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 1:
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 no virtual base classes,
  • has the same access control for 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.104
    [Note 2:
    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 (possibly inherited) non-static data members, the set M(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 1: 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 struct Q {}; struct S : Q { }; struct T : Q { }; struct U : S, T { }; // not a standard-layout class — end example]
A standard-layout struct is a standard-layout class defined with the class-key struct or the class-key class.
A standard-layout union is a standard-layout class defined with the class-key union.
[Note 3:
Standard-layout classes are useful for communicating with code written in other programming languages.
Their layout is specified in [class.mem].
— end note]
[Example 2: struct N { // neither trivial nor standard-layout int i; int j; virtual ~N(); }; struct T { // trivial but not standard-layout int i; private: int j; }; struct SL { // standard-layout but not trivial int i; int j; ~SL(); }; struct POD { // both trivial and standard-layout int i; int j; }; — end example]
[Note 4:
Aggregates of class type are described in [dcl.init.aggr].
— end note]
A class S is an implicit-lifetime class if it is an aggregate or has at least one trivial eligible constructor and a trivial, non-deleted destructor.
This ensures that two subobjects that have the same class type and that belong to the same most derived object are not allocated at the same address ([expr.eq]).