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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::pair

Member functions

pair::pair

pair::operator=

pair::swap (C++11)

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::pair) (C++11)

get(std::pair) (C++11)

Helper classes

tuple_size<std::pair> (C++11)

tuple_element<std::pair> (C++11)

basic_common_reference<std::pair> (C++23)

common_type<std::pair> (C++23)

formatter<std::pair> (C++23)

piecewise_construct_t (C++11)

Deduction guides(C++17)

 

void swap( pair& other ) noexcept(/* see below */);	(1)	(constexpr 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	swaps the contents of two tuples (public member function of std::tuple<Types...>)