C++ Primer, Fifth Edition

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 Pandavoid print(const Bear&);void highlight(const Endangered&);ostream& operator<<(ostream&, const ZooAnimal&);Panda ying_yang("ying_yang");print(ying_yang);     // passes Panda