class hierarchy
base class
derived class
subclass
superclass

    In object-oriented programming, a set of classes
   related by inheritance.  Each class is a "subclass" of another
   class - its "superclass".  The subclass contains all the features
   of its superclass, but may add new features or redefine existing
   features.  The features of a class are the set of attributes (or
   "properties") that an object of that class has and the methods
   that can be invoked on it.

   If each class has a just one superclass, this is called single
   inheritance.  The opposite is multiple inheritance, under which
   a class may have multiple superclasses.  Single inheritance gives
   the class hierarchy a tree structure whereas multiple
   inheritance gives a directed graph.  Typically there is one
   class at the top of the hierarchy which is the "object" class, the
   most general class that is an ancestor of all others and which has
   no superclass.

   In computing, as in genealogy, trees grow downwards, which is why
   subclasses are considered to be "below" their superclasses.

   When invoking a method on an object, the method is first
   looked for in the object's class, then the superclass of that
   class, and so on up the hierarchy until it is found.  Thus a class
   need only define those methods which are specific to it and it
   will inherit all other methods from all its superclasses.  An
   object of the subclass can do everything that an object of the
   superclass can and possible more.

   C++ calls the superclass the "base class" and the subclass the
   "derived class" (not to be confused with a derived type).

   (2014-09-06)