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

NAME
     std::expected::or_else - std::expected::or_else

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

	If  *this  contains an unexpected value, invokes f with the argument er-
     ror() and
	returns its result; otherwise, returns a std::expected object that  con-
     tains a copy
	of the contained expected value (obtained from operator*).

	1,2) Given type G as std::remove_cvref_t<std::invoke_result_t<F,
	decltype(error())>>.
	If    G    is	 not	a    specialization    of    std::expected,   or
     std::is_same_v<G::value_type, T>
	is false, the program is ill-formed.
	The effect is equivalent to

      if (has_value())
      {
	  if constexpr (std::is_void_v<T>)
	      return G();
	  else
	      return G(std::in_place, **this);
      }
      else
	  return std::invoke(std::forward<F>(f), error());

	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_cvref_t<std::invoke_result_t<F,
	decltype(std::move(error()))>>.
	If    G    is	 not	a    specialization    of    std::expected,   or
     std::is_same_v<G::value_type, T>
	is false, the program is ill-formed.
	The effect is equivalent to

      if (has_value())
      {
	  if constexpr (std::is_void_v<T>)
	      return G();
	  else
	      return G(std::in_place, std::move(**this));
      }
      else
	  return std::invoke(std::forward<F>(f), std::move(error()));

	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 that returns a std::expected

Return value
	The  result  of f, or a std::expected object that contains a copy of the
     expected
	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		 Correct
     behavior
	LWG  3938  C++23       or_else 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::or_else(3)
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