks: These functions shall be constexpr functions.
constexpr const T& operator*() const&; constexpr T& operator*() &;
Requires: *this contains a value.
Returns: *val.
Throws: Nothing.
Remarks: These functions shall be constexpr functions.
constexpr T&& operator*() &&; constexpr const T&& operator*() const&&;
Requires: *this contains a value.
Effects: Equivalent to: return std​::​move(*val);
constexpr explicit operator bool() const noexcept;
Returns: true if and only if *this contains a value.
Remarks: This function shall be a constexpr function.
constexpr bool has_value() const noexcept;
Returns: true if and only if *this contains a value.
Remarks: This function shall be a constexpr function.
constexpr const T& value() const&; constexpr T& value() &;
Effects: Equivalent to:
return bool(*this) ? *val : throw bad_optional_access();
constexpr T&& value() &&; constexpr const T&& value() const&&;
Effects: Equivalent to:
return bool(*this) ? std::move(*val) : throw bad_optional_access();
template <class U> constexpr T value_or(U&& v) const&;
Effects: Equivalent to:
return bool(*this) ? **this : static_cast<T>(std::forward<U>(v));
Remarks: If is_­copy_­constructible_­v<T> && is_­convertible_­v<U&&, T> is false, the program is ill-formed.
template <class U> constexpr T value_or(U&& v) &&;
Effects: Equivalent to:
return bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(v));
Remarks: If is_­move_­constructible_­v<T> && is_­convertible_­v<U&&, T> is false, the program is ill-formed.

23.6.3.6 Modifiers [optional.mod]

void reset() noexcept;
Effects: If *this contains a value, calls val->T​::​~T() to destroy the contained value; otherwise no effect.
Postconditions: *this does not contain a value.

23.6.4 No-value state indicator [optional.nullopt]

struct nullopt_t{see below}; inline constexpr nullopt_t nullopt(unspecified);
The struct nullopt_­t is an empty structure type used as a unique type to indicate the state of not containing a value for optional objects.
In particular, optional<T> has a constructor with nullopt_­t as a single argument; this indicates that an optional object not containing a value shall be constructed.
Type nullopt_­t shall not have a default constructor or an initializer-list constructor, and shall not be an aggregate.

23.6.5 Class bad_­optional_­access [optional.bad.access]

class bad_optional_access : public exception {
public:
  bad_optional_access();
};
The class bad_­optional_­access defines the type of objects thrown as exceptions to report the situation where an attempt is made to access the value of an optional object that does not contain a value.
bad_optional_access();
Effects: Constructs an object of class bad_­optional_­access.
Postconditions: what() returns an implementation-defined ntbs.

23.6.6 Relational operators [optional.relops]

template <class T, class U> constexpr bool operator==(const optional<T>& x, const optional<U>& y);
Requires: The expression *x == *y shall be well-formed and its result shall be convertible to bool.
[Note
:
T need not be EqualityComparable.
end note
]
Returns: If bool(x) != bool(y), false; otherwise if bool(x) == false, true; otherwise *x == *y.
Remarks: Specializations of this function template for which *x == *y is a core constant expression shall be constexpr functions.
template <class T, class U> constexpr bool operator!=(const optional<T>& x, const optional<U>& y);
Requires: The expression *x != *y shall be well-formed and its result shall be convertible to bool.
Returns: If bool(x) != bool(y), true; otherwise, if bool(x) == false, false; otherwise *x != *y.
Remarks: Specializations of this function template for which *x != *y is a core constant expression shall be constexpr functions.
template <class T, class U> constexpr bool operator<(const optional<T>& x, const optional<U>& y);
Requires: *x < *y shall be well-formed and its result shall be convertible to bool.
Returns: If !y, false; otherwise, if !x, true; otherwise *x < *y.
Remarks: Specializations of this function template for which *x < *y is a core constant expression shall be constexpr functions.
template <class T, class U> constexpr bool operator>(const optional<T>& x, const optional<U>& y);
Requires: The expression *x > *y shall be well-formed and its result shall be convertible to bool.
Returns: If !x, false; otherwise, if !y, true; otherwise *x > *y.
Remarks: Specializations of this function template for which *x > *y is a core constant expression shall be constexpr functions.
template <class T, class U> constexpr bool operator<=(const optional<T>& x, const optional<U>& y);
Requires: The expression *x <= *y shall be well-formed and its result shall be convertible to bool.
Returns: If !x, true; otherwise, if !y, false; otherwise *x <= *y.
Remarks: Specializations of this function template for which *x <= *y is a core constant expression shall be constexpr functions.
template <class T, class U> constexpr bool operator>=(const optional<T>& x, const optional<U>& y);
Requires: The expression *x >= *y shall be well-formed and its result shall be convertible to bool.
Returns: If !y, true; otherwise, if !x, false; otherwise *x >= *y.
Remarks: Specializations of this function template for which *x >= *y is a core constant expression shall be constexpr functions.

23.6.7 Comparison with nullopt [optional.nullops]

template <class T> constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept; template <class T> constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept;
Returns: !x.
template <class T> constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept; template <class T> constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept;
Returns: bool(x).
template <class T> constexpr bool operator<(const optional<T>& x, nullopt_t) noexcept;
Returns: false.
template <class T> constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept;
Returns: bool(x).
template <class T> constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept;
Returns: !x.
template <class T> constexpr bool operator<=(nullopt_t, const optional<T>& x) noexcept;
Returns: true.
template <class T> constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept;
Returns: bool(x).
template <class T> constexpr bool operator>(nullopt_t, const optional<T>& x) noexcept;
Returns: false.
template <class T> constexpr bool operator>=(const optional<T>& x, nullopt_t) noexcept;
Returns: true.
template <class T> constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept;
Returns: !x.

23.6.8 Comparison with T [optional.comp_with_t]

template <class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);
Requires: The expression *x == v shall be well-formed and its result shall be convertible to bool.
[Note
:
T need not be EqualityComparable.
end note
]
Effects: Equivalent to: return bool(x) ? *x == v : false;
template <class T, class U> constexpr bool operator==(const U& v, const optional<T>& x);
Requires: The expression v == *x shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? v == *x : false;
template <class T, class U> constexpr bool operator!=(const optional<T>& x, const U& v);
Requires: The expression *x != v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x != v : true;
template <class T, class U> constexpr bool operator!=(const U& v, const optional<T>& x);
Requires: The expression v != *x shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? v != *x : true;
template <class T, class U> constexpr bool operator<(const optional<T>& x, const U& v);
Requires: The expression *x < v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x < v : true;
template <class T, class U> constexpr bool operator<(const U& v, const optional<T>& x);
Requires: The expression v < *x shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? v < *x : false;
template <class T, class U> constexpr bool operator<=(const optional<T>& x, const U& v);
Requires: The expression *x <= v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x <= v : true;
template <class T, class U> constexpr bool operator<=(const U& v, const optional<T>& x);
Requires: The expression v <= *x shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? v <= *x : false;
template <class T, class U> constexpr bool operator>(const optional<T>& x, const U& v);
Requires: The expression *x > v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x > v : false;
template <class T, class U> constexpr bool operator>(const U& v, const optional<T>& x);
Requires: The expression v > *x shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? v > *x : true;
template <class T, class U> constexpr bool operator>=(const optional<T>& x, const U& v);
Requires: The expression *x >= v shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? *x >= v : false;
template <class T, class U> constexpr bool operator>=(const U& v, const optional<T>& x);
Requires: The expression v >= *x shall be well-formed and its result shall be convertible to bool.
Effects: Equivalent to: return bool(x) ? v >= *x : true;

23.6.9 Specialized algorithms [optional.specalg]

template <class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));
Effects: Calls x.swap(y).
Remarks: This function shall not participate in overload resolution unless is_­move_­constructible_­v<T> is true and is_­swappable_­v<T> is true.
template <class T> constexpr optional<decay_t<T>> make_optional(T&& v);
Returns: optional<decay_­t<T>>(std​::​forward<T>(v)).
template <class T, class...Args> constexpr optional<T> make_optional(Args&&... args);
Effects: Equivalent to: return optional<T>(in_­place, std​::​forward<Args>(args)...);
template <class T, class U, class... Args> constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);
Effects: Equivalent to: return optional<T>(in_­place, il, std​::​forward<Args>(args)...);

23.6.10 Hash support [optional.hash]

template <class T> struct hash<optional<T>>;
The specialization hash<optional<T>> is enabled ([unord.hash]) if and only if hash<remove_­const_­t<T>> is enabled.
When enabled, for an object o of type optional<T>, if bool(o) == true, then hash<optional<T>>()(o) shall evaluate to the same value as hash<remove_­const_­t<T>>()(*o); otherwise it evaluates to an unspecified value.
The member functions are not guaranteed to be noexcept.

23.7 Variants [variant]

23.7.1 In general [variant.general]

A variant object holds and manages the lifetime of a value.
If the variant holds a value, that value's type has to be one of the template argument types given to variant.
These template arguments are called alternatives.

23.7.2 Header <variant> synopsis [variant.syn]

namespace std {
  // [variant.variant], class template variant
  template <class... Types>
    class variant;

  // [variant.helper], variant helper classes
  template <class T> struct variant_size;                   // not defined
  template <class T> struct variant_size<const T>;
  template <class T> struct variant_size<volatile T>;
  template <class T> struct variant_size<const volatile T>;
  template <class T>
    inline constexpr size_t variant_size_v = variant_size<T>::value;

  template <class... Types>
    struct variant_size<variant<Types...>>;

  template <size_t I, class T> struct variant_alternative;  // not defined
  template <size_t I, class T> struct variant_alternative<I, const T>;
  template <size_t I, class T> struct variant_alternative<I, volatile T>;
  template <size_t I, class T> struct variant_alternative<I, const volatile T>;
  template <size_t I, class T>
    using variant_alternative_t = typename variant_alternative<I, T>::type;

  template <size_t I, class... Types>
    struct variant_alternative<I, variant<Types...>>;

  inline constexpr size_t variant_npos = -1;

  // [variant.get], value access
  template <class T, class... Types>
    constexpr bool holds_alternative(const variant<Types...>&) noexcept;

  template <size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
  template <size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&);
  template <size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>&);
  template <size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&&);

  template <class T, class... Types>
    constexpr T& get(variant<Types...>&);
  template <class T, class... Types>
    constexpr T&& get(variant<Types...>&&);
  template <class T, class... Types>
    constexpr const T& get(const variant<Types...>&);
  template <class T, class... Types>
    constexpr const T&& get(const variant<Types...>&&);

  template <size_t I, class... Types>
    constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
      get_if(variant<Types...>*) noexcept;
  template <size_t I, class... Types>
    constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
      get_if(const variant<Types...>*) noexcept;

  template <class T, class... Types>
    constexpr add_pointer_t<T>
      get_if(variant<Types...>*) noexcept;
  template <class T, class... Types>
    constexpr add_pointer_t<const T>
      get_if(const variant<Types...>*) noexcept;

  // [variant.relops], relational operators
  template <class... Types>
    constexpr bool operator==(const variant<Types...>&, const variant<Types...>&);
  template <class... Types>
    constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&);
  template <class... Types>
    constexpr bool operator<(const variant<Types...>&, const variant<Types...>&);
  template <class... Types>
    constexpr bool operator>(const variant<Types...>&, const variant<Types...>&);
  template <class... Types>
    constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&);
  template <class... Types>
    constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&);

  // [variant.visit], visitation
  template <class Visitor, class... Variants>
    constexpr see below visit(Visitor&&, Variants&&...);

  // [variant.monostate], class monostate
  struct monostate;

  // [variant.monostate.relops], monostate relational operators
  constexpr bool operator<(monostate, monostate) noexcept;
  constexpr bool operator>(monostate, monostate) noexcept;
  constexpr bool operator<=(monostate, monostate) noexcept;
  constexpr bool operator>=(monostate, monostate) noexcept;
  constexpr bool operator==(monostate, monostate) noexcept;
  constexpr bool operator!=(monostate, monostate) noexcept;

  // [variant.specalg], specialized algorithms
  template <class... Types>
    void swap(variant<Types...>&, variant<Types...>&) noexcept(see below);

  // [variant.bad.access], class bad_­variant_­access
  class bad_variant_access;

  // [variant.hash], hash support
  template <class T> struct hash;
  template <class... Types> struct hash<variant<Types...>>;
  template <> struct hash<monostate>;
}

23.7.3 Class template variant [variant.variant]

namespace std {
  template <class... Types>
    class variant {
    public:
      // [variant.ctor], constructors
      constexpr variant() noexcept(see below);
      variant(const variant&);
      variant(variant&&) noexcept(see below);

      template <class T>
        constexpr variant(T&&) noexcept(see below);

      template <class T, class... Args>
        constexpr explicit variant(in_place_type_t<T>, Args&&...);
      template <class T, class U, class... Args>
        constexpr explicit variant(in_place_type_t<T>, initializer_list<U>, Args&&...);

      template <size_t I, class... Args>
        constexpr explicit variant(in_place_index_t<I>, Args&&...);
      template <size_t I, class U, class... Args>
        constexpr explicit variant(in_place_index_t<I>, initializer_list<U>, Args&&...);

      // [variant.dtor], destructor
      ~variant();

      // [variant.assign], assignment
      variant& operator=(const variant&);
      variant& operator=(variant&&) noexcept(see below);

      template <class T> variant& operator=(T&&) noexcept(see below);

      // [variant.mod], modifiers
      template <class T, class... Args>
        T& emplace(Args&&...);
      template <class T, class U, class... Args>
        T& emplace(initializer_list<U>, Args&&...);
      template <size_t I, class... Args>
        variant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
      template <size_t I, class U, class... Args>
        variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U>, Args&&...);

      // [variant.status], value status
      constexpr bool valueless_by_exception() const noexcept;
      constexpr size_t index() const noexcept;

      // [variant.swap], swap
      void swap(variant&) noexcept(see below);
    };
}
Any instance of variant at any given time either holds a value of one of its alternative types, or it holds no value.
When an instance of variant holds a value of alternative type T, it means that a value of type T, referred to as the variant object's contained value, is allocated within the storage of the variant object.
Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.
The contained value shall be allocated in a region of the variant storage suitably aligned for all types in Types....
It is implementation-defined whether over-aligned types are supported.
All types in Types... shall be (possibly cv-qualified) object types that are not arrays.
A program that instantiates the definition of variant with no template arguments is ill-formed.

