std::swap (3) - Linux Manuals

std::swap: std::swap

NAME

std::swap - std::swap

Synopsis

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

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 && (since C++11)
std::is_nothrow_move_assignable<T>::value
)

2)

noexcept specification:
noexcept(noexcept(swap(*a, *b))) (until C++17)
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.
noexcept specification: (since C++17)
noexcept(std::is_nothrow_swappable_v<T2>)

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) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::tuple) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::shared_ptr) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::weak_ptr) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::unique_ptr) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::function) specializes the std::swap algorithm
                                    (function template)
(C++11)
                                    specializes the std::swap algorithm
std::swap(std::basic_string) (function template)

std::swap(std::array) specializes the std::swap algorithm
                                    (function template)
(C++11)
                                    specializes the std::swap algorithm
std::swap(std::deque) (function template)

std::swap(std::forward_list) specializes the std::swap algorithm
                                    (function template)
(C++11)
                                    specializes the std::swap algorithm
std::swap(std::list) (function template)
                                    specializes the std::swap algorithm
std::swap(std::vector) (function template)
                                    specializes the std::swap algorithm
std::swap(std::map) (function template)
                                    specializes the std::swap algorithm
std::swap(std::multimap) (function template)
                                    specializes the std::swap algorithm
std::swap(std::set) (function template)
                                    specializes the std::swap algorithm
std::swap(std::multiset) (function template)

std::swap(std::unordered_map) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::unordered_multimap) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::unordered_set) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::unordered_multiset) specializes the std::swap algorithm
                                    (function template)
(C++11)
                                    specializes the std::swap algorithm
std::swap(std::queue) (function template)
                                    specializes the std::swap algorithm
std::swap(std::priority_queue) (function template)
                                    specializes the std::swap algorithm
std::swap(std::stack) (function template)

std::swap(std::valarray) specializes the std::swap() algorithm
                                    (function template)
(C++11)

std::swap(std::basic_stringbuf) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_istringstream) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_ostringstream) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_stringstream) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_filebuf) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_ifstream) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_ofstream) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_fstream) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::basic_syncbuf) specializes the std::swap algorithm
                                    (function template)
(C++20)

std::swap(std::basic_regex) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::match_results) specializes the std::swap() algorithm
                                    (function template)
(C++11)

std::swap(std::thread) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::unique_lock) specialization of std::swap for unique_lock
                                    (function template)
(C++11)

std::swap(std::promise) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::packaged_task) specializes the std::swap algorithm
                                    (function template)
(C++11)

std::swap(std::optional) specializes the std::swap algorithm
                                    (function)
(C++17)

std::swap(std::any) specializes the std::swap algorithm
                                    (function)
(C++17)

std::swap(std::variant) specializes the std::swap algorithm
                                    (function)
(C++17)
                                    swaps two paths
swap(std::filesystem::path) (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