std::swap(std::variant)

Defined in header `<variant>`
template< class... Types > void swap( std::variant<Types...>& lhs, std::variant<Types...>& rhs ) noexcept(/* see below */);		(since C++17) (constexpr since C++20)

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)))

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_variant`	`202106L`	(C++20) (DR)	Fully constexpr `std::variant`

Example

#include <iostream>
#include <string>
#include <variant>

void 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
}

Output:

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