23.7.3.1 Constructors [variant.ctor]

In the descriptions that follow, let i be in the range [0, sizeof...(Types)), and be the type in Types....
constexpr variant() noexcept(see below);
Effects: Constructs a variant holding a value-initialized value of type .
Postconditions: valueless_­by_­exception() is false and index() is 0.
Throws: Any exception thrown by the value-initialization of .
Remarks: This function shall be constexpr if and only if the value-initialization of the alternative type would satisfy the requirements for a constexpr function.
The expression inside noexcept is equivalent to is_­nothrow_­default_­constructible_­v<>.
This function shall not participate in overload resolution unless is_­default_­constructible_­v<> is true.
[Note
:
See also class monostate.
end note
]
variant(const variant& w);
Effects: If w holds a value, initializes the variant to hold the same alternative as w and direct-initializes the contained value with get<j>(w), where j is w.index().
Otherwise, initializes the variant to not hold a value.
Throws: Any exception thrown by direct-initializing any for all i.
Remarks: This function shall not participate in overload resolution unless is_­copy_­constructible_­v<> is true for all i.
variant(variant&& w) noexcept(see below);
Effects: If w holds a value, initializes the variant to hold the same alternative as w and direct-initializes the contained value with get<j>(std​::​move(w)), where j is w.index().
Otherwise, initializes the variant to not hold a value.
Throws: Any exception thrown by move-constructing any for all i.
Remarks: The expression inside noexcept is equivalent to the logical AND of is_­nothrow_­move_­constructible_­v<> for all i.
This function shall not participate in overload resolution unless is_­move_­constructible_­v<> is true for all i.
template <class T> constexpr variant(T&& t) noexcept(see below);
Let be a type that is determined as follows: build an imaginary function FUN() for each alternative type .
The overload FUN() selected by overload resolution for the expression FUN(std​::​forward<T>(​t)) defines the alternative which is the type of the contained value after construction.
Effects: Initializes *this to hold the alternative type and direct-initializes the contained value as if direct-non-list-initializing it with std​::​forward<T>(t).
Postconditions: holds_­alternative<>(*this) is true.
Throws: Any exception thrown by the initialization of the selected alternative .
Remarks: This function shall not participate in overload resolution unless sizeof...(Types) is nonzero, unless is_­same_­v<decay_­t<T>, variant> is false, unless decay_­t<T> is neither a specialization of in_­place_­type_­t nor a specialization of in_­place_­index_­t, unless is_­constructible_­v<, T> is true, and unless the expression FUN(std​::​forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well formed.
[Note
:
variant<string, string> v("abc");
is ill-formed, as both alternative types have an equally viable constructor for the argument.
end note
]
The expression inside noexcept is equivalent to is_­nothrow_­constructible_­v<, T>.
If 's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
template <class T, class... Args> constexpr explicit variant(in_place_type_t<T>, Args&&... args);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std​::​forward<Args>(args)....
Postconditions: holds_­alternative<T>(*this) is true.
Throws: Any exception thrown by calling the selected constructor of T.
Remarks: This function shall not participate in overload resolution unless there is exactly one occurrence of T in Types... and is_­constructible_­v<T, Args...> is true.
If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
template <class T, class U, class... Args> constexpr explicit variant(in_place_type_t<T>, initializer_list<U> il, Args&&... args);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std​::​forward<Args>(args)....
Postconditions: holds_­alternative<T>(*this) is true.
Throws: Any exception thrown by calling the selected constructor of T.
Remarks: This function shall not participate in overload resolution unless there is exactly one occurrence of T in Types... and is_­constructible_­v<T, initializer_­list<U>&, Args...> is true.
If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
template <size_t I, class... Args> constexpr explicit variant(in_place_index_t<I>, Args&&... args);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type with the arguments std​::​forward<Args>(args)....
Postconditions: index() is I.
Throws: Any exception thrown by calling the selected constructor of .
Remarks: This function shall not participate in overload resolution unless
  • I is less than sizeof...(Types) and
  • is_­constructible_­v<, Args...> is true.
If 's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
template <size_t I, class U, class... Args> constexpr explicit variant(in_place_index_t<I>, initializer_list<U> il, Args&&... args);
Effects: Initializes the contained value as if direct-non-list-initializing an object of type with the arguments il, std​::​forward<Args>(args)....
Postconditions: index() is I.
Remarks: This function shall not participate in overload resolution unless
  • I is less than sizeof...(Types) and
  • is_­constructible_­v<, initializer_­list<U>&, Args...> is true.
If 's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.

23.7.3.2 Destructor [variant.dtor]

~variant();
Effects: If valueless_­by_­exception() is false, destroys the currently contained value.
Remarks: If is_­trivially_­destructible_­v<> == true for all then this destructor shall be a trivial destructor.

23.7.3.3 Assignment [variant.assign]

variant& operator=(const variant& rhs);
Let j be rhs.index().
Effects:
  • If neither *this nor rhs holds a value, there is no effect.
    Otherwise,
  • if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
    Otherwise,
  • if index() == j, assigns the value contained in rhs to the value contained in *this.
    Otherwise,
  • if either is_­nothrow_­copy_­constructible_­v<> or !is_­nothrow_­move_­constructible_­v<> is true, equivalent to emplace<j>(get<j>(rhs)).
    Otherwise,
  • equivalent to operator=(variant(rhs)).
Returns: *this.
Postconditions: index() == rhs.index().
Remarks: This function shall not participate in overload resolution unless is_­copy_­constructible_­v<> && is_­copy_­assignable_­v<> is true for all i.
variant& operator=(variant&& rhs) noexcept(see below);
Let j be rhs.index().
Effects:
  • If neither *this nor rhs holds a value, there is no effect.
    Otherwise,
  • if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
    Otherwise,
  • if index() == j, assigns get<j>(std​::​move(rhs)) to the value contained in *this.
    Otherwise,
  • equivalent to emplace<j>(get<j>(std​::​move(rhs))).
Returns: *this.
Remarks: This function shall not participate in overload resolution unless is_­move_­constructible_­v<> && is_­move_­assignable_­v<> is true for all i.
The expression inside noexcept is equivalent to: is_­nothrow_­move_­constructible_­v<> && is_­nothrow_­move_­assignable_­v<> for all i.
  • If an exception is thrown during the call to 's move construction (with j being rhs.index()), the variant will hold no value.
  • If an exception is thrown during the call to 's move assignment, the state of the contained value is as defined by the exception safety guarantee of 's move assignment; index() will be j.
template <class T> variant& operator=(T&& t) noexcept(see below);
Let be a type that is determined as follows: build an imaginary function FUN() for each alternative type .
The overload FUN() selected by overload resolution for the expression FUN(std​::​forward<T>(​t)) defines the alternative which is the type of the contained value after assignment.
Effects:
  • If *this holds a , assigns std​::​forward<T>(t) to the value contained in *this.
    Otherwise,
  • if is_­nothrow_­constructible_­v<, T> || !is_­nothrow_­move_­constructible_­v<> is true, equivalent to emplace<j>(std​::​forward<T>(t)).
    Otherwise,
  • equivalent to operator=(variant(std​::​forward<T>(t))).
Postconditions: holds_­alternative<>(*this) is true, with selected by the imaginary function overload resolution described above.
Returns: *this.
Remarks: This function shall not participate in overload resolution unless is_­same_­v<decay_­t<T>, variant> is false, unless is_­assignable_­v<&, T> && is_­constructible_­v<, T> is true, and unless the expression FUN(std​::​forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well formed.
[Note
:
variant<string, string> v;
v = "abc";
is ill-formed, as both alternative types have an equally viable constructor for the argument.
end note
]
The expression inside noexcept is equivalent to:
is_nothrow_assignable_v<T&, T> && is_nothrow_constructible_v<T, T>
  • If an exception is thrown during the assignment of std​::​forward<T>(t) to the value contained in *this, the state of the contained value and t are as defined by the exception safety guarantee of the assignment expression; valueless_­by_­exception() will be false.
  • If an exception is thrown during the initialization of the contained value, the variant object might not hold a value.

23.7.3.4 Modifiers [variant.mod]

template <class T, class... Args> T& emplace(Args&&... args);
Let I be the zero-based index of T in Types....
Effects: Equivalent to: return emplace<I>(std​::​forward<Args>(args)...);
Remarks: This function shall not participate in overload resolution unless is_­constructible_­v<T, Args...> is true, and T occurs exactly once in Types....
template <class T, class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);
Let I be the zero-based index of T in Types....
Effects: Equivalent to: return emplace<I>(il, std​::​forward<Args>(args)...);
Remarks: This function shall not participate in overload resolution unless is_­constructible_­v<T, initializer_­list<U>&, Args...> is true, and T occurs exactly once in Types....
template <size_t I, class... Args> variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);
Requires: I < sizeof...(Types).
Effects: Destroys the currently contained value if valueless_­by_­exception() is false.
Then initializes the contained value as if direct-non-list-initializing a value of type with the arguments std​::​forward<Args>(args)....
Postconditions: index() is I.
Returns: A reference to the new contained value.
Throws: Any exception thrown during the initialization of the contained value.
Remarks: This function shall not participate in overload resolution unless is_­constructible_­v<, Args...> is true.
If an exception is thrown during the initialization of the contained value, the variant might not hold a value.
template <size_t I, class U, class... Args> variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);
Requires: I < sizeof...(Types).
Effects: Destroys the currently contained value if valueless_­by_­exception() is false.
Then initializes the contained value as if direct-non-list-initializing a value of type with the arguments il, std​::​forward<Args>(args)....
Postconditions: index() is I.
Returns: A reference to the new contained value.
Throws: Any exception thrown during the initialization of the contained value.
Remarks: This function shall not participate in overload resolution unless is_­constructible_­v<, initializer_­list<U>&, Args...> is true.
If an exception is thrown during the initialization of the contained value, the variant might not hold a value.

23.7.3.5 Value status [variant.status]

constexpr bool valueless_by_exception() const noexcept;
Effects: Returns false if and only if the variant holds a value.
[Note
:
A variant might not hold a value if an exception is thrown during a type-changing assignment or emplacement.
The latter means that even a variant<float, int> can become valueless_­by_­exception(), for instance by
struct S { operator int() { throw 42; }};
variant<float, int> v{12.f};
v.emplace<1>(S());
end note
]
constexpr size_t index() const noexcept;
Effects: If valueless_­by_­exception() is true, returns variant_­npos.
Otherwise, returns the zero-based index of the alternative of the contained value.

23.7.3.6 Swap [variant.swap]

void swap(variant& rhs) noexcept(see below);
Requires: Lvalues of type shall be swappable ([swappable.requirements]) and is_­move_­constructible_­v<> shall be true for all i.
Effects:
  • if valueless_­by_­exception() && rhs.valueless_­by_­exception() no effect.
    Otherwise,
  • if index() == rhs.index(), calls swap(get<i>(*this), get<i>(rhs)) where i is index().
    Otherwise,
  • exchanges values of rhs and *this.
Throws: If index() == rhs.index(), any exception thrown by swap(get<i>(*this), get<i>(rhs)) with i being index().
Otherwise, any exception thrown by the move constructor of or with i being index() and j being rhs.index().
Remarks: If an exception is thrown during the call to function swap(get<i>(*this), get<i>(rhs)), the states of the contained values of *this and of rhs are determined by the exception safety guarantee of swap for lvalues of with i being index().
If an exception is thrown during the exchange of the values of *this and rhs, the states of the values of *this and of rhs are determined by the exception safety guarantee of variant's move constructor.
The expression inside noexcept is equivalent to the logical AND of is_­nothrow_­move_­constructible_­v<> && is_­nothrow_­swappable_­v<> for all i.

23.7.4 variant helper classes [variant.helper]

template <class T> struct variant_size;
Remarks: All specializations of variant_­size shall meet the UnaryTypeTrait requirements with a base characteristic of integral_­constant<size_­t, N> for some N.
template <class T> class variant_size<const T>; template <class T> class variant_size<volatile T>; template <class T> class variant_size<const volatile T>;
Let VS denote variant_­size<T> of the cv-unqualified type T.
Then each of the three templates shall meet the UnaryTypeTrait requirements with a base characteristic of integral_­constant<size_­t, VS​::​value>.
template <class... Types> struct variant_size<variant<Types...>> : integral_constant<size_t, sizeof...(Types)> { }; template <size_t I, class T> class variant_alternative<I, const T>; template <size_t I, class T> class variant_alternative<I, volatile T>; template <size_t I, class T> class variant_alternative<I, const volatile T>;
Let VA denote variant_­alternative<I, T> of the cv-unqualified type T.
Then each of the three templates shall meet the TransformationTrait requirements with a member typedef type that names the following type:
  • for the first specialization, add_­const_­t<VA​::​type>,
  • for the second specialization, add_­volatile_­t<VA​::​type>, and
  • for the third specialization, add_­cv_­t<VA​::​type>.
variant_alternative<I, variant<Types...>>::type
Requires: I < sizeof...(Types).
The program is ill-formed if I is out of bounds.
Value: The type .

23.7.5 Value access [variant.get]

template <class T, class... Types> constexpr bool holds_alternative(const variant<Types...>& v) noexcept;
Requires: The type T occurs exactly once in Types....
Otherwise, the program is ill-formed.
Returns: true if index() is equal to the zero-based index of T in Types....
template <size_t I, class... Types> constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>& v); template <size_t I, class... Types> constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&& v); template <size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>& v); template <size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&& v);
Requires: I < sizeof...(Types).
Otherwise the program is ill-formed.
Effects: If v.index() is I, returns a reference to the object stored in the variant.
Otherwise, throws an exception of type bad_­variant_­access.
template <class T, class... Types> constexpr T& get(variant<Types...>& v); template <class T, class... Types> constexpr T&& get(variant<Types...>&& v); template <class T, class... Types> constexpr const T& get(const variant<Types...>& v); template <class T, class... Types> constexpr const T&& get(const variant<Types...>&& v);
Requires: The type T occurs exactly once in Types....
Otherwise, the program is ill-formed.
Effects: If v holds a value of type T, returns a reference to that value.
Otherwise, throws an exception of type bad_­variant_­access.
template <size_t I, class... Types> constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>> get_if(variant<Types...>* v) noexcept; template <size_t I, class... Types> constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>> get_if(const variant<Types...>* v) noexcept;
Requires: I < sizeof...(Types).
Otherwise the program is ill-formed.
Returns: A pointer to the value stored in the variant, if v != nullptr and v->index() == I.
Otherwise, returns nullptr.
template <class T, class... Types> constexpr add_pointer_t<T> get_if(variant<Types...>* v) noexcept; template <class T, class... Types> constexpr add_pointer_t<const T> get_if(const variant<Types...>* v) noexcept;
Requires: The type T occurs exactly once in Types....
Otherwise, the program is ill-formed.
Effects: Equivalent to: return get_­if<i>(v); with i being the zero-based index of T in Types....

