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

	(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

#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