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

Helper variable template

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.

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.

See also