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          Abstract Classes in an Object-Based Hierarchy


     An object-based hierarchy  includes  data  items  derived  from a
     single  abstract  base  class.  In the Turbo C++ class libraries,
     the class Object serves as the root the hierarchy.   This chapter
     describes  the characteristics of the class  Object  and  of  the
     abstract classes derived from this class.

     An abstract class is one from which you can create other classes,
     but one from which you  would not normally create usable objects.
     Conversely, a  non-abstract class, or instance class, is one from
     which you can create usable objects.  An example of this class is
     the String class.  In other words, instance classes are  one step
     removed from  usable objects, whereas abstract classes are two or
     more steps removed from usable objects.