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

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)

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

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

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	`transform_error` was ill-formed if `T` is not (possibly cv-qualified) void and `E` is not copyable	made well-formed

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)

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

Parameters

Return value

Example

Defect reports

See also