< cpp‎ | types
Metaprogramming library
Type traits
Type categories
Type properties
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
Type trait constants
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
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.


[edit] Template parameters

T - a type to check

[edit] 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

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)
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 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

#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

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]