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C++ concepts: 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
  • Has no non-static data members of reference type
  • 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

See also

(C++11)
 checks if a type is standard-layout type
(class template)

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