std::optional<T>::or_else
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.
Contents |
[edit]Parameters
| f | - | a function or Callable object that returns an std::optional<T> |
[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
Run this code
#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
[edit]See also
| returns the contained value if available, another value otherwise (public member function) | |
(C++23) | returns the result of the given function on the contained value if it exists, or an empty optional otherwise (public member function) |
(C++23) | returns an optional containing the transformed contained value if it exists, or an empty optional otherwise (public member function) |
