NAME¶

std::expected::transform_error - std::expected::transform_error

Synopsis¶

template< class F > (1) (since C++23)
constexpr auto transform_error( F&& f ) &;
template< class F > (2) (since C++23)
constexpr auto transform_error( F&& f ) const&;
template< class F > (3) (since C++23)
constexpr auto transform_error( F&& f ) &&;
template< class F > (4) (since C++23)
constexpr auto transform_error( F&& f ) const&&;

If *this contains an error value, invokes f with the argument error() and returns a
std::expected object that contains its result; otherwise, returns a std::expected
object that contains a copy of the contained expected value (obtained from
operator*).

1,2) Given type G as std::remove_cv_t<std::invoke_result_t<F, decltype(error())>>.
If G is not a valid template argument for std::unexpected, or G
g(std::invoke(std::forward<F>(f), error())); is ill-formed, the program is
ill-formed.
The effect is equivalent to

if (has_value())
{
if (std::is_void_v<T>)
return std::expected<T, G>();
else
return std::expected<T, G>(std::in_place, **this);
}
else
// the returned std::expected object contains an unexpected value,
// which is direct-non-list-initialized with
// std::invoke(std::forward<F>(f), error())
return /* an std::expected<T, G> object */;

These overloads participate in overload resolution only if std::is_void_v<T> or
std::is_constructible_v<T, decltype(**this)> is true.
3,4) Given type G as std::remove_cv_t<std::invoke_result_t<F,
decltype(std::move(error()))>>.
If G is not a valid template argument for std::unexpected, or G
g(std::invoke(std::forward<F>(f), std::move(error()))); is ill-formed, the program
is ill-formed.
The effect is equivalent to

if (has_value())
{
if (std::is_void_v<T>)
return std::expected<T, G>();
else
return std::expected<T, G>(std::in_place, std::move(**this));
}
else
// the returned std::expected object contains an unexpected value,
// which is direct-non-list-initialized with
// std::invoke(std::forward<F>(f), std::move(error()))
return /* an std::expected<T, G> object */;

These overloads participate in overload resolution only if std::is_void_v<T> or
std::is_constructible_v<T, decltype(std::move(**this))> is true.

Parameters¶

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

Return value¶

A std::expected object containing either the result of f or an expected value, as
described above.

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
transform_error was ill-formed if T is not
LWG 3938 C++23 (possibly cv-qualified) void and E is not made well-formed
copyable

See also¶

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

Category:¶

* Todo no example