18.3.2. Conversions and Multiple Base Classes
Under single inheritance, a pointer or a reference to a derived class can be converted automatically to a pointer or a reference to an accessible base class (§ 15.2.2, p. 597, and § 15.5, p. 613). The same holds true with multiple inheritance. A pointer or reference to any of an object’s (accessible) base classes can be used to point or refer to a derived object. For example, a pointer or reference to ZooAnimal, Bear, or Endangered can be bound to a Panda object:
// operations that take references to base classes of type Panda
void print(const Bear&);
void highlight(const Endangered&);
ostream& operator<<(ostream&, const ZooAnimal&);
Panda ying_yang("ying_yang");
print(ying_yang); // passes Panda to a reference to Bear
highlight(ying_yang); // passes Panda to a reference to Endangered
cout << ying_yang << endl; // passes Panda to a reference to ZooAnimal
Exercises Section 18.3.1
Exercise 18.21: Explain the following declarations. Identify any that are in error and explain why they are incorrect:
(a) class CADVehicle : public CAD, Vehicle { ... };
(b) class DblList: public List, public List { ... };
(c) class iostream: public istream, public ostream { ... };
Exercise 18.22: Given the following class hierarchy, in which each class defines a default constructor:
class A { ... };
class B : public A { ... };
class C : public B { ... };
class X { ... };
class Y { ... };
class Z : public X, public Y { ... };
class MI : public C, public Z { ... };
what is the order of constructor execution for the following definition?
MI mi;
The compiler makes no attempt to distinguish between base classes in terms of a derived-class conversion. Converting to each base class is equally good. For example, if there was an overloaded version of print:
void print(const Bear&);
void print(const Endangered&);
an unqualified call to print with a Panda object would be a compile-time error:
Panda ying_yang("ying_yang");
print(ying_yang); // error: ambiguous
Lookup Based on Type of Pointer or Reference
As with single inheritance, the static type of the object, pointer, or reference determines which members we can use (§ 15.6, p. 617). If we use a ZooAnimal pointer, only the operations defined in that class are usable. The Bear-specific, Panda-specific, and Endangered portions of the Panda interface are invisible. Similarly, a Bear pointer or reference knows only about the Bear and ZooAnimal members; an Endangered pointer or reference is limited to the Endangered members.
As an example, consider the following calls, which assume that our classes define the virtual functions listed in Table 18.1:
Bear *pb = new Panda("ying_yang");
pb->print(); // ok: Panda::print()
pb->cuddle(); // error: not part of the Bear interface
pb->highlight(); // error: not part of the Bear interface
delete pb; // ok: Panda::~Panda()
When a Panda is used via an Endangered pointer or reference, the Panda-specific and Bear portions of the Panda interface are invisible:
Endangered *pe = new Panda("ying_yang");
pe->print(); // ok: Panda::print()
pe->toes(); // error: not part of the Endangered interface
pe->cuddle(); // error: not part of the Endangered interface
pe->highlight(); // ok: Panda::highlight()
delete pe; // ok: Panda::~Panda()
Table 18.1. Virtual Functions in the ZooAnimal/Endangered Classes
