Std::variant::swap (3) Linux Manual Page

std::variant<Types…>::swap – std::variant<Types…>::swap

Synopsis

void swap(variant &rhs) noexcept(/* see below */);
(1)(since C++ 17)

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<i>(*this), std:get<i>(rhs)) where i is index(). If an exception is thrown, the state of the values depends 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<i>(*this), std::get<i>(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 <variant>

#include <string>

#include <iostream>
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

std::variant::swap (3) Linux Manual Page