std::swap(3)

NAME
std::swap - std::swap

Synopsis
   Defined in header <algorithm> (until C++11)
   Defined in header <utility> (since C++11)
   Defined in header <string_view> (since C++17)
   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(/* (until C++20)
   see below */);
   template< class T >
   constexpr void swap( T& a, T& b ) (1) (since C++20)
   noexcept(/* see below */);
   template< class T2, std::size_t N > (since C++11)
   void swap( T2 (&a)[N], T2 (&b)[N]) (until C++20)
   noexcept(/* see below */); (2)
   template< class T2, std::size_t N >
   constexpr void swap( T2 (&a)[N], T2 (since C++20)
   (&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)))
   The lookup for the identifier swap in the exception specification (since C++11)
   finds this function template in addition to anything found by the (until C++17)
   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 (until C++20)
   std is not the associated namespace for the program-defined type).
   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
   (C++11) (function template)
   std::swap(std::tuple) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::shared_ptr) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::weak_ptr) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::unique_ptr) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::function) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_string) specializes the std::swap algorithm
                        (function template)
   std::swap(std::array) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::deque) specializes the std::swap algorithm
                        (function template)
   std::swap(std::forward_list) specializes the std::swap algorithm
   (C++11) (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) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::unordered_multimap) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::unordered_set) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::unordered_multiset) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::queue) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::priority_queue) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::stack) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::valarray) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_stringbuf) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_istringstream) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_ostringstream) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_stringstream) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_filebuf) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_ifstream) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_ofstream) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_fstream) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::basic_syncbuf) specializes the std::swap algorithm
   (C++20) (function template)
   std::swap(std::basic_spanbuf) specializes the std::swap algorithm
   (C++23) (function template)
   std::swap(std::basic_ispanstream) specializes the std::swap algorithm
   (C++23) (function template)
   std::swap(std::basic_ospanstream) specializes the std::swap algorithm
   (C++23) (function template)
   std::swap(std::basic_spanstream) specializes the std::swap algorithm
   (C++23) (function template)
   std::swap(std::basic_regex) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::match_results) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::thread) specializes the std::swap algorithm
   (C++11) (function)
   std::swap(std::unique_lock) specialization of std::swap for unique_lock
   (C++11) (function template)
   std::swap(std::promise) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::packaged_task) specializes the std::swap algorithm
   (C++11) (function template)
   std::swap(std::optional) specializes the std::swap algorithm
   (C++17) (function template)
   std::swap(std::any) specializes the std::swap algorithm
   (C++17) (function)
   std::swap(std::variant) specializes the std::swap algorithm
   (C++17) (function template)
   swap(std::filesystem::path) swaps two paths
   (C++17) (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 2554 C++11 swapping multi-dimensional arrays can never be made to work
                       noexcept due to name lookup problems

See also
   ranges::swap swaps the values of two objects
   (C++20) (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 replaces the argument with a new value and returns its previous value
   (C++14) (function template)