std::is_standard_layout
| 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
#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 incompleteclass 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) |