W3cubDocs

/C++

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

See also

swaps the values of two objects
(function template)
(C++11)
swaps the contents of two tuples
(public member function of std::tuple<Types...>)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/utility/pair/swap