C++ named requirements: StandardLayoutType

 

C++

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Named requirements

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Technical Specifications

 

C++ named requirements

Basic
DefaultConstructible CopyAssignable CopyConstructible	MoveAssignable (C++11) MoveConstructible (C++11) Destructible
Layout
TrivialType (C++11) TriviallyCopyable (C++11)	StandardLayoutType (C++11) PODType
Library-Wide
EqualityComparable LessThanComparable Swappable (C++11) ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11)	Allocator FunctionObject Callable Predicate BinaryPredicate Compare
Container
Container SequenceContainer ContiguousContainer (C++17) ReversibleContainer	AssociativeContainer AllocatorAwareContainer UnorderedAssociativeContainer (C++11)
Container Elements
DefaultInsertable (C++11) CopyInsertable (C++11) MoveInsertable (C++11)	EmplaceConstructible (C++11) Erasable (C++11)
Iterator
LegacyIterator LegacyOutputIterator LegacyInputIterator LegacyForwardIterator	LegacyBidirectionalIterator LegacyRandomAccessIterator LegacyContiguousIterator (C++17) ConstexprIterator (C++20)
Stream I/O
FormattedInputFunction UnformattedInputFunction	FormattedOutputFunction UnformattedOutputFunction
Random Numbers
SeedSequence (C++11) RandomNumberEngine (C++11)	RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11) RandomNumberEngineAdaptor (C++11)
Concurrency
BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11)	Mutex (C++11) TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14)
Other
CharTraits RegexTraits (C++11) BitmaskType NumericType LiteralType (C++11)	UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

 

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

Note: the standard doesn't define a named requirement with this name. This is a type category defined by the core language. It is included here as a named requirement 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 (possibly indirect) base class subobjects of the same type struct Q {}; struct S : Q { }; struct T : Q { }; struct U : S, T { }; // not a standard-layout class: two base class subobjects of type Q Has all non-static data members and bit-fields 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)