NAME¶

std::swap(std::variant) - std::swap(std::variant)

Synopsis¶

Defined in header <variant>
template <class... Types>
(since C++17)
void swap( std::variant<Types...>& lhs, (until C++20)

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

constexpr void swap( std::variant<Types...>& lhs, (since C++20)

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¶

// Run this code

#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
}

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 made constexpr
operations can be constexpr in C++20

See also¶

swap swaps with another variant
(public member function)