20 General utilities library [utilities]

20.3 Pairs [pairs]

20.3.2 Class template pair [pairs.pair]

// defined in header <utility>

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

    T1 first;
    T2 second;
    pair(const pair&) = default;
    pair(pair&&) = default;
    constexpr pair();
    constexpr pair(const T1& x, const T2& y);
    template<class U, class V> constexpr pair(U&& x, V&& y);
    template<class U, class V> constexpr pair(const pair<U, V>& p);
    template<class U, class V> constexpr pair(pair<U, V>&& 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 U, class V> pair& operator=(const pair<U, V>& p);
    pair& operator=(pair&& p) noexcept(see below);
    template<class U, class V> pair& operator=(pair<U, V>&& p);

    void swap(pair& p) noexcept(see below);
  };
}
1

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.

2

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.

constexpr pair();

3

Requires: is_default_constructible<first_type>::value is true and is_default_construct-
ible<second_type>::value is true.

4

Effects: Value-initializes first and second.

constexpr pair(const T1& x, const T2& y);

5

Requires: is_copy_constructible<first_type>::value is true and is_copy_constructible<second_type>::value is true.

6

Effects: The constructor initializes first with x and second with y.

template<class U, class V> constexpr pair(U&& x, V&& y);

7

Requires: is_constructible<first_type, U&&>::value is true and is_constructible<second_type, V&&>::value is true.

8

Effects: The constructor initializes first with std::forward<U>(x) and second with std::forward<V>(y).

9

Remarks: If U is not implicitly convertible to first_type or V is not implicitly convertible to second_type this constructor shall not participate in overload resolution.

template<class U, class V> constexpr pair(const pair<U, V>& p);

10

Requires: is_constructible<first_type, const U&>::value is true and is_constructible<second_type, const V&>::value is true.

11

Effects: Initializes members from the corresponding members of the argument.

12

Remark: This constructor shall not participate in overload resolution unless const U& is implicitly convertible to first_type and const V& is implicitly convertible to second_type.

template<class U, class V> constexpr pair(pair<U, V>&& p);

13

Requires: is_constructible<first_type, U&&>::value is true and is_constructible<second_type, V&&>::value is true.

14

Effects: The constructor initializes first with std::forward<U>(p.first) and second with std::forward<V>(p.second).

15

Remark: This constructor shall not participate in overload resolution unless U is implicitly convertible to first_type and V is implicitly convertible to second_type.

template<class... Args1, class... Args2> pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args);

16

Requires: is_constructible<first_type, Args1&&...>::value is true and is_constructible<second_type, Args2&&...>::value is true.

17

Effects: The constructor 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);

18

Requires: is_copy_assignable<first_type>::value is true and is_copy_assignable<second_type>::value is true.

19

Effects: Assigns p.first to first and p.second to second.

20

Returns: *this.

template<class U, class V> pair& operator=(const pair<U, V>& p);

21

Requires: is_assignable<first_type&, const U&>::value is true and is_assignable<second_type&, const V&>::value is true.

22

Effects: Assigns p.first to first and p.second to second.

23

Returns: *this.

pair& operator=(pair&& p) noexcept(see below);

24

Remarks: The expression inside noexcept is equivalent to:

is_nothrow_move_assignable<T1>::value &&
is_nothrow_move_assignable<T2>::value
25

Requires: is_move_assignable<first_type>::value is true and is_move_assignable<second_type>::value is true.

26

Effects: Assigns to first with std::forward<first_type>(p.first) and to second with
std::forward<second_type>(p.second).

27

Returns: *this.

template<class U, class V> pair& operator=(pair<U, V>&& p);

28

Requires: is_assignable<first_type&, U&&>::value is true and is_assignable<second_type&, V&&>::value is true.

29

Effects: Assigns to first with std::forward<U>(p.first) and to second with
std::forward<V>(p.second).

30

Returns: *this.

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

31

Remarks: The expression inside noexcept is equivalent to:

noexcept(swap(first, p.first)) &&
noexcept(swap(second, p.second))
32

Requires: first shall be swappable with ([swappable.requirements]) p.first and second shall be swappable with p.second.

33

Effects: Swaps first with p.first and second with p.second.