# C++ concepts: StandardLayoutType

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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

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

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