20 General utilities library [utilities]

20.4 Pairs [pairs]

20.4.2 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; constexpr explicit(see below) pair(); constexpr explicit(see below) pair(const T1& x, const T2& y); template<class U1 = T1, class U2 = T2> constexpr explicit(see below) pair(U1&& x, U2&& y); template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>& p); template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p); template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>&& p); template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>&& p); template<class... Args1, class... Args2> constexpr pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args); constexpr pair& operator=(const pair& p); constexpr const pair& operator=(const pair& p) const; template<class U1, class U2> constexpr pair& operator=(const pair<U1, U2>& p); template<class U1, class U2> constexpr const pair& operator=(const pair<U1, U2>& p) const; constexpr pair& operator=(pair&& p) noexcept(see below); constexpr const pair& operator=(pair&& p) const; template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p); template<class U1, class U2> constexpr const pair& operator=(pair<U1, U2>&& p) const; constexpr void swap(pair& p) noexcept(see below); constexpr void swap(const pair& p) const noexcept(see below); }; template<class T1, class T2> pair(T1, T2) -> pair<T1, T2>; }
Constructors and member functions of pair do 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 is a constexpr function if and only if all required element-wise initializations for move and copy, respectively, would satisfy the requirements for a constexpr function.
If (is_­trivially_­destructible_­v<T1> && is_­trivially_­destructible_­v<T2>) is true, then the destructor of pair is trivial.
pair<T, U> is a structural type ([temp.param]) if T and U are both structural types.
Two values p1 and p2 of type pair<T, U> are template-argument-equivalent ([temp.type]) if and only if p1.first and p2.first are template-argument-equivalent and p1.second and p2.second are template-argument-equivalent.
constexpr explicit(see below) pair();
Constraints:
  • is_­default_­constructible_­v<T1> is true and
  • is_­default_­constructible_­v<T2> is true.
Effects: Value-initializes first and second.
Remarks: The expression inside explicit evaluates to true if and only if either T1 or T2 is not implicitly default-constructible.
[Note 1:
This behavior can be implemented with a trait that checks whether a const T1& or a const T2& can be initialized with {}.
— end note]
constexpr explicit(see below) pair(const T1& x, const T2& y);
Constraints:
  • is_­copy_­constructible_­v<T1> is true and
  • is_­copy_­constructible_­v<T2> is true.
Effects: Initializes first with x and second with y.
Remarks: The expression inside explicit is equivalent to: !is_convertible_v<const T1&, T1> || !is_convertible_v<const T2&, T2>
template<class U1 = T1, class U2 = T2> constexpr explicit(see below) pair(U1&& x, U2&& y);
Constraints:
  • is_­constructible_­v<T1, U1> is true and
  • is_­constructible_­v<T2, U2> is true.
Effects: Initializes first with std​::​forward<U1>(x) and second with std​::​forward<U2>(y).
Remarks: The expression inside explicit is equivalent to: !is_convertible_v<U1, T1> || !is_convertible_v<U2, T2>
template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>& p); template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p); template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>&& p); template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>&& p);
Let FWD(u) be static_­cast<decltype(u)>(u).
Constraints:
  • is_­constructible_­v<T1, decltype(get<0>(FWD(p)))> is true and
  • is_­constructible_­v<T2, decltype(get<1>(FWD(p)))> is true.
Effects: Initializes first with get<0>(FWD(p)) and second with get<1>(FWD(p)).
Remarks: The expression inside explicit is equivalent to: !is_convertible_v<decltype(get<0>(FWD(p))), T1> || !is_convertible_v<decltype(get<1>(FWD(p))), T2>
template<class... Args1, class... Args2> constexpr pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args);
Mandates:
  • is_­constructible_­v<T1, Args1...> is true and
  • is_­constructible_­v<T2, 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.
constexpr pair& operator=(const pair& p);
Effects: Assigns p.first to first and p.second to second.
Returns: *this.
Remarks: This operator is defined as deleted unless is_­copy_­assignable_­v<T1> is true and is_­copy_­assignable_­v<T2> is true.
constexpr const pair& operator=(const pair& p) const;
Constraints:
  • is_­copy_­assignable<const T1> is true and
  • is_­copy_­assignable<const T2> is true.
Effects: Assigns p.first to first and p.second to second.
Returns: *this.
template<class U1, class U2> constexpr pair& operator=(const pair<U1, U2>& p);
Constraints:
  • is_­assignable_­v<T1&, const U1&> is true and
  • is_­assignable_­v<T2&, const U2&> is true.
Effects: Assigns p.first to first and p.second to second.
Returns: *this.
template<class U1, class U2> constexpr const pair& operator=(const pair<U1, U2>& p) const;
Constraints:
  • is_­assignable_­v<const T1&, const U1&> is true, and
  • is_­assignable_­v<const T2&, const U2&> is true.
Effects: Assigns p.first to first and p.second to second.
Returns: *this.
constexpr pair& operator=(pair&& p) noexcept(see below);
Constraints:
  • is_­move_­assignable_­v<T1> is true and
  • is_­move_­assignable_­v<T2> is true.
Effects: Assigns to first with std​::​forward<T1>(p.first) and to second with std​::​forward<T2>(​p.second).
Returns: *this.
Remarks: The exception specification is equivalent to: is_nothrow_move_assignable_v<T1> && is_nothrow_move_assignable_v<T2>
constexpr const pair& operator=(pair&& p) const;
Constraints:
  • is_­assignable<const T1&, T1> is true and
  • is_­assignable<const T2&, T2> is true.
Effects: Assigns std​::​forward<T1>(p.first) to first and std​::​forward<T2>(p.second) to second.
Returns: *this.
template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p);
Constraints:
  • is_­assignable_­v<T1&, U1> is true and
  • is_­assignable_­v<T2&, U2> is true.
Effects: Assigns to first with std​::​forward<U1>(p.first) and to second with
std​::​forward<U2>(p.second).
Returns: *this.
template<class U1, class U2> constexpr const pair& operator=(pair<U1, U2>&& p) const;
Constraints:
  • is_­assignable_­v<const T1&, U1> is true, and
  • is_­assignable_­v<const T2&, U2> is true.
Effects: Assigns std​::​forward<U1>(p.first) to first and std​::​forward<U2>(u.second) to second.
Returns: *this.
constexpr void swap(pair& p) noexcept(see below); constexpr void swap(const pair& p) const noexcept(see below);
Mandates:
  • For the first overload, is_­swappable_­v<T1> is true and is_­swappable_­v<T2> is true.
  • For the second overload, is_­swappable_­v<const T1> is true and is_­swappable_­v<const T2> is true.
Preconditions: first is swappable with ([swappable.requirements]) p.first and second is swappable with p.second.
Effects: Swaps first with p.first and second with p.second.
Remarks: The exception specification is equivalent to:
  • is_­nothrow_­swappable_­v<T1> && is_­nothrow_­swappable_­v<T2> for the first overload, and
  • is_­nothrow_­swappable_­v<const T1> && is_­nothrow_­swappable_­v<const T2> for the second overload.