CS 5535: Homework 3

1. One of the points in [Snyder86] is that instance variables of a class should not all be visible to its subclasses (i.e., not "child accesses all").  Give two examples, one in Modula-3 and one in Java of classes where this is enforced (i.e., the child cannot access all the instance variables of its superclass). (10 points per example.  Total: 20)
2. [Snyder86] distinguishes two kinds of inheritance: inheritance for subtypes and inheritance for reuse.  Indicate which category the following languages fall into: (a) Smalltalk (b) Java (c) Modula-3.  If a language does not cleanly fit into a single category, indicate that. (5 points per language.  Total: 15)
3. Consider the following class hierarchy in a single inheritance language.  Assume all methods are virtual: (total points: 20)

    class T { f() {...}
                   g() {...}
     };
     class S: public T {g() {...} }
Answer the following questions:
  (a) Give the virtual function table for each of the two classes. (10 points)
  (b) Give the steps involved in invoking the "g" method on a variable "o" declared to point to S (i.e., S *o) (10 points)
4. In a multiple inheritance language (such as C++) we noted in class (and in the Stroustrup paper) that one needs to put "deltas" in the virtual function table along with pointers to functions.  Explain why the "deltas" are needed in the virtual function table.  Illustrate with an example. (20 points)