std::optional<T>::or_else

Since C++23

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

# Declarations

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

(since C++23)

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

(since C++23)

# Parameters

f: a function or Callable object that returns an std::optional

# 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';
}