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std::swap(3) C++ Standard Libary 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 MoveCon- structible. - 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 pro- vide 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 li- brary: std::swap(std::pair) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::tuple) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::shared_ptr) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::weak_ptr) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::unique_ptr) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::function) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_string) specializes the std::swap algo- rithm (function template) std::swap(std::array) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::deque) specializes the std::swap algo- rithm (function template) std::swap(std::forward_list) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::list) specializes the std::swap algo- rithm (function template) std::swap(std::vector) specializes the std::swap algo- rithm (function template) std::swap(std::map) specializes the std::swap algo- rithm (function template) std::swap(std::multimap) specializes the std::swap algo- rithm (function template) std::swap(std::set) specializes the std::swap algo- rithm (function template) std::swap(std::multiset) specializes the std::swap algo- rithm (function template) std::swap(std::unordered_map) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::unordered_multimap) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::unordered_set) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::unordered_multiset) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::queue) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::priority_queue) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::stack) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::valarray) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_stringbuf) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_istringstream) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_ostringstream) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_stringstream) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_filebuf) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_ifstream) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_ofstream) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_fstream) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::basic_syncbuf) specializes the std::swap algo- rithm (C++20) (function template) std::swap(std::basic_spanbuf) specializes the std::swap algo- rithm (C++23) (function template) std::swap(std::basic_ispanstream) specializes the std::swap algo- rithm (C++23) (function template) std::swap(std::basic_ospanstream) specializes the std::swap algo- rithm (C++23) (function template) std::swap(std::basic_spanstream) specializes the std::swap algo- rithm (C++23) (function template) std::swap(std::basic_regex) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::match_results) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::thread) specializes the std::swap algo- rithm (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 algo- rithm (C++11) (function template) std::swap(std::packaged_task) specializes the std::swap algo- rithm (C++11) (function template) std::swap(std::optional) specializes the std::swap algo- rithm (C++17) (function template) std::swap(std::any) specializes the std::swap algo- rithm (C++17) (function) std::swap(std::variant) specializes the std::swap algo- rithm (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 retroac- tively 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) http://cppreference.com 2022.07.31 std::swap(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Exceptions | Complexity | Specializations | Example | Output: | See also
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