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

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< cpp‎ | utility‎ | pair

| (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 (C++11) | swaps the contents of two `tuple`s (public member function of `std::tuple<Types...>`)

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