std::pair<T1,T2>::swap

 

C++

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std::pair

Member functions

pair::pair

pair::operator=

pair::swap (C++11)

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::pair) (C++11)

get(std::pair) (C++11)

Helper classes

tuple_size<std::pair> (C++11)

tuple_element<std::pair> (C++11)

basic_common_reference<std::pair> (C++23)

common_type<std::pair> (C++23)

formatter<std::pair> (C++23)

piecewise_construct_t (C++11)

Deduction guides(C++17)

 

	(1)
void swap( pair& other ) noexcept(/* see below */);			(since C++11) (until C++20)
constexpr void swap( pair& other ) noexcept(/* see below */);			(since C++20)
constexpr void swap( const pair& other ) const noexcept(/* see below */);		(2)	(since C++23)

Swaps first with other.first and second with other.second, as if by using std::swap; swap(first, other.first); swap(second, other.second);.

If either selected swap function call is ill-formed or does not swap the value of the member, the behavior is undefined.	(until C++23)
1) The program is ill-formed if either std::is_swappable_v<T1> or std::is_swappable_v<T2> is not true. 2) The program is ill-formed if either std::is_swappable_v<const T1> or std::is_swappable_v<const T2> is not true. If either selected swap function call does not swap the value of the member, the behavior is undefined.	(since C++23)

Parameters

other

-

pair of values to swap

Return value

(none)

Exceptions

noexcept specification:   noexcept(      noexcept(swap(first, other.first)) &&      noexcept(swap(second, other.second)) ) In the expression above, the identifier swap is looked up in the same manner as the one used by the C++17 std::is_nothrow_swappable trait.	(until C++17)
1) noexcept specification:   noexcept(      std::is_nothrow_swappable_v<first_type> &&      std::is_nothrow_swappable_v<second_type> ) 2) noexcept specification:   noexcept(      std::is_nothrow_swappable_v<const first_type> &&      std::is_nothrow_swappable_v<const second_type> )	(since C++17)

Example

Run this code

#include <iostream>
#include <utility>
#include <string>
int main()
{
    std::pair<int, std::string> p1(10, "test"), p2;
    p2.swap(p1);
    std::cout << "(" << p2.first << ", " << p2.second << ")\n";
 
#if __cpp_lib_ranges_zip >= 202110L
    // Using the C++23 const qualified swap overload
    // (swap is no longer propagating pair constness)
    int i1 = 10, i2{};
    std::string s1("test"), s2;
    const std::pair<int&, std::string&> r1(i1, s1), r2(i2, s2);
    r2.swap(r1);
    std::cout << "(" << i2 << ", " << s2 << ")\n";
#endif
}

Possible output:

(10, test)
(10, test)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2456	C++11	the noexcept specification is ill-formed	made to work

See also

swap	swaps the values of two objects (function template)
swap	swaps the contents of two tuples (public member function of std::tuple<Types...>)