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

 

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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(std::variant)

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

variant_size

variant_alternative

hash<std::variant>

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variant_npos

 

void swap( variant& rhs )noexcept(/* see below */);		(since C++17) (constexpr 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_if<i>(this), *std::get_if<i>(std::addressof(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 program is ill-formed unless 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_if<i>(this), *std::get_if<i>(std::addressof(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>)&& ...))

Notes

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

Example

Run this code

#include <iostream>#include <string>#include <variant>   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 non-trivial destructors can be constexpr in C++20	made constexpr

See also

std::swap(std::variant) (C++17)

specializes the std::swap algorithm (function template)