std::variant<Types...>::swap

void swap( variant& rhs ) noexcept(/* see below */);		(since C++17) (until C++20)
constexpr void swap( variant& rhs ) noexcept(/* see below */);		(since C++20)

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

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

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