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std::variant::swap(3) C++ Standard Libary std::variant::swap(3) NAME std::variant::swap - std::variant::swap Synopsis void swap( variant& rhs ) noexcept(/* see below */); (since C++17) (un- til C++20) constexpr void swap( variant& rhs ) noexcept(/* see below */); (since C++20) Swaps two variant objects. * If both *this and rhs are valueless by exception, does nothing. * Otherwise, if both *this and rhs hold the same alternative, calls swap(*std::get_if<i>(this), *std::get_if<i>(std::addres- sof(rhs))) where i is index(). If an exception is thrown, the state of the values de- pends on the exception safety of the swap function called. * Otherwise, exchanges values of rhs and *this. If an exception is thrown, the state of *this and rhs depends on exception safety of variant's move constructor. The behavior is undefined unless lvalues of type T_i are Swappable and std::is_move_constructible_v<T_i> is true for all T_i in Types.... Parameters rhs - a variant object to swap with Return value (none) Exceptions If this->index() == rhs.index(), may throw any exception thrown by swap(*std::get_if<i>(this), *std::get_if<i>(std::addressof(rhs))) with i being index(). Otherwise, may throw any exception thrown by the move constructors of the alternatives currently held by *this and rhs. noexcept specification: noexcept(((std::is_nothrow_move_constructible_v<Types> && std::is_nothrow_swappable_v<Types>) && ...)) Example // Run this code #include <iostream> #include <string> #include <variant> int main() { std::variant<int, std::string> v1{2}, v2{"abc"}; std::visit([](auto&& x) { std::cout << x << ' '; }, v1); std::visit([](auto&& x) { std::cout << x << '\n'; }, v2); v1.swap(v2); std::visit([](auto&& x) { std::cout << x << ' '; }, v1); std::visit([](auto&& x) { std::cout << x << '\n'; }, v2); } Output: 2 abc abc 2 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 P2231R1 C++20 swap was not constexpr while the required made constexpr operations can be constexpr in C++20 Category: * conditionally noexcept http://cppreference.com 2024.06.10 std::variant::swap(3)
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