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std::optional(3)	       C++ Standard Libary		std::optional(3)

NAME
     std::optional - std::optional

Synopsis
	Defined in header <optional>
	template< class T >	      (since C++17)
	class optional;

	The  class  template  std::optional manages an optional contained 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 han-
     dles
	expensive-to-construct objects well and is more readable, as the  intent
     is expressed
	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
	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/assigned from a value of type T or
     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/assigned	from  a  value	of  type
     std::nullopt_t or
	    an optional object that does not contain a value.
	  * The member function reset() is called.

	There  are no optional references, functions, arrays, or cv void; a pro-
     gram is
	ill-formed if it instantiates an optional with such a type. In addition,
     a program
	is ill-formed if it instantiates an optional with the (possibly cv-qual-
     ified) tag
	types std::nullopt_t or std::in_place_t.

Template parameters
	    the type of the value to manage initialization state for.  The  type
     must meet the
	T  -  requirements  of	Destructible (in particular, array and reference
     types are not
	    allowed).

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
	has_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)
	      Monadic operations
	and_then       returns the result of the given function on the contained
     value if it
	(C++23)       exists, or an empty optional otherwise
		      (public member function)
	transform     returns an optional containing the  transformed  contained
     value if it
	(C++23)       exists, or an empty optional otherwise
		      (public member function)
	or_else       returns the optional itself if it contains a value, or the
     result of
	(C++23)       the given function otherwise
		      (public member function)

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

Non-member functions
	operator==
	operator!=
	operator<
	operator<=
	operator>
	operator>=
	operator<=>		 compares optional objects
	(C++17) 		 (function template)
	(C++17)
	(C++17)
	(C++17)
	(C++17)
	(C++17)
	(C++20)
	make_optional		 creates an optional object
	(C++17) 		 (function template)
	std::swap(std::optional) specializes the std::swap algorithm
	(C++17) 		 (function template)

Helper classes
	std::hash<std::optional> hash support for std::optional
	(C++17) 		 (class template specialization)
	nullopt_t		  indicator  of optional type with uninitialized
     state
	(C++17) 		 (class)
	bad_optional_access	 exception indicating checked access to  an  op-
     tional that
	(C++17) 		 doesn't contain a value
				 (class)

	Helpers

	nullopt 	 an object of type nullopt_t
	(C++17) 	 (constant)
	in_place
	in_place_type
	in_place_index	 in-place construction tag
	in_place_t	 (tag)
	in_place_type_t
	in_place_index_t
	(C++17)

	Deduction guides

Notes
	Feature-test macro  Value    Std	Feature
	__cpp_lib_optional 201606L (C++17) std::optional
	__cpp_lib_optional 202106L (C++20) Fully constexpr
				   (DR)
	__cpp_lib_optional 202110L (C++23) Monadic operations

Example
     // Run this code

      #include <iostream>
      #include <optional>
      #include <string>

      // optional can be used as the return type of a factory that may fail
      std::optional<std::string> create(bool b)
      {
	  if (b)
	      return "Godzilla";
	  return {};
      }

      // std::nullopt can be used to create any (empty) std::optional
      auto create2(bool b)
      {
	  return b ? std::optional<std::string>{"Godzilla"} : std::nullopt;
      }

      int main()
      {
	  std::cout << "create(false) returned "
		    << create(false).value_or("empty") << '\n';

	  //  optional-returning  factory  functions are usable as conditions of
     while and if
	  if (auto str = create2(true))
	      std::cout << "create2(true) returned " << *str << '\n';
      }

Output:
      create(false) returned empty
      create2(true) returned Godzilla

See also
	variant a type-safe discriminated union
	(C++17) (class template)
	any	objects that hold instances of any CopyConstructible type
	(C++17) (class)

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