std::swap(std::optional)

 

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std::optional

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optional::optional

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optional::emplace

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

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swap

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

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

This function template does not participate in overload resolution unless 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)))

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)

See also

swap	exchanges the contents (public member function)