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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;

	  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 a non-union class type, T shall be a complete	 type;	other-
       wise, the behavior
	  is undefined.

	  The behavior of a program that adds specializations for is_empty
	  or is_empty_v
	  (since C++17)	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 {};

	struct B {
	    int	m;
	};

	struct C {
	    static int m;
	};

	struct D {
	    virtual ~D();
	};

	union E	{};

	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."
	};

	int main()
	{
	    std::cout << std::boolalpha;
	    std::cout << "A " << std::is_empty<A>::value << '\n';
	    std::cout << "B " << std::is_empty<B>::value << '\n';
	    std::cout << "C " << std::is_empty<C>::value << '\n';
	    std::cout << "D " << std::is_empty<D>::value << '\n';
	    std::cout << "E " << std::is_empty<E>::value << '\n';
	    std::cout  <<  "F "	<< std::is_empty<F>::value << '\n'; // the re-
       sult is ABI-dependent
	    std::cout << "G " << std::is_empty<G>::value << '\n';  //  unnamed
       bit-fields of width of 0
	}

Possible output:
	A true
	B false
	C true
	D false
	E false
	F true
	G true

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

http://cppreference.com		  2022.07.31		      std::is_empty(3)

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