NAME¶

std::is_standard_layout - std::is_standard_layout

Synopsis¶

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

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 >
inline constexpr bool is_standard_layout_v = (since C++17)
is_standard_layout<T>::value;

Inherited from std::integral_constant

Member constants¶

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

Member functions¶

operator bool converts the object to bool, returns value
(public member function)
operator() returns value
(C++14) (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 the behavior is
class type with unknown bound undefined in this case

See also¶

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