11 Classes [class]

11.7 Derived classes [class.derived]

11.7.1 General [class.derived.general]

A list of base classes can be specified in a class definition using the notation:
The optional attribute-specifier-seq appertains to the base-specifier.
The component names of a class-or-decltype are those of its nested-name-specifier, type-name, and/or simple-template-id.
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.
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]
dag Base Base Derived1 Derived1 Derived1->Base Derived2 Derived2 Derived2->Derived1
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]