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    std::swap(std::variant)

    From cppreference.com
     
    C++
     
    Utilities library
    General utilities
    Relational operators (deprecated in C++20)
     
    std::variant
     
    Defined in header <variant> 
    template< class... Types >
void swap( std::variant<Types...>& lhs,
           std::variant<Types...>& rhs ) noexcept(/* see below */);
    		(since C++17)
    (constexpr since C++20)

    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)))

    Notes

    Feature-test macro Value Std Feature
    __cpp_lib_variant 202106L (C++20)
    (DR)    		 Fully constexpr std::variant

    Example

    Run this code
    #include <iostream>
#include <string>
#include <variant>

void 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 operations can be constexpr in C++20 made constexpr

    See also

    		 swaps with another variant
    (public member function) [edit]