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std::optional::swap(3)	      C++ Standard Libary	std::optional::swap(3)

NAME
       std::optional::swap - std::optional::swap

Synopsis
	  void	 swap(	 optional&   other   )	 noexcept(/*  see  below  */);
       (since C++17)
									     (un-
       til C++20)
	  constexpr void swap( optional& other ) noexcept(/*  see  below  */);
       (since C++20)

	  Swaps	the contents with those	of other.

	    *  If neither *this	nor other contain a value, the function	has no
       effect.

	    * If only one of *this and other contains a	value (let's call this
       object in and
	      the other	un), the contained value of un	is  direct-initialized
       from
	      std::move(*in),  followed	 by destruction	of the contained value
       of in as	if by
	      in->T::~T(). After this call, in does not	contain	 a  value;  un
       contains	a value.

	    * If both *this and	other contain values, the contained values are
       exchanged by
	      calling  using  std::swap;  swap(**this, *other).	T lvalues must
       satisfy Swappable.

	  The program  is  ill-formed  if  std::is_move_constructible_v<T>  is
       false.

Parameters
	  other	- the optional object to exchange the contents with

Return value
	  (none)

Exceptions
	  noexcept specification:
	  noexcept(std::is_nothrow_move_constructible_v<T> &&
	  std::is_nothrow_swappable_v<T>)

	  In  the case of thrown exception, the	states of the contained	values
       of *this	and
	  other	are determined by the exception	safety guarantees of  swap  of
       type T or T's
	  move	constructor, whichever is called. For both *this and other, if
       the object
	  contained a value, it	is left	containing a value, and	the other  way
       round.

Example
       // Run this code

	#include <iostream>
	#include <string>
	#include <optional>

	int main()
	{
	    std::optional<std::string> opt1("First example text");
	    std::optional<std::string> opt2("2nd text");

	    enum Swap {	Before,	After };
	    auto print_opts = [&](Swap e) {
		std::cout  <<  (e  ==  Before  ?  "Before  swap:\n"  :	"After
       swap:\n");
		std::cout << "opt1 contains '" << opt1.value_or("") << "'\n";
		std::cout << "opt2 contains '" << opt2.value_or("") << "'\n";
		std::cout << (e	== Before ? "---SWAP---\n": "\n");
	    };

	    print_opts(Before);
	    opt1.swap(opt2);
	    print_opts(After);

	    // Swap with only 1	set
	    opt1 = "Lorem ipsum	dolor sit amet,	consectetur tincidunt.";
	    opt2.reset();

	    print_opts(Before);
	    opt1.swap(opt2);
	    print_opts(After);
	}

Output:
	Before swap:
	opt1 contains 'First example text'
	opt2 contains '2nd text'
	---SWAP---
	After swap:
	opt1 contains '2nd text'
	opt2 contains 'First example text'

	Before swap:
	opt1 contains 'Lorem ipsum dolor sit amet, consectetur tincidunt.'
	opt2 contains ''
	---SWAP---
	After swap:
	opt1 contains ''
	opt2 contains 'Lorem ipsum dolor sit amet, consectetur tincidunt.'

	 Defect	reports

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

	    DR	     Applied	to		   Behavior    as    published
       Correct behavior
	  P2231R1  C++20       swap  was  not  constexpr  while	 the  required
       made constexpr
			     operations	can be constexpr in C++20

See also
	  std::swap(std::optional) specializes the std::swap algorithm
	  (C++17)		   (function template)

http://cppreference.com		  2022.07.31		std::optional::swap(3)

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