std::optional<T>:: or_else

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::optional
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

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

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)
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