std:: is_standard_layout

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

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 >
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

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