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std::expected::transform(3)    C++ Standard Libary   std::expected::transform(3)

NAME
     std::expected::transform - std::expected::transform

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

	If  *this  contains  an expected value, invokes f and returns a std::ex-
     pected 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  (obtained
     from
	operator*) is passed as an argument to f; otherwise f takes no argument.

	1,2) Given type U as
	  * std::remove_cv_t<std::invoke_result_t<F>> if T is (possibly cv-qual-
     ified) void,
	    or
	  *  std::remove_cv_t<std::invoke_result_t<F,  decltype(**this)>> other-
     wise,
	and expression /* invoke-expr */ as
	  * std::invoke(std::forward<F>(f))  if  T  is	(possibly  cv-qualified)
     void, or
	  * std::invoke(std::forward<F>(f), **this) otherwise.
	If  U is not a valid value type for std::expected, or U u(/* invoke-expr
     */); is
	ill-formed when std::is_void_v<U> is false, the program is ill-formed.
	The effect is equivalent to

      if (has_value())
      {
	  if constexpr (std::is_void_v<U>)
	  {
	      /* invoke-expr */;
	      return std::expected<U, E>();
	  }
	  else
	      // the returned std::expected object contains an expected value,
	      // which is direct-non-list-initialized with /* invoke-expr */
	      return /* an std::expected<U, E> object */;
      }
      else
	  return 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) Given type U as
	  * std::remove_cv_t<std::invoke_result_t<F>> if T is (possibly cv-qual-
     ified) void,
	    or
	  *		std::remove_cv_t<std::invoke_result_t<F,	     de-
     cltype(std::move(**this))>>
	    otherwise,
	and expression /* invoke-expr */ as
	  *  std::invoke(std::forward<F>(f))  if  T  is  (possibly cv-qualified)
     void, or
	  * std::invoke(std::forward<F>(f), std::move(**this)) otherwise.
	If U is not a valid value type for std::expected, or U u(/*  invoke-expr
     */); is
	ill-formed when std::is_void_v<U> is false, the program is ill-formed.
	The effect is equivalent to

      if (has_value())
      {
	  if constexpr (std::is_void_v<U>)
	  {
	      /* invoke-expr */;
	      return std::expected<U, E>();
	  }
	  else
	      // the returned std::expected object contains an expected value,
	      // which is direct-non-list-initialized with /* invoke-expr */
	      return /* an std::expected<U, E> object */;
      }
      else
	  return 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
	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.

Example
	 This section is incomplete
	 Reason: no example

	Defect reports

	The  following	behavior-changing  defect  reports were applied retroac-
     tively to
	previously published C++ standards.

	   DR	 Applied to		Behavior as published		    Cor-
     rect behavior
	LWG  3938 C++23      transform was ill-formed if T is not (possibly made
     well-formed
			    cv-qualified) void and E is not copyable

See also
			returns the expected itself if it contains  an	expected
     value;
	transform_error  otherwise,  returns  an  expected containing the trans-
     formed unexpected
			value
			(public member function)

Category:
	  * Todo no example

http://cppreference.com 	   2024.06.10	     std::expected::transform(3)
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