NAME¶

std::optional::or_else - std::optional::or_else

Synopsis¶

template< class F > (1) (since C++23)
constexpr optional or_else( F&& f ) const&;
template< class F > (2) (since C++23)
constexpr optional or_else( F&& f ) &&;

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¶

// 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

See also¶

value_or returns the contained value if available, another value otherwise
(public member function)
and_then returns the result of the given function on the contained value if it
(C++23) exists, or an empty optional otherwise
(public member function)
transform returns an optional containing the transformed contained value if it
(C++23) exists, or an empty optional otherwise
(public member function)