INTRODUCING VIRTUAL INSTANCE VARIABLES IN CLASSES TO PROVIDE SUFFICIENT SUPPORT FOR ENCAPSULATION

Encapsulation is a technique for minimizing interdependecies among sep arately-writen modules by defining strict external interfaces. Many po pular object-oriented languages (e.g, Smalltalk, Flavors, Objective-C and Eiffel) allow free access to inherited instances variables by desc endant classes, thus denying the designer the freedom to compatibly ch ange the representation of a class without affecting clients and sever ly compromising encapsulation. In this paper, by introducing virtual i nstance variables and providing two interfaces (client-interface and s ubclass-interface) for a class, we give a class mechanism which provid es sufficent support for encapsulation and is implemented in NDOOM, a object-oriented dialect of Modula-2 which we are developing.