23.7.6 Relational operators [variant.relops]

template <class... Types> constexpr bool operator==(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) == get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If v.index() != w.index(), false; otherwise if v.valueless_­by_­exception(), true; otherwise get<i>(v) == get<i>(w) with i being v.index().
template <class... Types> constexpr bool operator!=(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) != get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If v.index() != w.index(), true; otherwise if v.valueless_­by_­exception(), false; otherwise get<i>(v) != get<i>(w) with i being v.index().
template <class... Types> constexpr bool operator<(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) < get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If w.valueless_­by_­exception(), false; otherwise if v.valueless_­by_­exception(), true; otherwise, if v.index() < w.index(), true; otherwise if v.index() > w.index(), false; otherwise get<i>(v) < get<i>(w) with i being v.index().
template <class... Types> constexpr bool operator>(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) > get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If v.valueless_­by_­exception(), false; otherwise if w.valueless_­by_­exception(), true; otherwise, if v.index() > w.index(), true; otherwise if v.index() < w.index(), false; otherwise get<i>(v) > get<i>(w) with i being v.index().
template <class... Types> constexpr bool operator<=(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) <= get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If v.valueless_­by_­exception(), true; otherwise if w.valueless_­by_­exception(), false; otherwise, if v.index() < w.index(), true; otherwise if v.index() > w.index(), false; otherwise get<i>(v) <= get<i>(w) with i being v.index().
template <class... Types> constexpr bool operator>=(const variant<Types...>& v, const variant<Types...>& w);
Requires: get<i>(v) >= get<i>(w) is a valid expression returning a type that is convertible to bool, for all i.
Returns: If w.valueless_­by_­exception(), true; otherwise if v.valueless_­by_­exception(), false; otherwise, if v.index() > w.index(), true; otherwise if v.index() < w.index(), false; otherwise get<i>(v) >= get<i>(w) with i being v.index().

23.7.7 Visitation [variant.visit]

template <class Visitor, class... Variants> constexpr see below visit(Visitor&& vis, Variants&&... vars);
Requires: The expression in the Effects: element shall be a valid expression of the same type and value category, for all combinations of alternative types of all variants.
Otherwise, the program is ill-formed.
Effects: Let is... be vars.index()....
Returns INVOKE(forward<Visitor>(vis), get<is>(forward<Variants>(vars))...); ([func.require]).
Remarks: The return type is the common type of all possible INVOKE expressions of the Effects: element.
Throws: bad_­variant_­access if any variant in vars is valueless_­by_­exception().
Complexity: For sizeof...(Variants) <= 1, the invocation of the callable object is implemented in constant time, i.e. it does not depend on sizeof...(Types). For sizeof...(Variants) > 1, the invocation of the callable object has no complexity requirements.

23.7.8 Class monostate [variant.monostate]

struct monostate{};
The class monostate can serve as a first alternative type for a variant to make the variant type default constructible.

23.7.9 monostate relational operators [variant.monostate.relops]

constexpr bool operator<(monostate, monostate) noexcept { return false; } constexpr bool operator>(monostate, monostate) noexcept { return false; } constexpr bool operator<=(monostate, monostate) noexcept { return true; } constexpr bool operator>=(monostate, monostate) noexcept { return true; } constexpr bool operator==(monostate, monostate) noexcept { return true; } constexpr bool operator!=(monostate, monostate) noexcept { return false; }
[Note
:
monostate objects have only a single state; they thus always compare equal.
end note
]

23.7.10 Specialized algorithms [variant.specalg]

template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);
Effects: Equivalent to v.swap(w).
Remarks: This function shall not participate in overload resolution unless is_­move_­constructible_­v<> && is_­swappable_­v<> is true for all i.
The expression inside noexcept is equivalent to noexcept(v.swap(w)).

23.7.11 Class bad_­variant_­access [variant.bad.access]

class bad_variant_access : public exception {
public:
  bad_variant_access() noexcept;
  const char* what() const noexcept override;
};
Objects of type bad_­variant_­access are thrown to report invalid accesses to the value of a variant object.
bad_variant_access() noexcept;
Constructs a bad_­variant_­access object.
const char* what() const noexcept override;
Returns: An implementation-defined ntbs.

23.7.12 Hash support [variant.hash]

template <class... Types> struct hash<variant<Types...>>;
The specialization hash<variant<Types...>> is enabled ([unord.hash]) if and only if every specialization in hash<remove_­const_­t<Types>>... is enabled.
The member functions are not guaranteed to be noexcept.
template <> struct hash<monostate>;
The specialization is enabled ([unord.hash]).

23.8 Storage for any type [any]

This section describes components that C++ programs may use to perform operations on objects of a discriminated type.
[Note
:
The discriminated type may contain values of different types but does not attempt conversion between them, i.e. 5 is held strictly as an int and is not implicitly convertible either to "5" or to 5.0.
This indifference to interpretation but awareness of type effectively allows safe, generic containers of single values, with no scope for surprises from ambiguous conversions.
end note
]

23.8.1 Header <any> synopsis [any.synop]

namespace std {
  // [any.bad_any_cast], class bad_­any_­cast
  class bad_any_cast;

  // [any.class], class any
  class any;

  // [any.nonmembers], non-member functions
  void swap(any& x, any& y) noexcept;

  template <class T, class... Args>
    any make_any(Args&& ...args);
  template <class T, class U, class... Args>
    any make_any(initializer_list<U> il, Args&& ...args);

  template<class T>
    T any_cast(const any& operand);
  template<class T>
    T any_cast(any& operand);
  template<class T>
    T any_cast(any&& operand);

  template<class T>
    const T* any_cast(const any* operand) noexcept;
  template<class T>
    T* any_cast(any* operand) noexcept;
}

23.8.2 Class bad_­any_­cast [any.bad_any_cast]

class bad_any_cast : public bad_cast {
public:
  const char* what() const noexcept override;
};
Objects of type bad_­any_­cast are thrown by a failed any_­cast.
const char* what() const noexcept override;
Returns: An implementation-defined ntbs.
Remarks: The message may be a null-terminated multibyte string ([multibyte.strings]), suitable for conversion and display as a wstring ([string.classes], [locale.codecvt]).

23.8.3 Class any [any.class]

class any {
public:
  // [any.cons], construction and destruction
  constexpr any() noexcept;

  any(const any& other);
  any(any&& other) noexcept;

  template <class T> any(T&& value);

  template <class T, class... Args>
    explicit any(in_place_type_t<T>, Args&&...);
  template <class T, class U, class... Args>
    explicit any(in_place_type_t<T>, initializer_list<U>, Args&&...);

  ~any();

  // [any.assign], assignments
  any& operator=(const any& rhs);
  any& operator=(any&& rhs) noexcept;

  template <class T> any& operator=(T&& rhs);

  // [any.modifiers], modifiers
  template <class T, class... Args>
    decay_t<T>& emplace(Args&& ...);
  template <class T, class U, class... Args>
    decay_t<T>& emplace(initializer_list<U>, Args&&...);
  void reset() noexcept;
  void swap(any& rhs) noexcept;

  // [any.observers], observers
  bool has_value() const noexcept;
  const type_info& type() const noexcept;
};
An object of class any stores an instance of any type that satisfies the constructor requirements or it has no value, and this is referred to as the state of the class any object.
The stored instance is called the contained value, Two states are equivalent if either they both have no value, or both have a value and the contained values are equivalent.
The non-member any_­cast functions provide type-safe access to the contained value.
Implementations should avoid the use of dynamically allocated memory for a small contained value.
[Example
:
where the object constructed is holding only an int.
end example
]
Such small-object optimization shall only be applied to types T for which is_­nothrow_­move_­constructible_­v<T> is true.

23.8.3.1 Construction and destruction [any.cons]

constexpr any() noexcept;
Postconditions: has_­value() is false.
any(const any& other);
Effects: If other.has_­value() is false, constructs an object that has no value.
Otherwise, equivalent to any(in_­place<T>, any_­cast<const T&>(other)) where T is the type of the contained value.
Throws: Any exceptions arising from calling the selected constructor for the contained value.
any(any&& other) noexcept;
Effects: If other.has_­value() is false, constructs an object that has no value.
Otherwise, constructs an object of type any that contains either the contained value of other, or contains an object of the same type constructed from the contained value of other considering that contained value as an rvalue.
Postconditions: other is left in a valid but otherwise unspecified state.
template<class T> any(T&& value);
Let VT be decay_­t<T>.
Requires: VT shall satisfy the CopyConstructible requirements.
Effects: Constructs an object of type any that contains an object of type VT direct-initialized with std​::​forward<T>(value).
Remarks: This constructor shall not participate in overload resolution unless VT is not the same type as any, VT is not a specialization of in_­place_­type_­t, and is_­copy_­constructible_­v<VT> is true.
Throws: Any exception thrown by the selected constructor of VT.
template <class T, class... Args> explicit any(in_place_type_t<T>, Args&&... args);
Let VT be decay_­t<T>.
Requires: VT shall satisfy the CopyConstructible requirements.
Effects: Initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments std​::​forward<Args>(args)....
Postconditions: *this contains a value of type VT.
Throws: Any exception thrown by the selected constructor of VT.
Remarks: This constructor shall not participate in overload resolution unless is_­copy_­constructible_­v<VT> is true and is_­constructible_­v<VT, Args...> is true.
template <class T, class U, class... Args> explicit any(in_place_type_t<T>, initializer_list<U> il, Args&&... args);
Let VT be decay_­t<T>.
Requires: VT shall satisfy the CopyConstructible requirements.
Effects: Initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments il, std​::​forward<Args>(args)....
Postconditions: *this contains a value.
Throws: Any exception thrown by the selected constructor of VT.
Remarks: This constructor shall not participate in overload resolution unless is_­copy_­constructible_­v<VT> is true and is_­constructible_­v<VT, initializer_­list<U>&, Args...> is true.
~any();
Effects: As if by reset().

23.8.3.2 Assignment [any.assign]

any& operator=(const any& rhs);
Effects: As if by any(rhs).swap(*this).
No effects if an exception is thrown.
Returns: *this.
Throws: Any exceptions arising from the copy constructor for the contained value.
any& operator=(any&& rhs) noexcept;
Effects: As if by any(std​::​move(rhs)).swap(*this).
Returns: *this.
Postconditions: The state of *this is equivalent to the original state of rhs and rhs is left in a valid but otherwise unspecified state.
template<class T> any& operator=(T&& rhs);
Let VT be decay_­t<T>.
Requires: VT shall satisfy the CopyConstructible requirements.
Effects: Constructs an object tmp of type any that contains an object of type VT direct-initialized with std​::​forward<T>(rhs), and tmp.swap(*this).
No effects if an exception is thrown.
Returns: *this.
Remarks: This operator shall not participate in overload resolution unless VT is not the same type as any and is_­copy_­constructible_­v<VT> is true.
Throws: Any exception thrown by the selected constructor of VT.

23.8.3.3 Modifiers [any.modifiers]

template <class T, class... Args> decay_t<T>& emplace(Args&&... args);
Let VT be decay_­t<T>.
Requires: VT shall satisfy the CopyConstructible requirements.
Effects: Calls reset().
Then initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments std​::​forward<Args>(args)....
Postconditions: *this contains a value.
Returns: A reference to the new contained value.
Throws: Any exception thrown by the selected constructor of VT.
Remarks: If an exception is thrown during the call to VT's constructor, *this does not contain a value, and any previously contained value has been destroyed.
This function shall not participate in overload resolution unless is_­copy_­constructible_­v<VT> is true and is_­constructible_­v<VT, Args...> is true.
template <class T, class U, class... Args> decay_t<T>& emplace(initializer_list<U> il, Args&&... args);
Let VT be decay_­t<T>.
Requires: VT shall satisfy the CopyConstructible requirements.
Effects: Calls reset().
Then initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments il, std​::​forward<Args>(args)....
Postconditions: *this contains a value.
Returns: A reference to the new contained value.
Throws: Any exception thrown by the selected constructor of VT.
Remarks: If an exception is thrown during the call to VT's constructor, *this does not contain a value, and any previously contained value has been destroyed.
The function shall not participate in overload resolution unless is_­copy_­constructible_­v<VT> is true and is_­constructible_­v<VT, initializer_­list<U>&, Args...> is true.
void reset() noexcept;
Effects: If has_­value() is true, destroys the contained value.
Postconditions: has_­value() is false.
void swap(any& rhs) noexcept;
Effects: Exchanges the states of *this and rhs.

23.8.3.4 Observers [any.observers]

bool has_value() const noexcept;
Returns: true if *this contains an object, otherwise false.
const type_info& type() const noexcept;
Returns: typeid(T) if *this has a contained value of type T, otherwise typeid(void).
[Note
:
Useful for querying against types known either at compile time or only at runtime.
end note
]

23.8.4 Non-member functions [any.nonmembers]

void swap(any& x, any& y) noexcept;
Effects: As if by x.swap(y).
template <class T, class... Args> any make_any(Args&& ...args);
Effects: Equivalent to: return any(in_­place_­type<T>, std​::​forward<Args>(args)...);
template <class T, class U, class... Args> any make_any(initializer_list<U> il, Args&& ...args);
Effects: Equivalent to: return any(in_­place_­type<T>, il, std​::​forward<Args>(args)...);
template<class T> T any_cast(const any& operand); template<class T> T any_cast(any& operand); template<class T> T any_cast(any&& operand);
Let U be the type remove_­cv_­t<remove_­reference_­t<T>>.
Requires: For the first overload, is_­constructible_­v<T, const U&> is true.
For the second overload, is_­constructible_­v<T, U&> is true.
For the third overload, is_­constructible_­v<T, U> is true.
Otherwise the program is ill-formed.
Returns: For the first and second overload, static_­cast<T>(*any_­cast<U>(&operand)).
For the third overload, static_­cast<T>(std​::​move(*any_­cast<U>(&operand))).
Throws: bad_­any_­cast if operand.type() != typeid(remove_­reference_­t<T>).
[Example
:
any x(5);                                   // x holds int
assert(any_cast<int>(x) == 5);              // cast to value
any_cast<int&>(x) = 10;                     // cast to reference
assert(any_cast<int>(x) == 10);

x = "Meow";                                 // x holds const char*
assert(strcmp(any_cast<const char*>(x), "Meow") == 0);
any_cast<const char*&>(x) = "Harry";
assert(strcmp(any_cast<const char*>(x), "Harry") == 0);

x = string("Meow");                         // x holds string
string s, s2("Jane");
s = move(any_cast<string&>(x));             // move from any
assert(s == "Meow");
any_cast<string&>(x) = move(s2);            // move to any
assert(any_cast<const string&>(x) == "Jane");

string cat("Meow");
const any y(cat);                           // const y holds string
assert(any_cast<const string&>(y) == cat);

any_cast<string&>(y);                       // error; cannot
                                            // any_­cast away const
end example
]
template<class T> const T* any_cast(const any* operand) noexcept; template<class T> T* any_cast(any* operand) noexcept;
Returns: If operand != nullptr && operand->type() == typeid(T), a pointer to the object contained by operand; otherwise, nullptr.
[Example
:
bool is_string(const any& operand) {
  return any_cast<string>(&operand) != nullptr;
}
end example
]

23.9 Bitsets [bitset]

23.9.1 Header <bitset> synopsis [bitset.syn]

#include <string>
#include <iosfwd>   // for istream, ostream, see [iosfwd.syn]

namespace std {
  template <size_t N> class bitset;

  // [bitset.operators], bitset operators
  template <size_t N>
    bitset<N> operator&(const bitset<N>&, const bitset<N>&) noexcept;
  template <size_t N>
    bitset<N> operator|(const bitset<N>&, const bitset<N>&) noexcept;
  template <size_t N>
    bitset<N> operator^(const bitset<N>&, const bitset<N>&) noexcept;
  template <class charT, class traits, size_t N>
    basic_istream<charT, traits>&
      operator>>(basic_istream<charT, traits>& is, bitset<N>& x);
  template <class charT, class traits, size_t N>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& os, const bitset<N>& x);
}
The header <bitset> defines a class template and several related functions for representing and manipulating fixed-size sequences of bits.

