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std::optional<T>::or_else - cppreference.com

From cppreference.com

template< class F > constexpr optional or_else( F&& f ) const&;

 (1) (since C++23)

template< class F > constexpr optional or_else( F&& f ) &&;

 (2) (since C++23)

Returns *this if it contains a value. Otherwise, returns the result of f.

The program is ill-formed if std::remove_cvref_t<std::invoke_result_t<F>> is not same as std::optional<T>.

1) Equivalent to return *this ? *this : std::forward<F>(f)();. This overload participates in overload resolution only if both std::copy_constructible<T> and std::invocable<F> are modeled.

2) Equivalent to return *this ? std::move(*this) : std::forward<F>(f)();. This overload participates in overload resolution only if both std::move_constructible<T> and std::invocable<F> are modeled.

Parameters

f - a function or Callable object that returns an std::optional<T>

Return value

*this or the result of f, as described above.

Notes

Feature-test macro Value Std Feature
__cpp_lib_optional 202110L (C++23) Monadic operations in std::optional

Example

#include <iostream>
#include <optional>
#include <string>

int main()
{
    using maybe_int = std::optional<int>;

    auto valueless = []
    {
        std::cout << "Valueless: ";
        return maybe_int{0};
    };

    maybe_int x;
    std::cout << x.or_else(valueless).value() << '\n';

    x = 42;
    std::cout << "Has value: ";
    std::cout << x.or_else(valueless).value() << '\n';

    x.reset();
    std::cout << x.or_else(valueless).value() << '\n';
}

Output: 

Valueless: 0
Has value: 42
Valueless: 0

See also

returns the contained value if available, another value otherwise
(public member function) [edit]
returns the result of the given function on the contained value if it exists, or an empty optional otherwise
(public member function) [edit]
returns an optional containing the transformed contained value if it exists, or an empty optional otherwise
(public member function) [edit]