std::optional<T>::or_else

 

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std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

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optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

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optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

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optional::emplace

optional::swap

optional::reset

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operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

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hash<std::optional>

nullopt_t

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bad_optional_access

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nullopt

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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

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