23.9.2 Class template bitset [template.bitset]

namespace std {
  template<size_t N> class bitset {
  public:
    // bit reference:
    class reference {
      friend class bitset;
      reference() noexcept;

    public:
      ~reference() noexcept;
      reference& operator=(bool x) noexcept;            // for b[i] = x;
      reference& operator=(const reference&) noexcept;  // for b[i] = b[j];
      bool operator~() const noexcept;                  // flips the bit
      operator bool() const noexcept;                   // for x = b[i];
      reference& flip() noexcept;                       // for b[i].flip();
    };

    // [bitset.cons], constructors
    constexpr bitset() noexcept;
    constexpr bitset(unsigned long long val) noexcept;
    template<class charT, class traits, class Allocator>
      explicit bitset(
        const basic_string<charT, traits, Allocator>& str,
        typename basic_string<charT, traits, Allocator>::size_type pos = 0,
        typename basic_string<charT, traits, Allocator>::size_type n
          = basic_string<charT, traits, Allocator>::npos,
        charT zero = charT('0'),
        charT one = charT('1'));
    template <class charT>
      explicit bitset(
        const charT* str,
        typename basic_string<charT>::size_type n = basic_string<charT>::npos,
        charT zero = charT('0'),
        charT one = charT('1'));

    // [bitset.members], bitset operations
    bitset<N>& operator&=(const bitset<N>& rhs) noexcept;
    bitset<N>& operator|=(const bitset<N>& rhs) noexcept;
    bitset<N>& operator^=(const bitset<N>& rhs) noexcept;
    bitset<N>& operator<<=(size_t pos) noexcept;
    bitset<N>& operator>>=(size_t pos) noexcept;
    bitset<N>& set() noexcept;
    bitset<N>& set(size_t pos, bool val = true);
    bitset<N>& reset() noexcept;
    bitset<N>& reset(size_t pos);
    bitset<N>  operator~() const noexcept;
    bitset<N>& flip() noexcept;
    bitset<N>& flip(size_t pos);

    // element access:
    constexpr bool operator[](size_t pos) const;        // for b[i];
    reference operator[](size_t pos);                   // for b[i];

    unsigned long to_ulong() const;
    unsigned long long to_ullong() const;
    template <class charT = char,
              class traits = char_traits<charT>,
              class Allocator = allocator<charT>>
      basic_string<charT, traits, Allocator>
        to_string(charT zero = charT('0'), charT one = charT('1')) const;

    size_t count() const noexcept;
    constexpr size_t size() const noexcept;
    bool operator==(const bitset<N>& rhs) const noexcept;
    bool operator!=(const bitset<N>& rhs) const noexcept;
    bool test(size_t pos) const;
    bool all() const noexcept;
    bool any() const noexcept;
    bool none() const noexcept;
    bitset<N> operator<<(size_t pos) const noexcept;
    bitset<N> operator>>(size_t pos) const noexcept;
  };

  // [bitset.hash], hash support
  template <class T> struct hash;
  template <size_t N> struct hash<bitset<N>>;
}
The class template bitset<N> describes an object that can store a sequence consisting of a fixed number of bits, N.
Each bit represents either the value zero (reset) or one (set).
To toggle a bit is to change the value zero to one, or the value one to zero.
Each bit has a non-negative position pos.
When converting between an object of class bitset<N> and a value of some integral type, bit position pos corresponds to the bit value 1 << pos.
The integral value corresponding to two or more bits is the sum of their bit values.
The functions described in this subclause can report three kinds of errors, each associated with a distinct exception:

23.9.2.1 bitset constructors [bitset.cons]

constexpr bitset() noexcept;
Effects: Constructs an object of class bitset<N>, initializing all bits to zero.
constexpr bitset(unsigned long long val) noexcept;
Effects: Constructs an object of class bitset<N>, initializing the first M bit positions to the corresponding bit values in val.
M is the smaller of N and the number of bits in the value representation of unsigned long long.
If M < N, the remaining bit positions are initialized to zero.
template <class charT, class traits, class Allocator> explicit bitset( const basic_string<charT, traits, Allocator>& str, typename basic_string<charT, traits, Allocator>::size_type pos = 0, typename basic_string<charT, traits, Allocator>::size_type n = basic_string<charT, traits, Allocator>::npos, charT zero = charT('0'), charT one = charT('1'));
Throws: out_­of_­range if pos > str.size() or invalid_­argument if an invalid character is found (see below).
Effects: Determines the effective length rlen of the initializing string as the smaller of n and str.size() - pos.
The function then throws invalid_­argument if any of the rlen characters in str beginning at position pos is other than zero or one.
The function uses traits​::​eq() to compare the character values.
Otherwise, the function constructs an object of class bitset<N>, initializing the first M bit positions to values determined from the corresponding characters in the string str.
M is the smaller of N and rlen.
An element of the constructed object has value zero if the corresponding character in str, beginning at position pos, is zero.
Otherwise, the element has the value one.
Character position pos + M - 1 corresponds to bit position zero.
Subsequent decreasing character positions correspond to increasing bit positions.
If M < N, remaining bit positions are initialized to zero.
template <class charT> explicit bitset( const charT* str, typename basic_string<charT>::size_type n = basic_string<charT>::npos, charT zero = charT('0'), charT one = charT('1'));
Effects: Constructs an object of class bitset<N> as if by:
bitset(n == basic_string<charT>::npos
          ? basic_string<charT>(str)
          : basic_string<charT>(str, n),
       0, n, zero, one)

23.9.2.2 bitset members [bitset.members]

bitset<N>& operator&=(const bitset<N>& rhs) noexcept;
Effects: Clears each bit in *this for which the corresponding bit in rhs is clear, and leaves all other bits unchanged.
Returns: *this.
bitset<N>& operator|=(const bitset<N>& rhs) noexcept;
Effects: Sets each bit in *this for which the corresponding bit in rhs is set, and leaves all other bits unchanged.
Returns: *this.
bitset<N>& operator^=(const bitset<N>& rhs) noexcept;
Effects: Toggles each bit in *this for which the corresponding bit in rhs is set, and leaves all other bits unchanged.
Returns: *this.
bitset<N>& operator<<=(size_t pos) noexcept;
Effects: Replaces each bit at position I in *this with a value determined as follows:
  • If I < pos, the new value is zero;
  • If I >= pos, the new value is the previous value of the bit at position I - pos.
Returns: *this.
bitset<N>& operator>>=(size_t pos) noexcept;
Effects: Replaces each bit at position I in *this with a value determined as follows:
  • If pos >= N - I, the new value is zero;
  • If pos < N - I, the new value is the previous value of the bit at position I + pos.
Returns: *this.
bitset<N>& set() noexcept;
Effects: Sets all bits in *this.
Returns: *this.
bitset<N>& set(size_t pos, bool val = true);
Throws: out_­of_­range if pos does not correspond to a valid bit position.
Effects: Stores a new value in the bit at position pos in *this.
If val is true, the stored value is one, otherwise it is zero.
Returns: *this.
bitset<N>& reset() noexcept;
Effects: Resets all bits in *this.
Returns: *this.
bitset<N>& reset(size_t pos);
Throws: out_­of_­range if pos does not correspond to a valid bit position.
Effects: Resets the bit at position pos in *this.
Returns: *this.
bitset<N> operator~() const noexcept;
Effects: Constructs an object x of class bitset<N> and initializes it with *this.
Returns: x.flip().
bitset<N>& flip() noexcept;
Effects: Toggles all bits in *this.
Returns: *this.
bitset<N>& flip(size_t pos);
Throws: out_­of_­range if pos does not correspond to a valid bit position.
Effects: Toggles the bit at position pos in *this.
Returns: *this.
unsigned long to_ulong() const;
Throws: overflow_­error if the integral value x corresponding to the bits in *this cannot be represented as type unsigned long.
Returns: x.
unsigned long long to_ullong() const;
Throws: overflow_­error if the integral value x corresponding to the bits in *this cannot be represented as type unsigned long long.
Returns: x.
template <class charT = char, class traits = char_traits<charT>, class Allocator = allocator<charT>> basic_string<charT, traits, Allocator> to_string(charT zero = charT('0'), charT one = charT('1')) const;
Effects: Constructs a string object of the appropriate type and initializes it to a string of length N characters.
Each character is determined by the value of its corresponding bit position in *this.
Character position N - 1 corresponds to bit position zero.
Subsequent decreasing character positions correspond to increasing bit positions.
Bit value zero becomes the character zero, bit value one becomes the character one.
Returns: The created object.
size_t count() const noexcept;
Returns: A count of the number of bits set in *this.
constexpr size_t size() const noexcept;
Returns: N.
bool operator==(const bitset<N>& rhs) const noexcept;
Returns: true if the value of each bit in *this equals the value of the corresponding bit in rhs.
bool operator!=(const bitset<N>& rhs) const noexcept;
Returns: true if !(*this == rhs).
bool test(size_t pos) const;
Throws: out_­of_­range if pos does not correspond to a valid bit position.
Returns: true if the bit at position pos in *this has the value one.
bool all() const noexcept;
Returns: count() == size().
bool any() const noexcept;
Returns: count() != 0.
bool none() const noexcept;
Returns: count() == 0.
bitset<N> operator<<(size_t pos) const noexcept;
Returns: bitset<N>(*this) <<= pos.
bitset<N> operator>>(size_t pos) const noexcept;
Returns: bitset<N>(*this) >>= pos.
constexpr bool operator[](size_t pos) const;
Requires: pos shall be valid.
Returns: true if the bit at position pos in *this has the value one, otherwise false.
Throws: Nothing.
bitset<N>::reference operator[](size_t pos);
Requires: pos shall be valid.
Returns: An object of type bitset<N>​::​reference such that (*this)[pos] == this->test(pos), and such that (*this)[pos] = val is equivalent to this->set(pos, val).
Throws: Nothing.
Remarks: For the purpose of determining the presence of a data race, any access or update through the resulting reference potentially accesses or modifies, respectively, the entire underlying bitset.

23.9.3 bitset hash support [bitset.hash]

template <size_t N> struct hash<bitset<N>>;
The specialization is enabled ([unord.hash]).

23.9.4 bitset operators [bitset.operators]

bitset<N> operator&(const bitset<N>& lhs, const bitset<N>& rhs) noexcept;
Returns: bitset<N>(lhs) &= rhs.
bitset<N> operator|(const bitset<N>& lhs, const bitset<N>& rhs) noexcept;
Returns: bitset<N>(lhs) |= rhs.
bitset<N> operator^(const bitset<N>& lhs, const bitset<N>& rhs) noexcept;
Returns: bitset<N>(lhs) ^= rhs.
template <class charT, class traits, size_t N> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, bitset<N>& x);
Effects: Extracts up to N characters from is.
Stores these characters in a temporary object str of type basic_­string<charT, traits>, then evaluates the expression x = bitset<N>(str).
Characters are extracted and stored until any of the following occurs:
  • N characters have been extracted and stored;
  • end-of-file occurs on the input sequence;
  • the next input character is neither is.widen('0') nor is.widen('1') (in which case the input character is not extracted).
If no characters are stored in str, calls is.setstate(ios_­base​::​failbit) (which may throw ios_­base​::​failure ([iostate.flags])).
Returns: is.
template <class charT, class traits, size_t N> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const bitset<N>& x);
Returns:
os << x.template to_string<charT, traits, allocator<charT>>(
  use_facet<ctype<charT>>(os.getloc()).widen('0'),
  use_facet<ctype<charT>>(os.getloc()).widen('1'))
(see [ostream.formatted]).

23.10 Memory [memory]

23.10.1 In general [memory.general]

This subclause describes the contents of the header <memory> and some of the contents of the header <cstdlib>.

