std::swap

 

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Technical Specifications

 

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copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
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C library
qsort	bsearch

 

Defined in header <algorithm> Defined in header <utility>		(until C++11) (since C++11)
	(1)
template< class T > void swap( T& a, T& b );			(until C++11)
template< class T > void swap( T& a, T& b ) noexcept(/* see below */);			(since C++11)
template< class T2, std::size_t N > void swap( T2 (&a)[N], T2 (&b)[N]) noexcept(/* see below */);		(2)	(since C++11)

Exchanges the given values.

1) Swaps the values a and b. This overload does not participate in overload resolution unless std::is_move_constructible_v<T> && std::is_move_assignable_v<T> is true. (since C++17)

2) Swaps the arrays a and b. In effect calls std::swap_ranges(a, a+N, b). This overload does not participate in overload resolution unless 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 MoveAssignable and MoveConstructible.
- T2 must meet the requirements of Swappable.

Return value

(none)

Exceptions

1)

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

2)

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.	(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 user-defined types, but such specializations are not found by ADL (the namespace std is not the associated namespace for the user-defined type). The expected way to make a user-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)	specializes the std::swap algorithm (function template)
std::swap(std::priority_queue)	specializes the std::swap algorithm (function template)
std::swap(std::stack)	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_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 template)
std::swap(std::unique_lock) (C++11)	specialization of std::swap for unique_lock (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)
std::swap(std::any) (C++17)	specializes the std::swap algorithm (function)
std::swap(std::variant) (C++17)	specializes the std::swap algorithm (function)
swap(std::filesystem::path)	swaps two paths (function)

Example

Run this code

#include <algorithm>
#include <iostream>
 
int main()
{
   int a = 5, b = 3;
 
   // before
   std::cout << a << ' ' << b << '\n';
 
   std::swap(a,b);
 
   // after
   std::cout << a << ' ' << b << '\n';
}

Output:

5 3
3 5

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 2554	C++11	swapping multi-dimensional arrays can never be noexcept due to name lookup problems	made to work

See also

iter_swap	swaps the elements pointed to by two iterators (function template)
swap_ranges	swaps two ranges of elements (function template)