Single Inheritance
To specify a derived class, we define the class as we
learned but we also add the base class' name as part of the
derived class' definition.
We don't need to alter the base classes to specify which
classes are derived from them.
For public derivation where derived is the name of the
derived class and base is the name of the base class:
class derived : public base
{
  public:
    ...
};