23.10.2 Header <memory> synopsis [memory.syn]

The header <memory> defines several types and function templates that describe properties of pointers and pointer-like types, manage memory for containers and other template types, destroy objects, and construct multiple objects in uninitialized memory buffers ([pointer.traits][specialized.algorithms]).
The header also defines the templates unique_­ptr, shared_­ptr, weak_­ptr, and various function templates that operate on objects of these types ([smartptr]).
namespace std {
  // [pointer.traits], pointer traits
  template <class Ptr> struct pointer_traits;
  template <class T> struct pointer_traits<T*>;

  // [util.dynamic.safety], pointer safety
  enum class pointer_safety { relaxed, preferred, strict };
  void declare_reachable(void* p);
  template <class T> T* undeclare_reachable(T* p);
  void declare_no_pointers(char* p, size_t n);
  void undeclare_no_pointers(char* p, size_t n);
  pointer_safety get_pointer_safety() noexcept;

  // [ptr.align], pointer alignment function
  void* align(size_t alignment, size_t size, void*& ptr, size_t& space);

  // [allocator.tag], allocator argument tag
  struct allocator_arg_t { explicit allocator_arg_t() = default; };
  inline constexpr allocator_arg_t allocator_arg{};

  // [allocator.uses], uses_­allocator
  template <class T, class Alloc> struct uses_allocator;

  // [allocator.traits], allocator traits
  template <class Alloc> struct allocator_traits;

  // [default.allocator], the default allocator
  template <class T> class allocator;
  template <class T, class U>
    bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
  template <class T, class U>
    bool operator!=(const allocator<T>&, const allocator<U>&) noexcept;

  // [specialized.algorithms], specialized algorithms
  template <class T> constexpr T* addressof(T& r) noexcept;
  template <class T> const T* addressof(const T&&) = delete;
  template <class ForwardIterator>
    void uninitialized_default_construct(ForwardIterator first, ForwardIterator last);
  template <class ExecutionPolicy, class ForwardIterator>
    void uninitialized_default_construct(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                         ForwardIterator first, ForwardIterator last);
  template <class ForwardIterator, class Size>
    ForwardIterator uninitialized_default_construct_n(ForwardIterator first, Size n);
  template <class ExecutionPolicy, class ForwardIterator, class Size>
    ForwardIterator uninitialized_default_construct_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                                      ForwardIterator first, Size n);
  template <class ForwardIterator>
    void uninitialized_value_construct(ForwardIterator first, ForwardIterator last);
  template <class ExecutionPolicy, class ForwardIterator>
    void uninitialized_value_construct(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                       ForwardIterator first, ForwardIterator last);
  template <class ForwardIterator, class Size>
    ForwardIterator uninitialized_value_construct_n(ForwardIterator first, Size n);
  template <class ExecutionPolicy, class ForwardIterator, class Size>
    ForwardIterator uninitialized_value_construct_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                                    ForwardIterator first, Size n);
  template <class InputIterator, class ForwardIterator>
    ForwardIterator uninitialized_copy(InputIterator first, InputIterator last,
                                       ForwardIterator result);
  template <class ExecutionPolicy, class InputIterator, class ForwardIterator>
    ForwardIterator uninitialized_copy(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                       InputIterator first, InputIterator last,
                                       ForwardIterator result);
  template <class InputIterator, class Size, class ForwardIterator>
    ForwardIterator uninitialized_copy_n(InputIterator first, Size n,
                                         ForwardIterator result);
  template <class ExecutionPolicy, class InputIterator, class Size, class ForwardIterator>
    ForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                         InputIterator first, Size n,
                                         ForwardIterator result);
  template <class InputIterator, class ForwardIterator>
    ForwardIterator uninitialized_move(InputIterator first, InputIterator last,
                                       ForwardIterator result);
  template <class ExecutionPolicy, class InputIterator, class ForwardIterator>
    ForwardIterator uninitialized_move(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                       InputIterator first, InputIterator last,
                                       ForwardIterator result);
  template <class InputIterator, class Size, class ForwardIterator>
    pair<InputIterator, ForwardIterator>
      uninitialized_move_n(InputIterator first, Size n, ForwardIterator result);
  template <class ExecutionPolicy, class InputIterator, class Size, class ForwardIterator>
    pair<InputIterator, ForwardIterator>
      uninitialized_move_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                           InputIterator first, Size n, ForwardIterator result);
  template <class ForwardIterator, class T>
    void uninitialized_fill(ForwardIterator first, ForwardIterator last,
                            const T& x);
  template <class ExecutionPolicy, class ForwardIterator, class T>
    void uninitialized_fill(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                            ForwardIterator first, ForwardIterator last,
                            const T& x);
  template <class ForwardIterator, class Size, class T>
    ForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);
  template <class ExecutionPolicy, class ForwardIterator, class Size, class T>
    ForwardIterator uninitialized_fill_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                         ForwardIterator first, Size n, const T& x);
  template <class T>
    void destroy_at(T* location);
  template <class ForwardIterator>
    void destroy(ForwardIterator first, ForwardIterator last);
  template <class ExecutionPolicy, class ForwardIterator>
    void destroy(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                 ForwardIterator first, ForwardIterator last);
  template <class ForwardIterator, class Size>
    ForwardIterator destroy_n(ForwardIterator first, Size n);
  template <class ExecutionPolicy, class ForwardIterator, class Size>
    ForwardIterator destroy_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                              ForwardIterator first, Size n);

  // [unique.ptr], class template unique_­ptr
  template <class T> struct default_delete;
  template <class T> struct default_delete<T[]>;
  template <class T, class D = default_delete<T>> class unique_ptr;
  template <class T, class D> class unique_ptr<T[], D>;

  template <class T, class... Args> unique_ptr<T> make_unique(Args&&... args);
  template <class T> unique_ptr<T> make_unique(size_t n);
  template <class T, class... Args> unspecified make_unique(Args&&...) = delete;

  template <class T, class D>
    void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;

  template <class T1, class D1, class T2, class D2>
    bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template <class T1, class D1, class T2, class D2>
    bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template <class T1, class D1, class T2, class D2>
    bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template <class T1, class D1, class T2, class D2>
    bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template <class T1, class D1, class T2, class D2>
    bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template <class T1, class D1, class T2, class D2>
    bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

  template <class T, class D>
    bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
  template <class T, class D>
    bool operator==(nullptr_t, const unique_ptr<T, D>& y) noexcept;
  template <class T, class D>
    bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
  template <class T, class D>
    bool operator!=(nullptr_t, const unique_ptr<T, D>& y) noexcept;
  template <class T, class D>
    bool operator<(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator<(nullptr_t, const unique_ptr<T, D>& y);
  template <class T, class D>
    bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator<=(nullptr_t, const unique_ptr<T, D>& y);
  template <class T, class D>
    bool operator>(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator>(nullptr_t, const unique_ptr<T, D>& y);
  template <class T, class D>
    bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator>=(nullptr_t, const unique_ptr<T, D>& y);

  // [util.smartptr.weak.bad], class bad_­weak_­ptr
  class bad_weak_ptr;

  // [util.smartptr.shared], class template shared_­ptr
  template<class T> class shared_ptr;

  // [util.smartptr.shared.create], shared_­ptr creation
  template<class T, class... Args>
    shared_ptr<T> make_shared(Args&&... args);                                  // T is not array
  template<class T, class A, class... Args>
    shared_ptr<T> allocate_shared(const A& a, Args&&... args);                  // T is not array

  template<class T>
    shared_ptr<T> make_shared(size_t N);                                        // T is U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, size_t N);                        // T is U[]

  template<class T>
    shared_ptr<T> make_shared();                                                // T is U[N]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a);                                  // T is U[N]

  template<class T>
    shared_ptr<T> make_shared(size_t N, const remove_extent_t<T>& u);           // T is U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, size_t N,
                                  const remove_extent_t<T>& u);                 // T is U[]

  template<class T> shared_ptr<T>
    make_shared(const remove_extent_t<T>& u);                                   // T is U[N]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, const remove_extent_t<T>& u);     // T is U[N]

  // [util.smartptr.shared.cmp], shared_­ptr comparisons
  template<class T, class U>
    bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

  template <class T>
    bool operator==(const shared_ptr<T>& x, nullptr_t) noexcept;
  template <class T>
    bool operator==(nullptr_t, const shared_ptr<T>& y) noexcept;
  template <class T>
    bool operator!=(const shared_ptr<T>& x, nullptr_t) noexcept;
  template <class T>
    bool operator!=(nullptr_t, const shared_ptr<T>& y) noexcept;
  template <class T>
    bool operator<(const shared_ptr<T>& x, nullptr_t) noexcept;
  template <class T>
    bool operator<(nullptr_t, const shared_ptr<T>& y) noexcept;
  template <class T>
    bool operator<=(const shared_ptr<T>& x, nullptr_t) noexcept;
  template <class T>
    bool operator<=(nullptr_t, const shared_ptr<T>& y) noexcept;
  template <class T>
    bool operator>(const shared_ptr<T>& x, nullptr_t) noexcept;
  template <class T>
    bool operator>(nullptr_t, const shared_ptr<T>& y) noexcept;
  template <class T>
    bool operator>=(const shared_ptr<T>& x, nullptr_t) noexcept;
  template <class T>
    bool operator>=(nullptr_t, const shared_ptr<T>& y) noexcept;

  // [util.smartptr.shared.spec], shared_­ptr specialized algorithms
  template<class T>
    void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

  // [util.smartptr.shared.cast], shared_­ptr casts
  template<class T, class U>
    shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;

  // [util.smartptr.getdeleter], shared_­ptr get_­deleter
  template<class D, class T>
    D* get_deleter(const shared_ptr<T>& p) noexcept;

  // [util.smartptr.shared.io], shared_­ptr I/O
  template<class E, class T, class Y>
    basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const shared_ptr<Y>& p);

  // [util.smartptr.weak], class template weak_­ptr
  template<class T> class weak_ptr;

  // [util.smartptr.weak.spec], weak_­ptr specialized algorithms
  template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;

  // [util.smartptr.ownerless], class template owner_­less
  template<class T = void> struct owner_less;

  // [util.smartptr.enab], class template enable_­shared_­from_­this
  template<class T> class enable_shared_from_this;

  // [util.smartptr.shared.atomic], shared_­ptr atomic access
  template<class T>
    bool atomic_is_lock_free(const shared_ptr<T>* p);

  template<class T>
    shared_ptr<T> atomic_load(const shared_ptr<T>* p);
  template<class T>
    shared_ptr<T> atomic_load_explicit(const shared_ptr<T>* p, memory_order mo);

  template<class T>
    void atomic_store(shared_ptr<T>* p, shared_ptr<T> r);
  template<class T>
    void atomic_store_explicit(shared_ptr<T>* p, shared_ptr<T> r, memory_order mo);

  template<class T>
    shared_ptr<T> atomic_exchange(shared_ptr<T>* p, shared_ptr<T> r);
  template<class T>
    shared_ptr<T> atomic_exchange_explicit(shared_ptr<T>* p, shared_ptr<T> r, memory_order mo);

  template<class T>
    bool atomic_compare_exchange_weak(
      shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);
  template<class T>
    bool atomic_compare_exchange_strong(
      shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);
  template<class T>
    bool atomic_compare_exchange_weak_explicit(
      shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
      memory_order success, memory_order failure);
  template<class T>
    bool atomic_compare_exchange_strong_explicit(
      shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
      memory_order success, memory_order failure);

  // [util.smartptr.hash], hash support
  template <class T> struct hash;
  template <class T, class D> struct hash<unique_ptr<T, D>>;
  template <class T> struct hash<shared_ptr<T>>;

  // [allocator.uses.trait], uses_­allocator
  template <class T, class Alloc>
    inline constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;
}

23.10.3 Pointer traits [pointer.traits]

The class template pointer_­traits supplies a uniform interface to certain attributes of pointer-like types.
namespace std {
  template <class Ptr> struct pointer_traits {
    using pointer         = Ptr;
    using element_type    = see below;
    using difference_type = see below;

    template <class U> using rebind = see below;

    static pointer pointer_to(see below r);
  };

  template <class T> struct pointer_traits<T*> {
    using pointer         = T*;
    using element_type    = T;
    using difference_type = ptrdiff_t;

    template <class U> using rebind = U*;

    static pointer pointer_to(see below r) noexcept;
  };
}

23.10.3.1 Pointer traits member types [pointer.traits.types]

using element_type = see below;
Type: Ptr​::​element_­type if the qualified-id Ptr​::​element_­type is valid and denotes a type ([temp.deduct]); otherwise, T if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the specialization is ill-formed.
using difference_type = see below;
Type: Ptr​::​difference_­type if the qualified-id Ptr​::​difference_­type is valid and denotes a type ([temp.deduct]); otherwise, ptrdiff_­t.
template <class U> using rebind = see below;
Alias template: Ptr​::​rebind<U> if the qualified-id Ptr​::​rebind<U> is valid and denotes a type ([temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.

23.10.3.2 Pointer traits member functions [pointer.traits.functions]

static pointer pointer_traits::pointer_to(see below r); static pointer pointer_traits<T*>::pointer_to(see below r) noexcept;
Remarks: If element_­type is cv void, the type of r is unspecified; otherwise, it is element_­type&.
Returns: The first member function returns a pointer to r obtained by calling Ptr​::​pointer_­to(r) through which indirection is valid; an instantiation of this function is ill-formed if Ptr does not have a matching pointer_­to static member function.
The second member function returns addressof(r).

23.10.4 Pointer safety [util.dynamic.safety]

A complete object is declared reachable while the number of calls to declare_­reachable with an argument referencing the object exceeds the number of calls to undeclare_­reachable with an argument referencing the object.
void declare_reachable(void* p);
Requires: p shall be a safely-derived pointer or a null pointer value.
Effects: If p is not null, the complete object referenced by p is subsequently declared reachable ([basic.stc.dynamic.safety]).
Throws: May throw bad_­alloc if the system cannot allocate additional memory that may be required to track objects declared reachable.
template <class T> T* undeclare_reachable(T* p);
Requires: If p is not null, the complete object referenced by p shall have been previously declared reachable, and shall be live ([basic.life]) from the time of the call until the last undeclare_­reachable(p) call on the object.
Returns: A safely derived copy of p which shall compare equal to p.
Throws: Nothing.
[Note
:
It is expected that calls to declare_­reachable(p) will consume a small amount of memory in addition to that occupied by the referenced object until the matching call to undeclare_­reachable(p) is encountered.
Long running programs should arrange that calls are matched.
end note
]
void declare_no_pointers(char* p, size_t n);
Requires: No bytes in the specified range are currently registered with declare_­no_­pointers().
If the specified range is in an allocated object, then it must be entirely within a single allocated object.
The object must be live until the corresponding undeclare_­no_­pointers() call.
[Note
:
In a garbage-collecting implementation, the fact that a region in an object is registered with declare_­no_­pointers() should not prevent the object from being collected.
end note
]
Effects: The n bytes starting at p no longer contain traceable pointer locations, independent of their type.
Hence indirection through a pointer located there is undefined if the object it points to was created by global operator new and not previously declared reachable.
[Note
:
This may be used to inform a garbage collector or leak detector that this region of memory need not be traced.
end note
]
Throws: Nothing.
[Note
:
Under some conditions implementations may need to allocate memory.
However, the request can be ignored if memory allocation fails.
end note
]
void undeclare_no_pointers(char* p, size_t n);
Requires: The same range must previously have been passed to declare_­no_­pointers().
Effects: Unregisters a range registered with declare_­no_­pointers() for destruction.
It must be called before the lifetime of the object ends.
Throws: Nothing.
pointer_safety get_pointer_safety() noexcept;
Returns: pointer_­safety​::​strict if the implementation has strict pointer safety.
It is implementation-defined whether get_­pointer_­safety returns pointer_­safety​::​relaxed or pointer_­safety​::​preferred if the implementation has relaxed pointer safety.222
pointer_­safety​::​preferred might be returned to indicate that a leak detector is running so that the program can avoid spurious leak reports.

