NAME

std::variant::swap - std::variant::swap

Synopsis

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 made constexpr
operations can be constexpr in C++20