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

Primary template
template < class F > constexpr auto transform_error ( F && f ) & ;	(1)	(since C++23)
template < class F > constexpr auto transform_error ( F && f ) const & ;	(2)	(since C++23)
template < class F > constexpr auto transform_error ( F && f ) && ;	(3)	(since C++23)
template < class F > constexpr auto transform_error ( F && f ) const && ;	(4)	(since C++23)
void partial specialization
template < class F > constexpr auto transform_error ( F && f ) & ;	(5)	(since C++23)
template < class F > constexpr auto transform_error ( F && f ) const & ;	(6)	(since C++23)
template < class F > constexpr auto transform_error ( F && f ) && ;	(7)	(since C++23)
template < class F > constexpr auto transform_error ( 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 a std::expected object that contains an unexpected value, which is initialized with the result of f . Otherwise, returns a std::expected object that represents an expected value.

Given type G as:

If any of the following conditions is satisfied, the program is ill-formed:

• G is not a valid template argument for std::unexpected .
• The following corresponding declaration is ill-formed:

Parameters

f

-

a suitable function or Callable object whose call signature returns a non-reference type

Return value

Given expression expr as:

The return values are defined as follows:

Overload	Value of has_value()
	true	false
( 1,2 )	std:: expected < T, G > ( std:: in_place , val )	std:: expected < T, G > ( std:: unexpect , expr )
( 3,4 )	std:: expected < T, G > ( std:: in_place , std :: move ( val ) )
( 5,6 )	std:: expected < T, G > ( )
( 7,8 )

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

or_else	returns the expected itself if it contains an expected value; otherwise, returns the result of the given function on the unexpected value (public member function)
transform	returns an expected containing the transformed expected value if it exists; otherwise, returns the expected itself (public member function)