11 Classes [class]

11.7 Derived classes [class.derived]

11.7.2 Multiple base classes [class.mi]

A class can be derived from any number of base classes.

[Note 1:

The use of more than one direct base class is often called multiple inheritance.

— end note]

[Example 1: class A { /* ... */ }; class B { /* ... */ }; class C { /* ... */ }; class D : public A, public B, public C { /* ... */ }; — end example]

[Note 2:

The order of derivation is not significant except as specified by the semantics of initialization by constructor ([class.base.init]), cleanup ([class.dtor]), and storage layout ([class.mem], [class.access.spec]).

— end note]

A class shall not be specified as a direct base class of a derived class more than once.

[Note 3:

A class can be an indirect base class more than once and can be a direct and an indirect base class.

There are limited things that can be done with such a class; lookup that finds its non-static data members and member functions in the scope of the derived class will be ambiguous.

However, the static members, enumerations and types can be unambiguously referred to.

— end note]

[Example 2: class X { /* ... */ }; class Y : public X, public X { /* ... */ }; // error class L { public: int next; /* ... */ }; class A : public L { /* ... */ }; class B : public L { /* ... */ }; class C : public A, public B { void f(); /* ... */ }; // well-formed class D : public A, public L { void f(); /* ... */ }; // well-formed — end example]

A base class specifier that does not contain the keyword virtual specifies a non-virtual base class.

A base class specifier that contains the keyword virtual specifies a virtual base class.

For each distinct occurrence of a non-virtual base class in the class lattice of the most derived class, the most derived object ([intro.object]) shall contain a corresponding distinct base class subobject of that type.

For each distinct base class that is specified virtual, the most derived object shall contain a single base class subobject of that type.

[Note 4:

For an object of class type C, each distinct occurrence of a (non-virtual) base class L in the class lattice of C corresponds one-to-one with a distinct L subobject within the object of type C.

Given the class C defined above, an object of class C will have two subobjects of class L as shown in Figure 4 .

Figure 4: Non-virtual base [fig:class.nonvirt]

In such lattices, explicit qualification can be used to specify which subobject is meant.

The body of function C::f can refer to the member next of each L subobject: void C::f() { A::next = B::next; } // well-formed

Without the A:: or B:: qualifiers, the definition of C::f above would be ill-formed because of ambiguity ([class.member.lookup]).

— end note]

[Note 5:

In contrast, consider the case with a virtual base class: class V { /* ... */ }; class A : virtual public V { /* ... */ }; class B : virtual public V { /* ... */ }; class C : public A, public B { /* ... */ };

Figure 5: Virtual base [fig:class.virt]

For an object c of class type C, a single subobject of type V is shared by every base class subobject of c that has a virtual base class of type V.

Given the class C defined above, an object of class C will have one subobject of class V, as shown in Figure 5 .

— end note]

[Note 6:

A class can have both virtual and non-virtual base classes of a given type.

class B { /* ... */ }; class X : virtual public B { /* ... */ }; class Y : virtual public B { /* ... */ }; class Z : public B { /* ... */ }; class AA : public X, public Y, public Z { /* ... */ };

For an object of class AA, all virtual occurrences of base class B in the class lattice of AA correspond to a single B subobject within the object of type AA, and every other occurrence of a (non-virtual) base class B in the class lattice of AA corresponds one-to-one with a distinct B subobject within the object of type AA.

Given the class AA defined above, class AA has two subobjects of class B: Z's B and the virtual B shared by X and Y, as shown in Figure 6 .

Figure 6: Virtual and non-virtual base [fig:class.virtnonvirt]

— end note]