Difference between revisions of "cpp/utility/variant/swap"

Latest revision as of 13:58, 20 October 2024

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....

[edit] Parameters

[edit] Return value

(none)

[edit] 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.

[edit] Notes

[edit] Example

#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);
}

2 abc
abc 2

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.