home | help 
std::expected::and_then(3)     C++ Standard Libary    std::expected::and_then(3)

NAME
     std::expected::and_then - std::expected::and_then

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

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

	If T is not (possibly cv-qualified) void, the contained  expected  value
     (obtained
	from  operator*) is passed as an argument to f; otherwise f takes no ar-
     gument.

	1,2) Given type U as
	  * std::remove_cvref_t<std::invoke_result_t<F>> if T is  (possibly  cv-
     qualified)
	    void, or
	  *  std::remove_cvref_t<std::invoke_result_t<F, decltype(**this)>> oth-
     erwise.
	If U is not a specialization of std::expected, or  std::is_same_v<U::er-
     ror_type, E>
	is false, the program is ill-formed.
	The effect is equivalent to

      if (has_value())
      {
	  if constexpr (std::is_void_v<T>)
	      return std::invoke(std::forward<F>(f));
	  else
	      return std::invoke(std::forward<F>(f), **this);
      }
      else
	  return U(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_cvref_t<std::invoke_result_t<F>>  if T is (possibly cv-
     qualified)
	    void, or
	  *	      std::remove_cvref_t<std::invoke_result_t<F,	     de-
     cltype(std::move(**this))>>
	    otherwise.
	If  U is not a specialization of std::expected, or std::is_same_v<U::er-
     ror_type, E>
	is false, the program is ill-formed.
	The effect is equivalent to

      if (has_value())
      {
	  if constexpr (std::is_void_v<T>)
	      return std::invoke(std::forward<F>(f));
	  else
	      return std::invoke(std::forward<F>(f), std::move(**this));
      }
      else
	  return U(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 that returns a std::expected

Return value
	The result of f or a std::expected object that contains an error  value,
     as described
	above.

Notes
	Feature-test macro  Value    Std		 Feature
	__cpp_lib_expected 202211L (C++23) Monadic functions for std::expected

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

See also
	unexpect   in-place construction tag for unexpected value in expected
	unexpect_t (tag)
	(C++23)
		   returns an expected containing the transformed expected value
     if it
	transform  exists; otherwise, returns the expected itself
		   (public member function)

Category:
	  * Todo no example

http://cppreference.com 	   2024.06.10	      std::expected::and_then(3)
home | help