Polymorphism and virtual functions

When we discussed public inheritance earlier, we mentioned that a derived object can be used in any place where a base object is expected. Even with this requirement, it's often useful to know what the actual type of the object is—in other words, what type the object was created as:

Derived d;Base& b = d;...b.some_method(); // b is really a Derived object

some_method() is a part of the public interface of the Base class and has to be valid for the Derived class as well. But, within the flexibility allowed by the contract of the base class interface, it can do something different. As an example, we've already used the avian hierarchy before to represent different birds, in particular, birds that can fly. ...

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