std::pair::swap(3)

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

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...>)

Category:
     * conditionally noexcept