23.10.5 Align [ptr.align]

void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
Effects: If it is possible to fit size bytes of storage aligned by alignment into the buffer pointed to by ptr with length space, the function updates ptr to represent the first possible address of such storage and decreases space by the number of bytes used for alignment.
Otherwise, the function does nothing.
Requires:
  • alignment shall be a power of two
  • ptr shall represent the address of contiguous storage of at least space bytes
Returns: A null pointer if the requested aligned buffer would not fit into the available space, otherwise the adjusted value of ptr.
[Note
:
The function updates its ptr and space arguments so that it can be called repeatedly with possibly different alignment and size arguments for the same buffer.
end note
]

23.10.6 Allocator argument tag [allocator.tag]

namespace std { struct allocator_arg_t { explicit allocator_arg_t() = default; }; inline constexpr allocator_arg_t allocator_arg{}; }
The allocator_­arg_­t struct is an empty structure type used as a unique type to disambiguate constructor and function overloading.
Specifically, several types (see tuple [tuple]) have constructors with allocator_­arg_­t as the first argument, immediately followed by an argument of a type that satisfies the Allocator requirements.

23.10.7 uses_­allocator [allocator.uses]

23.10.7.1 uses_­allocator trait [allocator.uses.trait]

template <class T, class Alloc> struct uses_allocator;
Remarks: Automatically detects whether T has a nested allocator_­type that is convertible from Alloc.
Meets the BinaryTypeTrait requirements.
The implementation shall provide a definition that is derived from true_­type if the qualified-id T​::​allocator_­type is valid and denotes a type ([temp.deduct]) and is_­convertible_­v<Alloc, T​::​allocator_­type> != false, otherwise it shall be derived from false_­type.
A program may specialize this template to derive from true_­type for a user-defined type T that does not have a nested allocator_­type but nonetheless can be constructed with an allocator where either:
  • the first argument of a constructor has type allocator_­arg_­t and the second argument has type Alloc or
  • the last argument of a constructor has type Alloc.

23.10.7.2 Uses-allocator construction [allocator.uses.construction]

Uses-allocator construction with allocator Alloc refers to the construction of an object obj of type T, using constructor arguments v1, v2, ..., vN of types V1, V2, ..., VN, respectively, and an allocator alloc of type Alloc, according to the following rules:
  • if uses_­allocator_­v<T, Alloc> is false and is_­constructible_­v<T, V1, V2, ..., VN> is true, then obj is initialized as obj(v1, v2, ..., vN);
  • otherwise, if uses_­allocator_­v<T, Alloc> is true and is_­constructible_­v<T, allocator_­arg_­t, Alloc, V1, V2, ..., VN> is true, then obj is initialized as obj(allocator_­arg, alloc, v1, v2, ..., vN);
  • otherwise, if uses_­allocator_­v<T, Alloc> is true and is_­constructible_­v<T, V1, V2, ..., VN, Alloc> is true, then obj is initialized as obj(v1, v2, ..., vN, alloc);
  • otherwise, the request for uses-allocator construction is ill-formed.
    [Note
    :
    An error will result if uses_­allocator_­v<T, Alloc> is true but the specific constructor does not take an allocator.
    This definition prevents a silent failure to pass the allocator to an element.
    end note
    ]

23.10.8 Allocator traits [allocator.traits]

The class template allocator_­traits supplies a uniform interface to all allocator types.
An allocator cannot be a non-class type, however, even if allocator_­traits supplies the entire required interface.
[Note
:
Thus, it is always possible to create a derived class from an allocator.
end note
]
namespace std {
  template <class Alloc> struct allocator_traits {
    using allocator_type     = Alloc;

    using value_type         = typename Alloc::value_type;

    using pointer            = see below;
    using const_pointer      = see below;
    using void_pointer       = see below;
    using const_void_pointer = see below;

    using difference_type    = see below;
    using size_type          = see below;

    using propagate_on_container_copy_assignment = see below;
    using propagate_on_container_move_assignment = see below;
    using propagate_on_container_swap            = see below;
    using is_always_equal                        = see below;

    template <class T> using rebind_alloc = see below;
    template <class T> using rebind_traits = allocator_traits<rebind_alloc<T>>;

    static pointer allocate(Alloc& a, size_type n);
    static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);

    static void deallocate(Alloc& a, pointer p, size_type n);

    template <class T, class... Args>
      static void construct(Alloc& a, T* p, Args&&... args);

    template <class T>
      static void destroy(Alloc& a, T* p);

    static size_type max_size(const Alloc& a) noexcept;

    static Alloc select_on_container_copy_construction(const Alloc& rhs);
  };
}

23.10.8.1 Allocator traits member types [allocator.traits.types]

using pointer = see below;
Type: Alloc​::​pointer if the qualified-id Alloc​::​pointer is valid and denotes a type ([temp.deduct]); otherwise, value_­type*.
using const_pointer = see below;
Type: Alloc​::​const_­pointer if the qualified-id Alloc​::​const_­pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​rebind<​const value_­type>.
using void_pointer = see below;
Type: Alloc​::​void_­pointer if the qualified-id Alloc​::​void_­pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​rebind<​void>.
using const_void_pointer = see below;
Type: Alloc​::​const_­void_­pointer if the qualified-id Alloc​::​const_­void_­pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​​rebind<const void>.
using difference_type = see below;
Type: Alloc​::​difference_­type if the qualified-id Alloc​::​difference_­type is valid and denotes a type ([temp.deduct]); otherwise, pointer_­traits<pointer>​::​difference_­type.
using size_type = see below;
Type: Alloc​::​size_­type if the qualified-id Alloc​::​size_­type is valid and denotes a type ([temp.deduct]); otherwise, make_­unsigned_­t<difference_­type>.
using propagate_on_container_copy_assignment = see below;
Type: Alloc​::​propagate_­on_­container_­copy_­assignment if the qualified-id Alloc​::​propagate_­on_­container_­copy_­assignment is valid and denotes a type ([temp.deduct]); otherwise false_­type.
using propagate_on_container_move_assignment = see below;
Type: Alloc​::​propagate_­on_­container_­move_­assignment if the qualified-id Alloc​::​propagate_­on_­container_­move_­assignment is valid and denotes a type ([temp.deduct]); otherwise false_­type.
using propagate_on_container_swap = see below;
Type: Alloc​::​propagate_­on_­container_­swap if the qualified-id Alloc​::​propagate_­on_­container_­swap is valid and denotes a type ([temp.deduct]); otherwise false_­type.
using is_always_equal = see below;
Type: Alloc​::​is_­always_­equal if the qualified-id Alloc​::​is_­always_­equal is valid and denotes a type ([temp.deduct]); otherwise is_­empty<Alloc>​::​type.
template <class T> using rebind_alloc = see below;
Alias template: Alloc​::​rebind<T>​::​other if the qualified-id Alloc​::​rebind<T>​::​other is valid and denotes a type ([temp.deduct]); otherwise, Alloc<T, Args> if Alloc is a class template instantiation of the form Alloc<U, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind_­alloc is ill-formed.

23.10.8.2 Allocator traits static member functions [allocator.traits.members]

static pointer allocate(Alloc& a, size_type n);
Returns: a.allocate(n).
static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);
Returns: a.allocate(n, hint) if that expression is well-formed; otherwise, a.allocate(n).
static void deallocate(Alloc& a, pointer p, size_type n);
Effects: Calls a.deallocate(p, n).
Throws: Nothing.
template <class T, class... Args> static void construct(Alloc& a, T* p, Args&&... args);
Effects: Calls a.construct(p, std​::​forward<Args>(args)...) if that call is well-formed; otherwise, invokes ​::​new (static_­cast<void*>(p)) T(std​::​forward<Args>(args)...).
template <class T> static void destroy(Alloc& a, T* p);
Effects: Calls a.destroy(p) if that call is well-formed; otherwise, invokes p->~T().
static size_type max_size(const Alloc& a) noexcept;
Returns: a.max_­size() if that expression is well-formed; otherwise, numeric_­limits<size_­type>​::​​max()/sizeof(value_­type).
static Alloc select_on_container_copy_construction(const Alloc& rhs);
Returns: rhs.select_­on_­container_­copy_­construction() if that expression is well-formed; otherwise, rhs.

23.10.9 The default allocator [default.allocator]

All specializations of the default allocator satisfy the allocator completeness requirements ([allocator.requirements.completeness]).
namespace std {
  template <class T> class allocator {
   public:
    using value_type      = T;
    using propagate_on_container_move_assignment = true_type;
    using is_always_equal = true_type;

    allocator() noexcept;
    allocator(const allocator&) noexcept;
    template <class U> allocator(const allocator<U>&) noexcept;
    ~allocator();

    T* allocate(size_t n);
    void deallocate(T* p, size_t n);
  };
}

23.10.9.1 allocator members [allocator.members]

Except for the destructor, member functions of the default allocator shall not introduce data races as a result of concurrent calls to those member functions from different threads.
Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.
T* allocate(size_t n);
Returns: A pointer to the initial element of an array of storage of size n * sizeof(T), aligned appropriately for objects of type T.
Remarks: the storage is obtained by calling ​::​operator new, but it is unspecified when or how often this function is called.
Throws: bad_­alloc if the storage cannot be obtained.
void deallocate(T* p, size_t n);
Requires: p shall be a pointer value obtained from allocate().
n shall equal the value passed as the first argument to the invocation of allocate which returned p.
Effects: Deallocates the storage referenced by p .
Remarks: Uses ​::​operator delete, but it is unspecified when this function is called.

23.10.9.2 allocator globals [allocator.globals]

template <class T, class U> bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
Returns: true.
template <class T, class U> bool operator!=(const allocator<T>&, const allocator<U>&) noexcept;
Returns: false.

23.10.10 Specialized algorithms [specialized.algorithms]

Throughout this subclause, the names of template parameters are used to express type requirements.
  • If an algorithm's template parameter is named InputIterator, the template argument shall satisfy the requirements of an input iterator.
  • If an algorithm's template parameter is named ForwardIterator, the template argument shall satisfy the requirements of a forward iterator, and is required to have the property that no exceptions are thrown from increment, assignment, comparison, or indirection through valid iterators.
Unless otherwise specified, if an exception is thrown in the following algorithms there are no effects.

23.10.10.1 addressof [specialized.addressof]

template <class T> constexpr T* addressof(T& r) noexcept;
Returns: The actual address of the object or function referenced by r, even in the presence of an overloaded operator&.
Remarks: An expression addressof(E) is a constant subexpression if E is an lvalue constant subexpression.

23.10.10.2 uninitialized_­default_­construct [uninitialized.construct.default]

template <class ForwardIterator> void uninitialized_default_construct(ForwardIterator first, ForwardIterator last);
Effects: Equivalent to:
for (; first != last; ++first)
  ::new (static_cast<void*>(addressof(*first)))
    typename iterator_traits<ForwardIterator>::value_type;
template <class ForwardIterator, class Size> ForwardIterator uninitialized_default_construct_n(ForwardIterator first, Size n);
Effects: Equivalent to:
for (; n>0; (void)++first, --n)
  ::new (static_cast<void*>(addressof(*first)))
    typename iterator_traits<ForwardIterator>::value_type;
return first;

23.10.10.3 uninitialized_­value_­construct [uninitialized.construct.value]

template <class ForwardIterator> void uninitialized_value_construct(ForwardIterator first, ForwardIterator last);
Effects: Equivalent to:
for (; first != last; ++first)
  ::new (static_cast<void*>(addressof(*first)))
    typename iterator_traits<ForwardIterator>::value_type();
template <class ForwardIterator, class Size> ForwardIterator uninitialized_value_construct_n(ForwardIterator first, Size n);
Effects: Equivalent to:
for (; n>0; (void)++first, --n)
  ::new (static_cast<void*>(addressof(*first)))
    typename iterator_traits<ForwardIterator>::value_type();
return first;

23.10.10.4 uninitialized_­copy [uninitialized.copy]

template <class InputIterator, class ForwardIterator> ForwardIterator uninitialized_copy(InputIterator first, InputIterator last, ForwardIterator result);
Effects: As if by:
for (; first != last; ++result, (void) ++first)
  ::new (static_cast<void*>(addressof(*result)))
    typename iterator_traits<ForwardIterator>::value_type(*first);
Returns: result.
template <class InputIterator, class Size, class ForwardIterator> ForwardIterator uninitialized_copy_n(InputIterator first, Size n, ForwardIterator result);
Effects: As if by:
for ( ; n > 0; ++result, (void) ++first, --n) {
  ::new (static_cast<void*>(addressof(*result)))
    typename iterator_traits<ForwardIterator>::value_type(*first);
}
Returns: result.

23.10.10.5 uninitialized_­move [uninitialized.move]

template <class InputIterator, class ForwardIterator> ForwardIterator uninitialized_move(InputIterator first, InputIterator last, ForwardIterator result);
Effects: Equivalent to:
for (; first != last; (void)++result, ++first)
  ::new (static_cast<void*>(addressof(*result)))
    typename iterator_traits<ForwardIterator>::value_type(std::move(*first));
return result;
Remarks: If an exception is thrown, some objects in the range [first, last) are left in a valid but unspecified state.
template <class InputIterator, class Size, class ForwardIterator> pair<InputIterator, ForwardIterator> uninitialized_move_n(InputIterator first, Size n, ForwardIterator result);
Effects: Equivalent to:
for (; n > 0; ++result, (void) ++first, --n)
  ::new (static_cast<void*>(addressof(*result)))
    typename iterator_traits<ForwardIterator>::value_type(std::move(*first));
return {first,result};
Remarks: If an exception is thrown, some objects in the range [first, std​::​next(first,n)) are left in a valid but unspecified state.

