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

NAME
       std::is_standard_layout - std::is_standard_layout

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

	  If  T	 is a standard-layout type, provides the member	constant value
       equal to	true.
	  For any other	type, value is false.

	  The behavior is undefined if std::remove_all_extents_t<T> is an  in-
       complete	type and
	  not (possibly	cv-qualified) void.

	  The  behavior	 of  a	program	that adds specializations for is_stan-
       dard_layout
	  or is_standard_layout_v
	  (since C++17)	is undefined.

Template parameters
	  T - a	type to	check

	 Helper	variable template

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

       Inherited from std::integral_constant

Member constants
	  value	   true	if T is	a standard-layout 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
	  A pointer to a standard-layout class may be converted	(with reinter-
       pret_cast) to a
	  pointer to its first non-static data member and vice versa.

	  If  a	 standard-layout  union	 holds	two  or	 more  standard-layout
       structs,	it is
	  permitted to inspect the common initial part of them.

	  The macro offsetof is	only guaranteed	to be  usable  with  standard-
       layout classes.

Example
       // Run this code

	#include <iostream>
	#include <type_traits>

	struct A
	{
	    int	m;
	};

	struct B
	{
	    int	m1;
	private:
	    int	m2;
	};

	struct C
	{
	    virtual void foo();
	};

	int main()
	{
	    std::cout << std::boolalpha;
	    std::cout << std::is_standard_layout<A>::value << '\n';
	    std::cout << std::is_standard_layout<B>::value << '\n';
	    std::cout << std::is_standard_layout<C>::value << '\n';
	}

Output:
	true
	false
	false

See also
	  is_trivially_copyable	       checks if a type	is trivially copyable
	  (C++11)		       (class template)
	  is_pod			checks	if  a type is a	plain-old data
       (POD) type
	  (C++11)(deprecated in	C++20) (class template)
				       byte offset from	 the  beginning	 of  a
       standard-layout
	  offsetof		       type to specified member
				       (function macro)

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

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