std:: swap

From cppreference.com

Defined in header `<algorithm>`		(until C++11)
Defined in header `<utility>`		(since C++11)
Defined in header `<string_view>`
template < class T > void swap ( T & a, T & b ) ;	(1)	(conditionally noexcept since C++11) (constexpr since C++20)
template < class T2, std:: size_t N > void swap ( T2 ( & a ) [ N ] , T2 ( & b ) [ N ] ) ;	(2)	(conditionally noexcept since C++11) (constexpr since C++20)

Exchanges the given values.

1) Swaps the values a and b .

This overload participates in overload resolution only if std:: is_move_constructible_v < T > && std:: is_move_assignable_v < T > is true .

(since C++17)

2) Swaps the arrays a and b . Equivalent to std:: swap_ranges ( a, a + N, b ) .

This overload participates in overload resolution only if std:: is_swappable_v < T2 > is true .

(since C++17)

Parameters

a, b	-	the values to be swapped
Type requirements
- `T` must meet the requirements of CopyConstructible and CopyAssignable (until C++11) MoveConstructible and MoveAssignable (since C++11) .
- `T2` must meet the requirements of Swappable .

Return value

(none)

Exceptions

(none)	(until C++11)
`noexcept` specification: noexcept ( std:: is_nothrow_move_constructible < T > :: value && std:: is_nothrow_move_assignable < T > :: value )	(since C++11)

`noexcept` specification: noexcept ( noexcept ( swap ( * a, * b ) ) ) The lookup for the identifier `swap` in the exception specification finds this function template in addition to anything found by the usual lookup rules, making the exception specification equivalent to C++17 std::is_nothrow_swappable .	(since C++11) (until C++17)
`noexcept` specification: noexcept ( std:: is_nothrow_swappable_v < T2 > )	(since C++17)

Complexity

1) Constant.

2) Linear in N .

Specializations

std::swap may be specialized in namespace std for program-defined types, but such specializations are not found by ADL (the namespace std is not the associated namespace for the program-defined type).

(until C++20)

The expected way to make a program-defined type swappable is to provide a non-member function swap in the same namespace as the type: see Swappable for details.

The following overloads are already provided by the standard library:

std::swap (std::pair) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::tuple) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::shared_ptr) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::weak_ptr) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::unique_ptr) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::function) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_string)	specializes the std::swap algorithm (function template)
std::swap (std::array) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::deque)	specializes the std::swap algorithm (function template)
std::swap (std::forward_list) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::list)	specializes the std::swap algorithm (function template)
std::swap (std::vector)	specializes the std::swap algorithm (function template)
std::swap (std::map)	specializes the std::swap algorithm (function template)
std::swap (std::multimap)	specializes the std::swap algorithm (function template)
std::swap (std::set)	specializes the std::swap algorithm (function template)
std::swap (std::multiset)	specializes the std::swap algorithm (function template)
std::swap (std::unordered_map) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::unordered_multimap) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::unordered_set) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::unordered_multiset) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::queue) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::priority_queue) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::stack) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::valarray) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_stringbuf) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_istringstream) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_ostringstream) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_stringstream) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_filebuf) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_ifstream) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_ofstream) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_fstream) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::basic_syncbuf) (C++20)	specializes the std::swap algorithm (function template)
std::swap (std::basic_spanbuf) (C++23)	specializes the std::swap algorithm (function template)
std::swap (std::basic_ispanstream) (C++23)	specializes the std::swap algorithm (function template)
std::swap (std::basic_ospanstream) (C++23)	specializes the std::swap algorithm (function template)
std::swap (std::basic_spanstream) (C++23)	specializes the std::swap algorithm (function template)
std::swap (std::basic_regex) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::match_results) (C++11)	specializes the `std::swap` algorithm (function template)
std::swap (std::thread) (C++11)	specializes the std::swap algorithm (function)
std::swap (std::unique_lock) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::shared_lock) (C++14)	specializes the std::swap algorithm (function template)
std::swap (std::promise) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::packaged_task) (C++11)	specializes the std::swap algorithm (function template)
std::swap (std::optional) (C++17)	specializes the std::swap algorithm (function template)
std::swap (std::any) (C++17)	specializes the std::swap algorithm (function)
std::swap (std::variant) (C++17)	specializes the std::swap algorithm (function template)
std::swap (std::basic_stacktrace) (C++23)	specializes the std::swap algorithm (function template)
swap (std::filesystem::path) (C++17)	specializes the std::swap algorithm (function)
swap (std::expected) (C++23)	specializes the std::swap algorithm (function)
swap (std::jthread) (C++20)	specializes the std::swap algorithm (function)
swap (std::move_only_function) (C++23)	specializes the std::swap algorithm (function)
swap (std::stop_source) (C++20)	specializes the std::swap algorithm (function)
swap (std::stop_token) (C++20)	specializes the std::swap algorithm (function)

Example

Run this code

#include <algorithm>
#include <iostream>
 
namespace Ns
{
    class A
    {
        int id {};
 
        friend void swap(A& lhs, A& rhs)
        {
            std::cout << "swap(" << lhs << ", " << rhs << ")\n";
            std::swap(lhs.id, rhs.id);
        }
 
        friend std::ostream& operator<<(std::ostream& os, A const& a)
        {
            return os << "A::id=" << a.id;
        }
 
    public:
        A(int i) : id {i} {}
        A(A const&) = delete;
        A& operator = (A const&) = delete;
    };
}
 
int main()
{
    int a = 5, b = 3;
    std::cout << a << ' ' << b << '\n';
    std::swap(a, b);
    std::cout << a << ' ' << b << '\n';
 
    Ns::A p {6}, q {9};
    std::cout << p << ' ' << q << '\n';
//  std::swap(p, q); // error, type requirements are not satisfied
    swap(p, q);      // OK, ADL finds the appropriate friend `swap`
    std::cout << p << ' ' << q << '\n';
}

Output:

5 3
3 5
A::id=6 A::id=9
swap(A::id=6, A::id=9)
A::id=9 A::id=6

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 227	C++98	`T` was not required to be CopyConstructible or DefaultConstructible (a temporary object of type `T` might not be able to be constructed)	`T` is also required to be CopyConstructible
LWG 809	C++98	arrays could not be swapped	added overload (2)
LWG 2554	C++11	swapping multi-dimensional arrays can never be noexcept due to name lookup problems	made to work

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)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::swap (C++20)	swaps the values of two objects (customization point object)
iter_swap	swaps the elements pointed to by two iterators (function template)
swap_ranges	swaps two ranges of elements (function template)
exchange (C++14)	replaces the argument with a new value and returns its previous value (function template)