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

NAME
     std::is_empty - std::is_empty

Synopsis
	Defined in header <type_traits>
	template< class T >		 (since C++11)
	struct is_empty;

	std::is_empty is a UnaryTypeTrait.

	If  T is an empty type (that is, a non-union class type with no non-sta-
     tic data
	members other than bit-fields of size 0, no virtual functions,	no  vir-
     tual base
	classes,  and  no  non-empty base classes), provides the member constant
     value equal to
	true. For any other type, value is false.

	If T is an incomplete non-union class type, the behavior is undefined.

	If   the   program   adds   specializations   for    std::is_empty    or
     std::is_empty_v, the
	behavior is undefined.

Template parameters
	T - a type to check

	Helper variable template

	template< class T >					(since C++17)
	inline constexpr bool is_empty_v = is_empty<T>::value;

     Inherited from std::integral_constant

Member constants
	value	 true if T is an empty class type, false otherwise
	[static] (public static member constant)

Member functions
	operator bool converts the object to bool, returns value
		      (public member function)
	operator()    returns value
	(C++14)       (public member function)

Member types
	Type	   Definition
	value_type bool
	type	   std::integral_constant<bool, value>

Notes
	Inheriting from empty base classes usually does not increase the size of
     a class due
	to empty base optimization.

	std::is_empty<T> and all other type traits are empty classes.

Example
     // Run this code

      #include <iostream>
      #include <type_traits>

      struct A {};
      static_assert(std::is_empty_v<A> == true);

      struct B { int m; };
      static_assert(std::is_empty_v<B> == false);

      struct C { static int m; };
      static_assert(std::is_empty_v<C> == true);

      struct D { virtual ~D(); };
      static_assert(std::is_empty_v<D> == false);

      union E {};
      static_assert(std::is_empty_v<E> == false);

      struct F { [[no_unique_address]] E e; };

      struct G
      {
	  int:0;
	  //  C++ standard allow "as a special case, an unnamed bit-field with a
     width of zero
	  // specifies alignment of the next bit-field	at  an	allocation  unit
     boundary.
	  // Only when declaring an unnamed bit-field may the width be zero."
      };
      static_assert(std::is_empty_v<G>);  //  holds  only  unnamed bit-fields of
     zero width

      int main()
      {
	  std::cout << std::boolalpha;
	  std::cout << "F: " << std::is_empty_v<F> << '\n';  //  the  result  is
     ABI-dependent
      }

Possible output:
      F: true

	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 2015 C++11	    the behavior was undefined if the base  characteris-
     tic is
			    T  is  an  incomplete  union type std::false_type in
     this case

See also
	is_class checks if a type is a non-union class type
	(C++11)  (class template)

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