NAME

std::pair::swap - std::pair::swap

Synopsis

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

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 (until C++23)
the value of the member, the behavior is undefined.
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. (since C++23)

If either selected swap function call does not swap the value of the
member, the behavior is undefined.

Parameters

other - pair of values to swap

Return value

(none)

Exceptions

noexcept specification:
noexcept(

noexcept(swap(first, other.first)) &&
noexcept(swap(second, other.second)) (until C++17)

)

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.
1)
noexcept specification:
noexcept(

std::is_nothrow_swappable_v<first_type> &&
std::is_nothrow_swappable_v<second_type>

) (since C++17)
2)
noexcept specification:
noexcept(

std::is_nothrow_swappable_v<const first_type> &&
std::is_nothrow_swappable_v<const second_type>

)

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