std::optional<T>::swap

 

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std::optional

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optional::optional

optional::~optional

optional::operator=

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optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

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optional::begin (C++26)

optional::end (C++26)

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optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

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optional::emplace

optional::swap

optional::reset

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operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

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hash<std::optional>

nullopt_t

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

The program is ill-formed unless type T is Swappable and std::is_move_constructible_v<T> is true.

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.

Feature-test macro	Value	Std	Feature
__cpp_lib_optional	202106L	(C++20) (DR20)	Fully constexpr

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.'

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 the required operations can be constexpr in C++20	made constexpr

See also

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

specializes the std::swap algorithm (function template)