std::swap

< cpp‎ | algorithm
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Concepts and utilities: std::Sortable, std::projected, ...
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
(C++11)
Minimum/maximum operations
(C++11)
(C++11)
(C++17)
(C++20)
Permutations
Numeric operations
C library
 
Defined in header <algorithm>
(until C++11)
Defined in header <utility>
(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)
(until C++20)
template< class T >
constexpr void swap( T& a, T& b ) noexcept(/* see below */);
 (since C++20)
(2)
template< class T2, std::size_t N >
void swap( T2 (&a)[N], T2 (&b)[N]) noexcept(/* see below */);
 (since C++11)
(until C++20)
template< class T2, std::size_t N >
constexpr void swap( T2 (&a)[N], T2 (&b)[N]) noexcept(/* see below */);
 (since C++20)

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 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:

(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap() algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++20)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap() algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specialization of std::swap for unique_lock
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)
(C++17)
 specializes the std::swap algorithm
(function)
(C++17)
 specializes the std::swap algorithm
(function)
(C++17)
 specializes the std::swap algorithm
(function)
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

 swaps the elements pointed to by two iterators
(function template)
swaps two ranges of elements
(function template)
(C++14)
 replaces the argument with a new value and returns its previous value
(function template)