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

 

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Utilities library

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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

 

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 (operator*()) 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(operator*())>>;
  • for overloads (3-4), std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(operator*()))>>;
• 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) These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(error())> is true.

3-4) These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(std::move(error()))> is true.

Formally, these functions perform the following steps:

• If *this contains an expected value val:

1. Invoke f as if by
   • std::invoke(std::forward<F>(f), val) for overloads (1,2) if std::is_void_v<T> is false;
   • std::invoke(std::forward<F>(f), std::move(val)) for overloads (3,4) if std::is_void_v<T> is false;
   • std::invoke(std::forward<F>(f)) if std::is_void_v<T> is true.
2. 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()).

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 error value, as described above.

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(), which requires E to be copy constructible	changed to **this
LWG 3973	C++23	the expected value was obtained by **this, which can involve argument-dependent lookup	changed to name the member directly

See also

transform_error

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