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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;

	std::is_standard_layout is a UnaryTypeTrait.

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

	If std::remove_all_extents_t<T> is an incomplete type and not (possibly
	cv-qualified) void, the behavior is undefined.

	If the program adds specializations for std::is_standard_layout or
	std::is_standard_layout_v, the behavior 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 <type_traits>

      struct A { int m; };
      static_assert(std::is_standard_layout_v<A> == true);

      class B: public A { int m; };
      static_assert(std::is_standard_layout_v<B> == false);

      struct C { virtual void foo(); };
      static_assert(std::is_standard_layout_v<C> == false);

      int main() {}

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	  Applied to	   Behavior as published	  Correct behav-
     ior
	LWG 2015 C++11	    T could be an array of incomplete the behavior is
			    class type with unknown bound     undefined in  this
     case

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  stan-
     dard-layout
	offsetof		     type to specified member
				     (function macro)

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