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

NAME
       std::experimental::optional::optional	  -	std::experimental::op-
       tional::optional

Synopsis
	  constexpr optional() noexcept;				  (li-
       brary fundamentals
	  constexpr optional( std::experimental::nullopt_t )	     (1) TS)
	  noexcept;
	  optional(  const optional& other );			      (2) (li-
       brary fundamentals
									 TS)
	  optional( optional&& other ) noexcept(/* see below */);    (3)  (li-
       brary fundamentals
									 TS)
	  constexpr  optional( const T&	value );		      (4) (li-
       brary fundamentals
									 TS)
	  constexpr optional( T&& value	);			     (5)  (li-
       brary fundamentals
									 TS)
	  template<  class... Args >					  (li-
       brary fundamentals
	  constexpr explicit optional(				     (6) TS)
	  std::experimental::in_place_t, Args&&... args	);
	  template< class U, class... Args >

	  constexpr explicit optional(					  (li-
       brary fundamentals
	  std::experimental::in_place_t,			     (7) TS)
	  std::initializer_list<U> ilist,

	  Args&&... args );

	  Constructs a new optional object.

	  1) Constructs	the object that	does not contain a value.
	  2) Copy constructor: If other	contains a value, initializes the con-
       tained value as
	  if  direct-initializing (but not direct-list-initializing) an	object
       of type T with
	  the expression *other. If other does not contain a value, constructs
       an object that
	  does not contain a value.
	  3) Move constructor: If other	contains a value, initializes the con-
       tained value as
	  if direct-initializing (but not direct-list-initializing) an	object
       of type T with
	  the  expression  std::move(*other)  and does not make	other empty: a
       moved-from
	  optional still contains a value, but the value itself	is moved from.
       If other	does
	  not contain a	value, constructs an object that does  not  contain  a
       value.
	  4)  Constructs an optional object that contains a value, initialized
       as if
	  direct-initializing (but not direct-list-initializing) an object  of
       type T with the
	  expression  value.  This constructor is constexpr if the constructor
       of T selected by
	  direct-initialization	is constexpr.
	  5) Constructs	an optional object that	contains a value,  initialized
       as if
	  direct-initializing  (but not	direct-list-initializing) an object of
       type T with the
	  expression std::move(value). This constructor	is  constexpr  if  the
       constructor of T
	  selected by direct-initialization is constexpr.
	  6)  Constructs an optional object that contains a value, initialized
       as if
	  direct-initializing (but not direct-list-initializing) an object  of
       type T from the
	  arguments std::forward<Args>(args)...
	  7)  Constructs an optional object that contains a value, initialized
       as if
	  direct-initializing (but not direct-list-initializing) an object  of
       type T from the
	  arguments  ilist, std::forward<Args>(args).... The function does not
       participate in
	  the overload resolution  if  std::is_constructible<T,	 std::initial-
       izer_list<U>&,
	  Args&&...>::value != true

Parameters
	  other		 -	 another optional object whose contained value
       to copy
	  value		-	value to initialize the	contained value	with
	  args...	-	arguments to initialize	 the  contained	 value
       with
	  ilist		 -	  initializer list to initialize the contained
       value with

Type requirements
	  -
	  T must meet the requirements of CopyConstructible in	order  to  use
       overloads (2,4).
	  -
	  T  must  meet	 the requirements of MoveConstructible in order	to use
       overloads (3,5).

Exceptions
	  2) Throws any	exception thrown by the	constructor of T.
	  3) Throws any	exception thrown by the	constructor of T. Has the fol-
       lowing noexcept
	  declaration:
	  noexcept specification:
	  noexcept(std::is_nothrow_move_constructible<T>::value)
	  .
	  4-7) Throws any exception thrown by the constructor of T.

Example
       // Run this code

	#include <experimental/optional>
	#include <iostream>
	#include <string>
	int main()
	{
	    std::experimental::optional<int> o1, // empty
				 o2 = 1, // init from rvalue
				 o3 = o2; // copy-constructor

	    std::experimental::optional<std::string>	    o4(std::experimen-
       tal::in_place,
							{'a', 'b', 'c'});

	    std::cout << *o2 <<	' ' << *o3 << '	' << *o4 << '\n';
	}

Output:
	1 1 abc

See also
	  make_optional	creates	an optional object
			(function template)

http://cppreference.com		  2022.07.31	 std::experi...al::optional(3)

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