StandardLayoutType

Specifies that a type is standard layout type. Standard layout types are useful for communicating with code written in other programming languages.

Note, that the standard doesn't define a named requirement or concept with this name. This is a type category defined by the core language. It is included here as concept only for consistency.

Requirements

All non-static data members have the same access control
Has no virtual functions or virtual base classes
All non-static data members and base classes are themselves standard layout types

Either
- has no base classes with non-static data members, or
- has no non-static data members in the most derived class and at most one base class with non-static data members
Has no base classes of the same type as the first non-static data member (see empty base optimization)

(until C++14)

Has no two base class subobjects of the same type

struct Q {};
struct S : Q { };
struct T : Q { };
struct U : S, T { };         // not a standard-layout class

Has all non-static data members declared in the same class (either all in the derived or all in some base)

struct B { int i; };         // standard-layout class
struct C : B { };            // standard-layout class
struct D : C { };            // standard-layout class
struct E : D { char : 4; };  // not a standard-layout class

None of the base class subobjects has the same type as
- for non-union types, as the first non-static data member (see empty base optimization), and, recursively, the first non-static data member of that data member if it has non-union class type, or all non-static data members of that data member if it has union type, or an element of that data member if it has array type, etc.
- for union types, as any non-static data members, and, recursively, the first non-static data member of every member of non-union class type, and all non-static data members of all members of union type, and element type of all non-static data members of array type, etc.
- for array types, as the type of the array element, and, recursively, the first non-static data member of the array element if it has non-union class type, or as any non-static data member of the array element if it has union type, or as the element type of the array element if it has array type, etc.

(since C++14)

Properties

See Standard layout.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 1672	C++14	first non-static data member rule ignored existence of empty base classes	first non-static data member rule made recursive
CWG 1813	C++14	class with a member defined in an indirect base wasn't technically standard-layout	all member declarations must be in the same class
CWG 2120	C++14	array as first member wasn't considered when comparing the first element type with type of a base	array members are considered

StandardLayoutType

Requirements

Properties

Defect reports

See also