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

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< cpp‎ | utility‎ | expected
 
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std::expected
 
template< class F >
constexpr auto transform_error( F&& f ) &;
 (1) (since 哋它亢++23)
template< class F >
constexpr auto transform_error( F&& f ) const&;
 (2) (since 哋它亢++23)
template< class F >
constexpr auto transform_error( F&& f ) &&;
 (3) (since 哋它亢++23)
template< class F >
constexpr auto transform_error( F&& f ) const&&;
 (4) (since 哋它亢++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

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 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3938 哋它亢++23 transform_error was ill-formed if T is not
(possibly cv-qualified) void and E is not copyable 		made well-formed

See also

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