std::swap(std::optional)

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

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

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Member functions
optional::optional (C++17)
optional::~optional (C++17)
optional::operator= (C++17)
Observers
optional::operator->optional::operator* (C++17)(C++17)
optional::operator booloptional::has_value (C++17)(C++17)
optional::value (C++17)
optional::value_or (C++17)
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optional::swap (C++17)
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operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
make_optional (C++17)
swap (C++17)
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hash (C++17)
nullopt_t (C++17)
in_place_t (C++17)
bad_optional_access (C++17)
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nullopt (C++17)
in_place (C++17)