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std::experi...al::optional(3)  C++ Standard Libary std::experi...al::optional(3)

NAME
     std::experimental::optional - std::experimental::optional

Synopsis
	Defined in header <experimental/optional>
	template< class T >			   (library fundamentals TS)
	class optional;

	The  class template std::experimental::optional manages an optional con-
     tained value,
	i.e. a value that may or may not be present.

	A common use case for optional is the return value of  a  function  that
     may fail. As
	opposed to other approaches, such as std::pair<T,bool>, optional handles
     expensive
	to  construct  objects	well  and is more readable, as the intent is ex-
     pressed
	explicitly.

	Any instance of optional<T> at any given point in time either contains a
     value or
	does not contain a value.

	If an optional<T> contains a value, the value is guaranteed to be  allo-
     cated as part
	of the optional object footprint, i.e. no dynamic memory allocation ever
     takes
	place.	Thus,  an  optional object models an object, not a pointer, even
     though the
	operator*() and operator->() are defined.

	When an object of type optional<T> is contextually  converted  to  bool,
     the conversion
	returns  true  if  the	object contains a value and false if it does not
     contain a
	value.

	The optional object contains a value in the following conditions:

	  * The object is initialized with a value of type T.
	  * The object is assigned from another optional that contains a value.

	The object does not contain a value in the following conditions:

	  * The object is default-initialized.
	  * The object is initialized with a  value  of  std::experimental::nul-
     lopt_t or an
	    optional object that does not contain a value.
	  *  The object is assigned from a value of std::experimental::nullopt_t
     or from an
	    optional that does not contain a value.

Template parameters
	T - the type of the value to manage initialization state for.  The  type
     must meet the
	    requirements of Destructible.

Member types
	Member type Definition
	value_type  T

Member functions
	constructor   constructs the optional object
		      (public member function)
	destructor    destroys the contained value, if there is one
		      (public member function)
	operator=     assigns contents
		      (public member function)

Observers
	operator->    accesses the contained value
	operator*     (public member function)
	operator bool checks whether the object contains a value
		      (public member function)
	value	      returns the contained value
		      (public member function)
	value_or       returns	the  contained value if available, another value
     otherwise
		      (public member function)

Modifiers
	swap	      exchanges the contents
		      (public member function)
	emplace       constructs the contained value in-place
		      (public member function)

Member objects
	Member name   Definition
	val (private) pointer to the contained value (which  points  at  a  data
     member of the
		      same object), the name is for exposition only

Non-member functions
	operator==
	operator!=
	operator<			       compares optional objects
	operator<=			       (function template)
	operator>
	operator>=
	make_optional			       creates an optional object
					       (function template)
	std::swap(std::experimental::optional)	specializes  the std::swap algo-
     rithm
					       (function)

Helper classes
	std::hash<std::experimental::optional> specializes the	std::hash  algo-
     rithm
					       (class template specialization)
	nullopt_t				indicator  of optional type with
     uninitialized
	(library fundamentals TS)	       state
					       (class)
	in_place_t			       disambiguation tag type	for  in-
     place
	(library fundamentals TS)	       construction of optional types
					       (class)
	bad_optional_access			exception indicating checked ac-
     cess to an
	(library fundamentals TS)	       optional that doesn't  contain  a
     value
					       (class)

	Helper objects

	nullopt 		  an object of type nullopt_t
	(library fundamentals TS) (function)
	in_place		     an    object    of   type	 std::experimen-
     tal::in_place_t
	(library fundamentals TS) (function)

Category:
	  * Noindexed pages

http://cppreference.com 	   2024.06.10	   std::experi...al::optional(3)
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