std::optional::or_else - cppreference.com (original) (raw)
| 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>.
- 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.
[edit] Parameters
[edit] Return value
*this or the result of f, as described above.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
| __cpp_lib_optional | 202110L | (C++23) | Monadic operations in std::optional |
[edit] Example
#include #include #include int main() { using maybe_int = std::optional; 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
[edit] 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] |