11 Classes [class]

A class-or-decltype shall denote a (possibly cv-qualified) class type that is not an incompletely defined class ([class.mem]); any cv-qualifiers are ignored.

The class denoted by the class-or-decltype of a base-specifier is called a direct base class for the class being defined; for each such base-specifier, the corresponding direct base class relationship is the ordered pair (D, B) where D is the class being defined and B is the direct base class.

The lookup for the component name of the type-name or simple-template-id is type-only ([basic.lookup]).

A class B is a base class of a class D if it is a direct base class of D or a direct base class of one of D's base classes.

A class is an indirect base class of another if it is a base class but not a direct base class.

A class is said to be (directly or indirectly) derived from its (direct or indirect) base classes.

[Note 1:

See [class.access] for the meaning of access-specifier.

— end note]

Members of a base class are also members of the derived class.

[Note 2:

Constructors of a base class can be explicitly inherited ([namespace.udecl]).

Base class members can be referred to in expressions in the same manner as other members of the derived class, unless their names are hidden or ambiguous ([class.member.lookup]).

The scope resolution operator :: ([expr.prim.id.qual]) can be used to refer to a direct or indirect base member explicitly, even if it is hidden in the derived class.

A derived class can itself serve as a base class subject to access control; see [class.access.base].

A pointer to a derived class can be implicitly converted to a pointer to an accessible unambiguous base class ([conv.ptr]).

An lvalue of a derived class type can be bound to a reference to an accessible unambiguous base class ([dcl.init.ref]).

— end note]

The base-specifier-list specifies the type of the base class subobjects contained in an object of the derived class type.

[Example 1: struct Base { int a, b, c; };

struct Derived : Base { int b; };

struct Derived2 : Derived { int c; };

Here, an object of class Derived2 will have a subobject of class Derived which in turn will have a subobject of class Base.

— end example]

A base-specifier followed by an ellipsis is a pack expansion ([temp.variadic]).

The order in which the base class subobjects are allocated in the most derived object ([intro.object]) is unspecified.

[Note 3:

A derived class and its base class subobjects can be represented by a directed acyclic graph (DAG) where an arrow means “directly derived from” (see Figure 3).

An arrow need not have a physical representation in memory.

A DAG of subobjects is often referred to as a “subobject lattice”.

— end note]

Figure 3 — Directed acyclic graph [fig:class.dag]

[Note 4:

Initialization of objects representing base classes can be specified in constructors; see [class.base.init].

— end note]

[Note 5:

A base class subobject can have a layout different from the layout of a most derived object of the same type.

A base class subobject can have a polymorphic behavior ([class.cdtor]) different from the polymorphic behavior of a most derived object of the same type.

A base class subobject can be of zero size; however, two subobjects that have the same class type and that belong to the same most derived object cannot be allocated at the same address ([intro.object]).

— end note]