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std::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.

Contents

[edit] Template parameters

T - a type to check

[edit] Helper variable template

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

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>

[edit] Notes

A pointer to a standard-layout class may be converted (with reinterpret_cast) to a pointer to its first non-static data member (see empty base optimization) 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.

[edit] 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() {}

[edit] 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

[edit] See also

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