std::is_standard_layout

 

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integer_sequence (C++14)

 

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

std::is_standard_layout is an UnaryTypeTrait.

If T is a standard-layout type, the base characteristic is std::true_type. For any other type, the base characteristic is std::false_type.

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

The behavior of a program that adds specializations for std::is_standard_layout or std::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 = is_standard_layout<T>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

true if T is a standard-layout type, false otherwise (public static member constant)

Member functions

operator bool	converts the object to bool, returns value (public member function)
operator() (C++14)	returns value (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 reinterpret_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 retroactively to previously published C++ standards.

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

See also

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