std::swap(std::variant)

Defined in header <variant>.

Declarations

C++17

template <class... Types>void swap( std::variant<Types...>& lhs,           std::variant<Types...>& rhs ) noexcept(/* see below */);

C++20

template <class... Types>constexpr void swap( std::variant<Types...>& lhs,                     std::variant<Types...>& rhs ) noexcept(/* see below */);

Overloads the std::swap algorithm for std::variant. Effectively calls lhs.swap(rhs).

This overload participates in overload resolution only if std::is_move_constructible_v<T_i> and std::is_swappable_v<T_i> are both true for all T_i in Types...

Parameters

lhs, rhs - variant objects whose values to swap

Return value

(none)

Exceptions

noexcept specification:
noexcept(noexcept(lhs.swap(rhs)))

Example

#include <variant>#include <string>#include <iostream> auto print = [](auto const& v, char term = '\n') {    std::visit([](auto&& o) { std::cout << o; }, v);    std::cout << term;}; int main(){    std::variant<int, std::string> v1{123}, v2{"XYZ"};    print(v1, ' ');    print(v2);     std::swap(v1, v2);    print(v1, ' ');    print(v2);     std::variant<double, std::string> v3{3.14};    // std::swap(v1, v3); // ERROR: ~ inconsistent parameter packs}

Result

123 XYZXYZ 123

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2231R1	C++20	swap was not constexpr while the required operations can be constexpr in C++20	made constexpr