std::swap(std::optional)

 

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std::optional

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optional::optional

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optional::operator->optional::operator*

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optional::begin (C++26)

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optional::emplace

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

make_optional

swap(std::optional)

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Defined in header <optional>
template< class T > void swap( std::optional<T>& lhs,            std::optional<T>& rhs ) noexcept(/* see below */);		(since C++17) (constexpr since C++20)

Overloads the std::swap algorithm for std::optional. Exchanges the state of lhs with that of rhs. Effectively calls lhs.swap(rhs).

This overload participates in overload resolution only if std::is_move_constructible_v<T> and std::is_swappable_v<T> are both true.

Parameters

lhs, rhs

-

optional objects whose states to swap

Return value

(none)

Exceptions

noexcept specification:  

noexcept(noexcept(lhs.swap(rhs)))

Notes

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> a{"██████"}, b{"▒▒▒▒▒▒"};
 
    auto print = [&](auto const& s)
    {
        std::cout << s << "\t"
                     "a = " << a.value_or("(null)") << "  "
                     "b = " << b.value_or("(null)") << '\n';
    };
 
    print("Initially:");
    std::swap(a, b);
    print("swap(a, b):");
    a.reset();
    print("\n""a.reset():");
    std::swap(a, b);
    print("swap(a, b):");
}

Output:

Initially:   a = ██████  b = ▒▒▒▒▒▒
swap(a, b):  a = ▒▒▒▒▒▒  b = ██████
 
a.reset():   a = (null)  b = ██████
swap(a, b):  a = ██████  b = (null)

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

swap	exchanges the contents (public member function)