C++ concepts: StandardLayoutType - cppreference.com

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# C++ concepts: StandardLayoutType

From cppreference.com
< cpp‎ | concept

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.

### [edit] 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)

### [edit] Properties

See Standard layout.

### [edit] 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

### [edit] See also

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

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