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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-
       static data
	  members other	than bit-fields	of size	0, no  virtual	functions,  no
       virtual base
	  classes,  and	 no  non-empty base classes), provides the member con-
       stant value equal to
	  true.	For any	other type, value is false.

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

	  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  be-
       havior
	  LWG  2015  C++11	the behavior was undefined if the base charac-
       teristic	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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