std::variant<Types...>::swap (3) - Linux Manuals

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

NAME

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

Synopsis

void swap( variant& rhs ) noexcept(/* see below */); (1) (since C++17)

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

// Run this code

  #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);
  }

Output:

  2 abc
  abc 2