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

 

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std::expected

Member functions

expected::expected

expected::~expected

expected::operator=

Observers

expected::operator->expected::operator*

expected::operator boolexpected::has_value

expected::value

expected::error

expected::value_or

expected::error_or

Monadic operations

expected::and_then

expected::or_else

expected::transform

expected::transform_error

Modifiers

expected::emplace

expected::swap

Non-member functions

operator==(std::expected)

swap(std::expected)

Helper classes

unexpected

bad_expected_access

unexpect_tunexpect

 

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)

See More

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.

1-4) The expected value is initialized with the expected value val of *this.

Given type G as:

1,2) std::remove_cv_t<std::invoke_result_t<F, decltype(error())>>

3,4) std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(error()))>>

5,6) std::remove_cv_t<std::invoke_result_t<F, decltype(error())>>

7,8) std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(error()))>>

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:

1,2) G g(std::invoke(std::forward<F>(f), error()));

3,4) G g(std::invoke(std::forward<F>(f), std::move(error()));

5,6) G g(std::invoke(std::forward<F>(f), error()));

7,8) G g(std::invoke(std::forward<F>(f), std::move(error()));

1,2) This overload participates in overload resolution only if std::is_constructible_v<T, decltype((val))> is true.

3,4) This overload participates in overload resolution only if std::is_constructible_v<T, decltype(std::move(val))> is true.

Parameters

f

-

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

Return value

Given expression expr as:

1,2) std::invoke(std::forward<F>(f), error())

3,4) std::invoke(std::forward<F>(f), std::move(error()))

5,6) std::invoke(std::forward<F>(f), error())

7,8) std::invoke(std::forward<F>(f), std::move(error()))

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

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()	changed to **this
LWG 3973	C++23	the expected value was obtained by **this	changed to val

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)