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std::optional<T>::or_else

From cppreference.com
< cpp‎ | utility‎ | optional
 
 
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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.

[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) [edit]
(C++23)
returns the result of the given function on the contained value if it exists, or an empty optional otherwise
(public member function) [edit]
(C++23)
returns an optional containing the transformed contained value if it exists, or an empty optional otherwise
(public member function) [edit]

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