std::optional<T>::swap

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

Swaps the contents with those ofother.

If neither*this norother contain a value, the function has no effect.

If only one of*this andother contains a value (let's call this objectin and the otherun), the contained value ofun isdirect-initialized fromstd::move(*in), followed by destruction of the contained value ofin as if byin->T::~T(). After this call,in does not contain a value;un contains a value.

If both*this andother contain values, the contained values are exchanged by callingusingstd::swap; swap(**this,*other).

The program is ill-formed unless typeT isSwappable andstd::is_move_constructible_v<T> istrue.

[edit]Parameters

other - theoptional object to exchange the contents with

[edit]Return value

(none)

[edit]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 andother are determined by the exception safety guarantees ofswap of typeT orT's move constructor, whichever is called. For both*this andother, if the object contained a value, it is left containing a value, and the other way round.

Feature-test macro	Value	Std	Feature
`__cpp_lib_optional`	`202106L`	(C++20) (DR20)	Fullyconstexpr

[edit]Example

Run this code

#include <iostream>#include <optional>#include <string> 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.'