23.10.10.6 uninitialized_­fill [uninitialized.fill]

template <class ForwardIterator, class T> void uninitialized_fill(ForwardIterator first, ForwardIterator last, const T& x);
Effects: As if by:
for (; first != last; ++first)
  ::new (static_cast<void*>(addressof(*first)))
    typename iterator_traits<ForwardIterator>::value_type(x);
template <class ForwardIterator, class Size, class T> ForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);
Effects: As if by:
for (; n--; ++first)
  ::new (static_cast<void*>(addressof(*first)))
    typename iterator_traits<ForwardIterator>::value_type(x);
return first;

23.10.10.7 destroy [specialized.destroy]

template <class T> void destroy_at(T* location);
Effects: Equivalent to:
location->~T();
template <class ForwardIterator> void destroy(ForwardIterator first, ForwardIterator last);
Effects: Equivalent to:
for (; first!=last; ++first)
  destroy_at(addressof(*first));
template <class ForwardIterator, class Size> ForwardIterator destroy_n(ForwardIterator first, Size n);
Effects: Equivalent to:
for (; n > 0; (void)++first, --n)
  destroy_at(addressof(*first));
return first;

23.10.11 C library memory allocation [c.malloc]

[Note
:
The header <cstdlib> declares the functions described in this subclause.
end note
]
void* aligned_­alloc(size_t alignment, size_t size); void* calloc(size_t nmemb, size_t size); void* malloc(size_t size); void* realloc(void* ptr, size_t size);
Effects: These functions have the semantics specified in the C standard library.
Remarks: These functions do not attempt to allocate storage by calling ​::​operator new() ([support.dynamic]).
Storage allocated directly with these functions is implicitly declared reachable (see [basic.stc.dynamic.safety]) on allocation, ceases to be declared reachable on deallocation, and need not cease to be declared reachable as the result of an undeclare_­reachable() call.
[Note
:
This allows existing C libraries to remain unaffected by restrictions on pointers that are not safely derived, at the expense of providing far fewer garbage collection and leak detection options for malloc()-allocated objects.
It also allows malloc() to be implemented with a separate allocation arena, bypassing the normal declare_­reachable() implementation.
The above functions should never intentionally be used as a replacement for declare_­reachable(), and newly written code is strongly encouraged to treat memory allocated with these functions as though it were allocated with operator new.
end note
]
void free(void* ptr);
Effects: This function has the semantics specified in the C standard library.
Remarks: This function does not attempt to deallocate storage by calling ​::​operator delete().
See also: ISO C 7.22.3

23.11 Smart pointers [smartptr]

23.11.1 Class template unique_­ptr [unique.ptr]

A unique pointer is an object that owns another object and manages that other object through a pointer.
More precisely, a unique pointer is an object u that stores a pointer to a second object p and will dispose of p when u is itself destroyed (e.g., when leaving block scope ([stmt.dcl])).
In this context, u is said to own p.
The mechanism by which u disposes of p is known as p's associated deleter, a function object whose correct invocation results in p's appropriate disposition (typically its deletion).
Let the notation u.p denote the pointer stored by u, and let u.d denote the associated deleter.
Upon request, u can reset (replace) u.p and u.d with another pointer and deleter, but must properly dispose of its owned object via the associated deleter before such replacement is considered completed.
Additionally, u can, upon request, transfer ownership to another unique pointer u2.
Upon completion of such a transfer, the following postconditions hold:
  • u2.p is equal to the pre-transfer u.p,
  • u.p is equal to nullptr, and
  • if the pre-transfer u.d maintained state, such state has been transferred to u2.d.
As in the case of a reset, u2 must properly dispose of its pre-transfer owned object via the pre-transfer associated deleter before the ownership transfer is considered complete.
[Note
:
A deleter's state need never be copied, only moved or swapped as ownership is transferred.
end note
]
Each object of a type U instantiated from the unique_­ptr template specified in this subclause has the strict ownership semantics, specified above, of a unique pointer.
In partial satisfaction of these semantics, each such U is MoveConstructible and MoveAssignable, but is not CopyConstructible nor CopyAssignable.
The template parameter T of unique_­ptr may be an incomplete type.
[Note
:
The uses of unique_­ptr include providing exception safety for dynamically allocated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function.
end note
]
namespace std {
  template<class T> struct default_delete;
  template<class T> struct default_delete<T[]>;

  template<class T, class D = default_delete<T>> class unique_ptr;
  template<class T, class D> class unique_ptr<T[], D>;

  template<class T, class... Args> unique_ptr<T> make_unique(Args&&... args);
  template<class T> unique_ptr<T> make_unique(size_t n);
  template<class T, class... Args> unspecified make_unique(Args&&...) = delete;

  template<class T, class D>
    void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;

  template<class T1, class D1, class T2, class D2>
    bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

  template <class T, class D>
    bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
  template <class T, class D>
    bool operator==(nullptr_t, const unique_ptr<T, D>& y) noexcept;
  template <class T, class D>
    bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
  template <class T, class D>
    bool operator!=(nullptr_t, const unique_ptr<T, D>& y) noexcept;
  template <class T, class D>
    bool operator<(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator<(nullptr_t, const unique_ptr<T, D>& y);
  template <class T, class D>
    bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator<=(nullptr_t, const unique_ptr<T, D>& y);
  template <class T, class D>
    bool operator>(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator>(nullptr_t, const unique_ptr<T, D>& y);
  template <class T, class D>
    bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
  template <class T, class D>
    bool operator>=(nullptr_t, const unique_ptr<T, D>& y);
}

23.11.1.1 Default deleters [unique.ptr.dltr]

23.11.1.1.1 In general [unique.ptr.dltr.general]

The class template default_­delete serves as the default deleter (destruction policy) for the class template unique_­ptr.
The template parameter T of default_­delete may be an incomplete type.

23.11.1.1.2 default_­delete [unique.ptr.dltr.dflt]

namespace std {
  template <class T> struct default_delete {
    constexpr default_delete() noexcept = default;
    template <class U> default_delete(const default_delete<U>&) noexcept;
    void operator()(T*) const;
  };
}
template <class U> default_delete(const default_delete<U>& other) noexcept;
Effects: Constructs a default_­delete object from another default_­delete<U> object.
Remarks: This constructor shall not participate in overload resolution unless U* is implicitly convertible to T*.
void operator()(T* ptr) const;
Effects: Calls delete on ptr.
Remarks: If T is an incomplete type, the program is ill-formed.

23.11.1.1.3 default_­delete<T[]> [unique.ptr.dltr.dflt1]

namespace std {
  template <class T> struct default_delete<T[]> {
    constexpr default_delete() noexcept = default;
    template <class U> default_delete(const default_delete<U[]>&) noexcept;
    template <class U> void operator()(U* ptr) const;
  };
}
template <class U> default_delete(const default_delete<U[]>& other) noexcept;
Effects: Constructs a default_­delete object from another default_­delete<U[]> object.
Remarks: This constructor shall not participate in overload resolution unless U(*)[] is convertible to T(*)[].
template <class U> void operator()(U* ptr) const;
Effects: Calls delete[] on ptr.
Remarks: If U is an incomplete type, the program is ill-formed.
This function shall not participate in overload resolution unless U(*)[] is convertible to T(*)[].

23.11.1.2 unique_­ptr for single objects [unique.ptr.single]

namespace std {
  template <class T, class D = default_delete<T>> class unique_ptr {
  public:
    using pointer      = see below;
    using element_type = T;
    using deleter_type = D;

    // [unique.ptr.single.ctor], constructors
    constexpr unique_ptr() noexcept;
    explicit unique_ptr(pointer p) noexcept;
    unique_ptr(pointer p, see below d1) noexcept;
    unique_ptr(pointer p, see below d2) noexcept;
    unique_ptr(unique_ptr&& u) noexcept;
    constexpr unique_ptr(nullptr_t) noexcept;
    template <class U, class E>
      unique_ptr(unique_ptr<U, E>&& u) noexcept;

    // [unique.ptr.single.dtor], destructor
    ~unique_ptr();

    // [unique.ptr.single.asgn], assignment
    unique_ptr& operator=(unique_ptr&& u) noexcept;
    template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
    unique_ptr& operator=(nullptr_t) noexcept;

    // [unique.ptr.single.observers], observers
    add_lvalue_reference_t<T> operator*() const;
    pointer operator->() const noexcept;
    pointer get() const noexcept;
    deleter_type& get_deleter() noexcept;
    const deleter_type& get_deleter() const noexcept;
    explicit operator bool() const noexcept;

    // [unique.ptr.single.modifiers], modifiers
    pointer release() noexcept;
    void reset(pointer p = pointer()) noexcept;
    void swap(unique_ptr& u) noexcept;

    // disable copy from lvalue
    unique_ptr(const unique_ptr&) = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
  };
}
The default type for the template parameter D is default_­delete.
A client-supplied template argument D shall be a function object type, lvalue reference to function, or lvalue reference to function object type for which, given a value d of type D and a value ptr of type unique_­ptr<T, D>​::​pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter.
If the deleter's type D is not a reference type, D shall satisfy the requirements of Destructible.
If the qualified-id remove_­reference_­t<D>​::​pointer is valid and denotes a type ([temp.deduct]), then unique_­ptr<T, D>​::​pointer shall be a synonym for remove_­reference_­t<D>​::​pointer.
Otherwise unique_­ptr<T, D>​::​pointer shall be a synonym for element_­type*.
The type unique_­ptr<T, D>​::​pointer shall satisfy the requirements of NullablePointer.
[Example
:
Given an allocator type X ([allocator.requirements]) and letting A be a synonym for allocator_­traits<X>, the types A​::​pointer, A​::​const_­pointer, A​::​void_­pointer, and A​::​const_­void_­pointer may be used as unique_­ptr<T, D>​::​pointer.
end example
]

23.11.1.2.1 unique_­ptr constructors [unique.ptr.single.ctor]

constexpr unique_ptr() noexcept; constexpr unique_ptr(nullptr_t) noexcept;
Requires: D shall satisfy the requirements of DefaultConstructible, and that construction shall not throw an exception.
Effects: Constructs a unique_­ptr object that owns nothing, value-initializing the stored pointer and the stored deleter.
Postconditions: get() == nullptr.
get_­deleter() returns a reference to the stored deleter.
Remarks: If is_­pointer_­v<deleter_­type> is true or is_­default_­constructible_­v<deleter_­type> is false, this constructor shall not participate in overload resolution.
explicit unique_ptr(pointer p) noexcept;
Requires: D shall satisfy the requirements of DefaultConstructible, and that construction shall not throw an exception.
Effects: Constructs a unique_­ptr which owns p, initializing the stored pointer with p and value-initializing the stored deleter.
Postconditions: get() == p.
get_­deleter() returns a reference to the stored deleter.
Remarks: If is_­pointer_­v<deleter_­type> is true or is_­default_­constructible_­v<deleter_­type> is false, this constructor shall not participate in overload resolution.
If class template argument deduction ([over.match.class.deduct]) would select the function template corresponding to this constructor, then the program is ill-formed.
unique_ptr(pointer p, see below d1) noexcept; unique_ptr(pointer p, see below d2) noexcept;
The signature of these constructors depends upon whether D is a reference type.
If D is a non-reference type A, then the signatures are:
unique_ptr(pointer p, const A& d) noexcept;
unique_ptr(pointer p, A&& d) noexcept;
If D is an lvalue reference type A&, then the signatures are:
unique_ptr(pointer p, A& d) noexcept;
unique_ptr(pointer p, A&& d) = delete;
If D is an lvalue reference type const A&, then the signatures are:
unique_ptr(pointer p, const A& d) noexcept;
unique_ptr(pointer p, const A&& d) = delete;
Effects: Constructs a unique_­ptr object which owns p, initializing the stored pointer with p and initializing the deleter from std​::​forward<decltype(d)>(d).
Remarks: These constructors shall not participate in overload resolution unless is_­constructible_­v<D, decltype(d)> is true.
Postconditions: get() == p.
get_­deleter() returns a reference to the stored deleter.
If D is a reference type then get_­deleter() returns a reference to the lvalue d.
Remarks: If class template argument deduction ([over.match.class.deduct]) would select a function template corresponding to either of these constructors, then the program is ill-formed.
[Example
:
D d;
unique_ptr<int, D> p1(new int, D());        // D must be MoveConstructible
unique_ptr<int, D> p2(new int, d);          // D must be CopyConstructible
unique_ptr<int, D&> p3(new int, d);         // p3 holds a reference to d
unique_ptr<int, const D&> p4(new int, D()); // error: rvalue deleter object combined
                                            // with reference deleter type
end example
]
unique_ptr(unique_ptr&& u) noexcept;
Requires: If D is not a reference type, D shall satisfy the requirements of MoveConstructible.
Construction of the deleter from an rvalue of type D shall not throw an exception.
Effects: Constructs a unique_­ptr by transferring ownership from u to *this.
If D is a reference type, this deleter is copy constructed from u's deleter; otherwise, this deleter is move constructed from u's deleter.
[Note
:
The deleter constructor can be implemented with std​::​forward<D>.
end note
]
Postconditions: get() yields the value u.get() yielded before the construction.
get_­deleter() returns a reference to the stored deleter that was constructed from u.get_­deleter().
If D is a reference type then get_­deleter() and u.get_­deleter() both reference the same lvalue deleter.
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;
Requires: If E is not a reference type, construction of the deleter from an rvalue of type E shall be well formed and shall not throw an exception.
Otherwise, E is a reference type and construction of the deleter from an lvalue of type E shall be well formed and shall not throw an exception.
Remarks: This constructor shall not participate in overload resolution unless:
  • unique_­ptr<U, E>​::​pointer is implicitly convertible to pointer,
  • U is not an array type, and
  • either D is a reference type and E is the same type as D, or D is not a reference type and E is implicitly convertible to D.
Effects: Constructs a unique_­ptr by transferring ownership from u to *this.
If E is a reference type, this deleter is copy constructed from u's deleter; otherwise, this deleter is move constructed from u's deleter.
[Note
:
The deleter constructor can be implemented with std​::​forward<E>.
end note
]
Postconditions: get() yields the value u.get() yielded before the construction.
get_­deleter() returns a reference to the stored deleter that was constructed from u.get_­deleter().

23.11.1.2.2 unique_­ptr destructor [unique.ptr.single.dtor]

~unique_ptr();
Requires: The expression get_­deleter()(get()) shall be well formed, shall have well-defined behavior, and shall not throw exceptions.
[Note
:
The use of default_­delete requires T to be a complete type.
end note
]
Effects: If get() == nullptr there are no effects.
Otherwise get_­deleter()(get()).

23.11.1.2.3 unique_­ptr assignment [unique.ptr.single.asgn]

unique_ptr& operator=(unique_ptr&& u) noexcept;
Requires: If D is not a reference type, D shall satisfy the requirements of MoveAssignable and assignment of the deleter from an rvalue of type D shall not throw an exception.
Otherwise, D is a reference type; remove_­reference_­t<D> shall satisfy the CopyAssignable requirements and assignment of the deleter from an lvalue of type D shall not throw an exception.
Effects: Transfers ownership from u to *this as if by calling reset(u.release()) followed by get_­deleter() = std​::​forward<D>(u.get_­deleter()).
Returns: *this.
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
Requires: If E is not a reference type, assignment of the deleter from an rvalue of type E shall be well-formed and shall not throw an exception.
Otherwise, E is a reference type and assignment of the deleter from an lvalue of type E shall be well-formed and shall not throw an exception.
Remarks: This operator shall not participate in overload resolution unless:
  • unique_­ptr<U, E>​::​pointer is implicitly convertible to pointer, and
  • U is not an array type, and
  • is_­assignable_­v<D&, E&&> is true.
Effects: Transfers ownership from u to *this as if by calling reset(u.release()) followed by get_­deleter() = std​::​forward<E>(u.get_­deleter()).
Returns: *this.
unique_ptr& operator=(nullptr_t) noexcept;
Effects: As if by reset().
Postconditions: get() == nullptr.
Returns: *this.

