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std::swap(std::optional)

Defined in header <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)

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

#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.