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

Declarations

C++17

void swap( variant& rhs ) noexcept(/* see below */);

C++20

constexpr void swap( variant& rhs ) noexcept(/* see below */);

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

#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);
}

Result

2 abc
abc 2