23.11.1.2.4 unique_­ptr observers [unique.ptr.single.observers]

add_lvalue_reference_t<T> operator*() const;
Requires: get() != nullptr.
Returns: *get().
pointer operator->() const noexcept;
Requires: get() != nullptr.
Returns: get().
[Note
:
The use of this function typically requires that T be a complete type.
end note
]
pointer get() const noexcept;
Returns: The stored pointer.
deleter_type& get_deleter() noexcept; const deleter_type& get_deleter() const noexcept;
Returns: A reference to the stored deleter.
explicit operator bool() const noexcept;
Returns: get() != nullptr.

23.11.1.2.5 unique_­ptr modifiers [unique.ptr.single.modifiers]

pointer release() noexcept;
Postconditions: get() == nullptr.
Returns: The value get() had at the start of the call to release.
void reset(pointer p = pointer()) noexcept;
Requires: The expression get_­deleter()(get()) shall be well formed, shall have well-defined behavior, and shall not throw exceptions.
Effects: Assigns p to the stored pointer, and then if and only if the old value of the stored pointer, old_­p, was not equal to nullptr, calls get_­deleter()(old_­p).
[Note
:
The order of these operations is significant because the call to get_­deleter() may destroy *this.
end note
]
Postconditions: get() == p.
[Note
:
The postcondition does not hold if the call to get_­deleter() destroys *this since this->get() is no longer a valid expression.
end note
]
void swap(unique_ptr& u) noexcept;
Requires: get_­deleter() shall be swappable and shall not throw an exception under swap.
Effects: Invokes swap on the stored pointers and on the stored deleters of *this and u.

23.11.1.3 unique_­ptr for array objects with a runtime length [unique.ptr.runtime]

namespace std {
  template <class T, class D> class unique_ptr<T[], D> {
  public:
    using pointer      = see below;
    using element_type = T;
    using deleter_type = D;

    // [unique.ptr.runtime.ctor], constructors
    constexpr unique_ptr() noexcept;
    template <class U> explicit unique_ptr(U p) noexcept;
    template <class U> unique_ptr(U p, see below d) noexcept;
    template <class U> unique_ptr(U p, see below d) noexcept;
    unique_ptr(unique_ptr&& u) noexcept;
    template <class U, class E>
      unique_ptr(unique_ptr<U, E>&& u) noexcept;
    constexpr unique_ptr(nullptr_t) noexcept;

    // destructor
    ~unique_ptr();

    // assignment
    unique_ptr& operator=(unique_ptr&& u) noexcept;
    template <class U, class E>
      unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
    unique_ptr& operator=(nullptr_t) noexcept;

    // [unique.ptr.runtime.observers], observers
    T& operator[](size_t i) const;
    pointer get() const noexcept;
    deleter_type& get_deleter() noexcept;
    const deleter_type& get_deleter() const noexcept;
    explicit operator bool() const noexcept;

    // [unique.ptr.runtime.modifiers], modifiers
    pointer release() noexcept;
    template <class U> void reset(U p) noexcept;
    void reset(nullptr_t = nullptr) noexcept;
    void swap(unique_ptr& u) noexcept;

    // disable copy from lvalue
    unique_ptr(const unique_ptr&) = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
  };
}
A specialization for array types is provided with a slightly altered interface.
  • Conversions between different types of unique_­ptr<T[], D> that would be disallowed for the corresponding pointer-to-array types, and conversions to or from the non-array forms of unique_­ptr, produce an ill-formed program.
  • Pointers to types derived from T are rejected by the constructors, and by reset.
  • The observers operator* and operator-> are not provided.
  • The indexing observer operator[] is provided.
  • The default deleter will call delete[].
Descriptions are provided below only for members that differ from the primary template.
The template argument T shall be a complete type.

23.11.1.3.1 unique_­ptr constructors [unique.ptr.runtime.ctor]

template <class U> explicit unique_ptr(U p) noexcept;
This constructor behaves the same as the constructor in the primary template that takes a single parameter of type pointer except that it additionally shall not participate in overload resolution unless
  • U is the same type as pointer, or
  • pointer is the same type as element_­type*, U is a pointer type V*, and V(*)[] is convertible to element_­type(*)[].
template <class U> unique_ptr(U p, see below d) noexcept; template <class U> unique_ptr(U p, see below d) noexcept;
These constructors behave the same as the constructors in the primary template that take a parameter of type pointer and a second parameter except that they shall not participate in overload resolution unless either
  • U is the same type as pointer,
  • U is nullptr_­t, or
  • pointer is the same type as element_­type*, U is a pointer type V*, and V(*)[] is convertible to element_­type(*)[].
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;
This constructor behaves the same as in the primary template, except that it shall not participate in overload resolution unless all of the following conditions hold, where UP is unique_­ptr<U, E>:
  • U is an array type, and
  • pointer is the same type as element_­type*, and
  • UP​::​pointer is the same type as UP​::​element_­type*, and
  • UP​::​element_­type(*)[] is convertible to element_­type(*)[], and
  • either D is a reference type and E is the same type as D, or D is not a reference type and E is implicitly convertible to D.
[Note
:
This replaces the overload-resolution specification of the primary template
end note
]

23.11.1.3.2 unique_­ptr assignment [unique.ptr.runtime.asgn]

template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u)noexcept;
This operator behaves the same as in the primary template, except that it shall not participate in overload resolution unless all of the following conditions hold, where UP is unique_­ptr<U, E>:
  • U is an array type, and
  • pointer is the same type as element_­type*, and
  • UP​::​pointer is the same type as UP​::​element_­type*, and
  • UP​::​element_­type(*)[] is convertible to element_­type(*)[], and
  • is_­assignable_­v<D&, E&&> is true.
[Note
:
This replaces the overload-resolution specification of the primary template
end note
]

23.11.1.3.3 unique_­ptr observers [unique.ptr.runtime.observers]

T& operator[](size_t i) const;
Requires: i < the number of elements in the array to which the stored pointer points.
Returns: get()[i].

23.11.1.3.4 unique_­ptr modifiers [unique.ptr.runtime.modifiers]

void reset(nullptr_t p = nullptr) noexcept;
Effects: Equivalent to reset(pointer()).
template <class U> void reset(U p) noexcept;
This function behaves the same as the reset member of the primary template, except that it shall not participate in overload resolution unless either
  • U is the same type as pointer, or
  • pointer is the same type as element_­type*, U is a pointer type V*, and V(*)[] is convertible to element_­type(*)[].

23.11.1.4 unique_­ptr creation [unique.ptr.create]

template <class T, class... Args> unique_ptr<T> make_unique(Args&&... args);
Remarks: This function shall not participate in overload resolution unless T is not an array.
Returns: unique_­ptr<T>(new T(std​::​forward<Args>(args)...)).
template <class T> unique_ptr<T> make_unique(size_t n);
Remarks: This function shall not participate in overload resolution unless T is an array of unknown bound.
Returns: unique_­ptr<T>(new remove_­extent_­t<T>[n]()).
template <class T, class... Args> unspecified make_unique(Args&&...) = delete;
Remarks: This function shall not participate in overload resolution unless T is an array of known bound.

23.11.1.5 unique_­ptr specialized algorithms [unique.ptr.special]

template <class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;
Remarks: This function shall not participate in overload resolution unless is_­swappable_­v<D> is true.
Effects: Calls x.swap(y).
template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: x.get() == y.get().
template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: x.get() != y.get().
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Requires: Let CT denote
common_type_t<typename unique_ptr<T1, D1>::pointer,
              typename unique_ptr<T2, D2>::pointer>
Then the specialization less<CT> shall be a function object type that induces a strict weak ordering on the pointer values.
Returns: less<CT>()(x.get(), y.get()).
Remarks: If unique_­ptr<T1, D1>​::​pointer is not implicitly convertible to CT or unique_­ptr<T2, D2>​::​pointer is not implicitly convertible to CT, the program is ill-formed.
template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: !(y < x).
template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: y < x.
template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: !(x < y).
template <class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;
Returns: !x.
template <class T, class D> bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;
Returns: (bool)x.
template <class T, class D> bool operator<(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<(nullptr_t, const unique_ptr<T, D>& x);
Requires: The specialization less<unique_­ptr<T, D>​::​pointer> shall be a function object type that induces a strict weak ordering on the pointer values.
Returns: The first function template returns less<unique_­ptr<T, D>​::​pointer>()(x.get(),
nullptr)
.
The second function template returns less<unique_­ptr<T, D>​::​pointer>()(nullptr, x.get()).
template <class T, class D> bool operator>(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>(nullptr_t, const unique_ptr<T, D>& x);
Returns: The first function template returns nullptr < x.
The second function template returns x < nullptr.
template <class T, class D> bool operator<=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<=(nullptr_t, const unique_ptr<T, D>& x);
Returns: The first function template returns !(nullptr < x).
The second function template returns !(x < nullptr).
template <class T, class D> bool operator>=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& x);
Returns: The first function template returns !(x < nullptr).
The second function template returns !(nullptr < x).

23.11.2 Shared-ownership pointers [util.smartptr]

23.11.2.1 Class bad_­weak_­ptr [util.smartptr.weak.bad]

namespace std {
  class bad_weak_ptr : public exception {
  public:
    bad_weak_ptr() noexcept;
  };
}
An exception of type bad_­weak_­ptr is thrown by the shared_­ptr constructor taking a weak_­ptr.
bad_weak_ptr() noexcept;
Postconditions: what() returns an implementation-defined ntbs.

23.11.2.2 Class template shared_­ptr [util.smartptr.shared]

The shared_­ptr class template stores a pointer, usually obtained via new.
shared_­ptr implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer.
A shared_­ptr is said to be empty if it does not own a pointer.
namespace std {
  template<class T> class shared_ptr {
  public:
    using element_type = remove_extent_t<T>;
    using weak_type    = weak_ptr<T>;

    // [util.smartptr.shared.const], constructors
    constexpr shared_ptr() noexcept;
    template<class Y> explicit shared_ptr(Y* p);
    template<class Y, class D> shared_ptr(Y* p, D d);
    template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
    template <class D> shared_ptr(nullptr_t p, D d);
    template <class D, class A> shared_ptr(nullptr_t p, D d, A a);
    template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
    shared_ptr(const shared_ptr& r) noexcept;
    template<class Y> shared_ptr(const shared_ptr<Y>& r) noexcept;
    shared_ptr(shared_ptr&& r) noexcept;
    template<class Y> shared_ptr(shared_ptr<Y>&& r) noexcept;
    template<class Y> explicit shared_ptr(const weak_ptr<Y>& r);
    template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);
    constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }

    // [util.smartptr.shared.dest], destructor
    ~shared_ptr();

    // [util.smartptr.shared.assign], assignment
    shared_ptr& operator=(const shared_ptr& r) noexcept;
    template<class Y> shared_ptr& operator=(const shared_ptr<Y>& r) noexcept;
    shared_ptr& operator=(shared_ptr&& r) noexcept;
    template<class Y> shared_ptr& operator=(shared_ptr<Y>&& r) noexcept;
    template <class Y, class D> shared_ptr& operator=(unique_ptr<Y, D>&& r);

    // [util.smartptr.shared.mod], modifiers
    void swap(shared_ptr& r) noexcept;
    void reset() noexcept;
    template<class Y> void reset(Y* p);
    template<class Y, class D> void reset(Y* p, D d);
    template<class Y, class D, class A> void reset(Y* p, D d, A a);

    // [util.smartptr.shared.obs], observers
    element_type* get() const noexcept;
    T& operator*() const noexcept;
    T* operator->() const noexcept;
    element_type& operator[](ptrdiff_t i) const;
    long use_count() const noexcept;
    explicit operator bool() const noexcept;
    template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept;
    template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;
  };

  template<class T> shared_ptr(weak_ptr<T>) -> shared_ptr<T>;
  template<class T, class D> shared_ptr(unique_ptr<T, D>) -> shared_ptr<T>;

  // [util.smartptr.shared.create], shared_­ptr creation
  template<class T, class... Args>
    shared_ptr<T> make_shared(Args&&... args);                                  // T is not array
  template<class T, class A, class... Args>
    shared_ptr<T> allocate_shared(const A& a, Args&&... args);                  // T is not array

  template<class T>
    shared_ptr<T> make_shared(size_t N);                                        // T is U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, size_t N);                        // T is U[]

  template<class T>
    shared_ptr<T> make_shared();                                                // T is U[N]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a);                                  // T is U[N]

  template<class T>
    shared_ptr<T> make_shared(size_t N, const remove_extent_t<T>& u);           // T is U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, size_t N,
                                  const remove_extent_t<T>& u);                 // T is U[]

  template<class T> shared_ptr<T>
    make_shared(const remove_extent_t<T>& u);                                   // T is U[N]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, const remove_extent_t<T>& u);     // T is U[N]

  // [util.smartptr.shared.cmp], shared_­ptr comparisons
  template<class T, class U>
    bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

  template <class T>
    bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;
  template <class T>
    bool operator==(nullptr_t, const shared_ptr<T>& b) noexcept;
  template <class T>
    bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept;
  template <class T>
    bool operator!=(nullptr_t, const shared_ptr<T>& b) noexcept;
  template <class T>
    bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept;
  template <class T>
    bool operator<(nullptr_t, const shared_ptr<T>& b) noexcept;
  template <class T>
    bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept;
  template <class T>
    bool operator<=(nullptr_t, const shared_ptr<T>& b) noexcept;
  template <class T>
    bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept;
  template <class T>
    bool operator>(nullptr_t, const shared_ptr<T>& b) noexcept;
  template <class T>
    bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept;
  template <class T>
    bool operator>=(nullptr_t, const shared_ptr<T>& b) noexcept;

  // [util.smartptr.shared.spec], shared_­ptr specialized algorithms
  template<class T>
    void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

  // [util.smartptr.shared.cast], shared_­ptr casts
  template<class T, class U>
    shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;

  // [util.smartptr.getdeleter], shared_­ptr get_­deleter
  template<class D, class T>
    D* get_deleter(const shared_ptr<T>& p) noexcept;

  // [util.smartptr.shared.io], shared_­ptr I/O
  template<class E, class T, class Y>
    basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const shared_ptr<Y>& p);
}
Specializations of shared_­ptr shall be CopyConstructible, CopyAssignable, and LessThanComparable, allowing their use in standard containers.