[class.prop] (original) (raw)

11 Classes [class]

11.2 Properties of classes [class.prop]

A trivially copyable class is a class:

• that has at least one eligible copy constructor, move constructor, copy assignment operator, or move assignment operator ([special], [class.copy.ctor], [class.copy.assign]),
• where each eligible copy constructor, move constructor, copy assignment operator, and move assignment operator is trivial, and
• that has a trivial, non-deleted destructor ([class.dtor]).

A trivial class is a class that is trivially copyable and has one or more eligible default constructors ([class.default.ctor]), all of which are trivial.

[ Note

:

In particular, a trivially copyable or trivial class does not have virtual functions or virtual base classes.

— end note

]

A class S is a standard-layout class if it:

• has no non-static data members of type non-standard-layout class (or array of such types) or reference,
• has no virtual functions and novirtual base classes,
• has the same access controlfor all non-static data members,
• has no non-standard-layout base classes,
• has at most one base class subobject of any given type,
• has all non-static data members and bit-fields in the class and its base classes first declared in the same class, and
• has no element of the set M(S) of types as a base class, where for any type X, M(X) is defined as follows.101
  [ Note
  : M(X) is the set of the types of all non-base-class subobjects that may be at a zero offset in X. — end note
  ]
  • If X is a non-union class type with no (possiblyinherited) non-static data members, the setM(X) is empty.
  • If X is a non-union class type with a non-static data member of type that is either of zero size or is the first non-static data member of X(where said member may be an anonymous union), the set M(X) consists of and the elements of.
  • If X is a union type, the set M(X) is the union of all and the set containing all , where each is the type of the non-static data member of X.
  • If X is an array type with element type , the set M(X) consists of and the elements of .
  • If X is a non-class, non-array type, the set M(X) is empty.

[ Example

:

struct B { int i; };
struct C : B { };
struct D : C { };
struct E : D { char : 4; };

struct Q {}; struct S : Q { }; struct T : Q { }; struct U : S, T { };

— end example

]

[ Note

:

Standard-layout classes are useful for communicating with code written in other programming languages.

— end note

]

[ Example

:

struct N {
int i; int j; virtual ~N(); };

struct T {
int i; private: int j; };

struct SL {
int i; int j; ~SL(); };

struct POD {
int i; int j; };

— end example

]

A class S is an implicit-lifetime classif it is an aggregate or has at least one trivial eligible constructor and a trivial, non-deleted destructor.