std::swap(std::variant)

 

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std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Visitation

variant::visit (C++26)

Non-member functions

visit

holds_alternative

get(std::variant)

get_if

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

swap(std::variant)

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monostate

bad_variant_access

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hash<std::variant>

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variant_npos

 

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

swap	swaps with another variant (public member function)