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

< cpp‎ | utility‎ | expected

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

If *this contains an expected value, invokes f and returns a std::expected object that contains its result; otherwise, returns a std::expected object that contains a copy of error().

If T is not (possibly cv-qualified) void, the contained value (**this) is passed as an argument to f; otherwise f takes no argument.

Let U be:

if T is not (possibly cv-qualified) void:
- for overloads (1-2), std::remove_cv_t<std::invoke_result_t<F, decltype(**this)>>;
- for overloads (3-4), std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(**this))>>;
otherwise (T is possibly cv-qualified void), std::remove_cv_t<std::invoke_result_t<F>>.

U must be a valid value type for std::expected. A variable of type U must be constructible from the result of invocation (but does not need to be move-constructible). The return type is std::expected<U, E>.

1-2) If *this contains an expected value:

Invokes f as if by
- std::invoke(std::forward<F>(f), **this) if std::is_void_v<T> is false;
- std::invoke(std::forward<F>(f)) otherwise.
Then
- if std::is_void_v<U> is false, returns a std::expected object that contains an expected value, direct-initialized from the result of invocation;
- otherwise, returns std::expected<U, E>().

Otherwise (*this contains an error value), returns std::expected<U, E>(std::unexpect, error()).

These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(error())> is true.

3-4) If *this contains an expected value:

Invokes f as if by
- std::invoke(std::forward<F>(f), std::move(**this)) if std::is_void_v<T> is false;
- std::invoke(std::forward<F>(f)) otherwise.
Then
- if std::is_void_v<U> is false, returns a std::expected object that contains an expected value, direct-initialized from the result of invocation;
- otherwise, returns std::expected<U, E>().

Otherwise (*this contains an error value), returns std::expected<U, E>(std::unexpect, std::move(error())).

These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(std::move(error()))> 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 error value, as described above.

Example

This section is incomplete
Reason: no example

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

Parameters

Return value

Example

See also