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[class.derived.general]

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+ ### General <a id="class.derived.general">[[class.derived.general]]</a>
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+
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+ A list of base classes can be specified in a class definition using the
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+ notation:
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+
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+ ``` bnf
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+ base-clause:
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+ ':' base-specifier-list
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+ ```
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+
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+ ``` bnf
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+ base-specifier-list:
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+ base-specifier '...'ₒₚₜ
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+ base-specifier-list ',' base-specifier '...'ₒₚₜ
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+ ```
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+
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+ ``` bnf
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+ base-specifier:
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+ attribute-specifier-seqₒₚₜ class-or-decltype
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+ attribute-specifier-seqₒₚₜ virtual access-specifierₒₚₜ class-or-decltype
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+ attribute-specifier-seqₒₚₜ access-specifier virtualₒₚₜ class-or-decltype
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+ ```
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+
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+ ``` bnf
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+ class-or-decltype:
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+ nested-name-specifierₒₚₜ type-name
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+ nested-name-specifier template simple-template-id
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+ decltype-specifier
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+ ```
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+
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+ ``` bnf
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+ access-specifier:
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+ private
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+ protected
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+ public
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+ ```
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+
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+ The optional *attribute-specifier-seq* appertains to the
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+ *base-specifier*.
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+
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+ The component names of a *class-or-decltype* are those of its
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+ *nested-name-specifier*, *type-name*, and/or *simple-template-id*. A
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+ *class-or-decltype* shall denote a (possibly cv-qualified) class type
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+ that is not an incompletely defined class [[class.mem]]; any
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+ cv-qualifiers are ignored. The class denoted by the *class-or-decltype*
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+ of a *base-specifier* is called a *direct base class* for the class
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+ being defined. The lookup for the component name of the *type-name* or
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+ *simple-template-id* is type-only [[basic.lookup]]. A class `B` is a
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+ base class of a class `D` if it is a direct base class of `D` or a
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+ direct base class of one of `D`’s base classes. A class is an
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+ *indirect base class* of another if it is a base class but not a direct
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+ base class. A class is said to be (directly or indirectly) *derived*
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+ from its (direct or indirect) base classes.
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+
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+ [*Note 1*: See [[class.access]] for the meaning of
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+ *access-specifier*. — *end note*]
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+
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+ Members of a base class are also members of the derived class.
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+
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+ [*Note 2*: Constructors of a base class can be explicitly inherited
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+ [[namespace.udecl]]. Base class members can be referred to in
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+ expressions in the same manner as other members of the derived class,
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+ unless their names are hidden or ambiguous [[class.member.lookup]]. The
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+ scope resolution operator `::` [[expr.prim.id.qual]] can be used to
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+ refer to a direct or indirect base member explicitly, even if it is
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+ hidden in the derived class. A derived class can itself serve as a base
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+ class subject to access control; see  [[class.access.base]]. A pointer
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+ to a derived class can be implicitly converted to a pointer to an
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+ accessible unambiguous base class [[conv.ptr]]. An lvalue of a derived
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+ class type can be bound to a reference to an accessible unambiguous base
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+ class [[dcl.init.ref]]. — *end note*]
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+
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+ The *base-specifier-list* specifies the type of the *base class
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+ subobjects* contained in an object of the derived class type.
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ struct Base {
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+ int a, b, c;
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+ };
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+ ```
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+
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+ ``` cpp
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+ struct Derived : Base {
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+ int b;
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+ };
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+ ```
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+
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+ ``` cpp
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+ struct Derived2 : Derived {
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+ int c;
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+ };
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+ ```
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+
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+ Here, an object of class `Derived2` will have a subobject of class
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+ `Derived` which in turn will have a subobject of class `Base`.
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+
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+ — *end example*]
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+
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+ A *base-specifier* followed by an ellipsis is a pack expansion
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+ [[temp.variadic]].
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+
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+ The order in which the base class subobjects are allocated in the most
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+ derived object [[intro.object]] is unspecified.
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+
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+ [*Note 3*: A derived class and its base class subobjects can be
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+ represented by a directed acyclic graph (DAG) where an arrow means
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+ “directly derived from” (see Figure [[fig:class.dag]]). An arrow
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+ need not have a physical representation in memory. A DAG of subobjects
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+ is often referred to as a “subobject lattice”. — *end note*]
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+
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+ <a id="fig:class.dag"></a>
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+
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+ ![Directed acyclic graph \[fig:class.dag\]](images/figdag.svg)
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+
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+ [*Note 4*: Initialization of objects representing base classes can be
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+ specified in constructors; see  [[class.base.init]]. — *end note*]
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+
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+ [*Note 5*: A base class subobject can have a layout different from the
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+ layout of a most derived object of the same type. A base class subobject
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+ can have a polymorphic behavior [[class.cdtor]] different from the
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+ polymorphic behavior of a most derived object of the same type. A base
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+ class subobject can be of zero size; however, two subobjects that have
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+ the same class type and that belong to the same most derived object
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+ cannot be allocated at the same address [[intro.object]]. — *end note*]
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+