std::expected<T,E>:: or_else

Primary template
template < class F > constexpr auto or_else ( F && f ) & ;	(1)	(since C++23)
template < class F > constexpr auto or_else ( F && f ) const & ;	(2)	(since C++23)
template < class F > constexpr auto or_else ( F && f ) && ;	(3)	(since C++23)
template < class F > constexpr auto or_else ( F && f ) const && ;	(4)	(since C++23)
void partial specialization
template < class F > constexpr auto or_else ( F && f ) & ;	(5)	(since C++23)
template < class F > constexpr auto or_else ( F && f ) const & ;	(6)	(since C++23)
template < class F > constexpr auto or_else ( F && f ) && ;	(7)	(since C++23)
template < class F > constexpr auto or_else ( F && f ) const && ;	(8)	(since C++23)

If * this contains an unexpected value, invokes f with the unexpected value of * this as the argument and returns its result. Otherwise, returns a std::expected object that represents an expected value.

Given type G as:

If G is not a specialization of std::expected , or std:: is_same_v < G :: value_type , T > is false , the program is ill-formed.

Parameters

f

-

a suitable function or Callable object that returns a std::expected

Return value

Overload	Value of has_value()
	true	false
( 1,2 )	G ( std:: in_place , val )	std:: invoke ( std:: forward < F > ( f ) , error ( ) )
( 3,4 )	G ( std:: in_place , std :: move ( val ) )	std:: invoke ( std:: forward < F > ( f ) , std :: move ( error ( ) ) )
( 5,6 )	G ( )	std:: invoke ( std:: forward < F > ( f ) , error ( ) )
( 7,8 )		std:: invoke ( std:: forward < F > ( f ) , std :: move ( error ( ) ) )

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_expected	202211L	(C++23)	Monadic functions for std::expected

Example

This section is incomplete Reason: no example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3938	C++23	the expected value was obtained by value ( ) [1]	changed to ** this
LWG 3973	C++23	the expected value was obtained by ** this [2]	changed to val

1. ↑ value() requires E to be copy constructible (see LWG issue 3843 ), where operator* does not.
2. ↑ ** this can trigger argument-dependent lookup .

See also

transform_error

returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function)