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 XYZ
XYZ 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