A Uniform Model for Object-Oriented Languages Using the Class Abstraction

Abstract

One of the main goals of object-oriented languages is to unify their universe: "every entity of the language is an object." The class concept, however, usually does not follow this wish: a class being not a real object, i.e., created from a class. The metaclass concept introduced in Smalltalk80, attempts to give classes a first class citizenship but complicates the instantiation scheme without solving its fundamental limitations: the only partial specification of the class at the metaclass level, and the fixed number of meta-levels. Some more recent approaches, as in Loops and then CommonLoops, tend towards a better unification, but reveal the same limitations. We propose to go one step further and present an ultimate new model for an optimal simplification of the class/object concept, while keeping the class abstraction. In this model, implemented by ObjVlisp 1 , every object is an instance of a class, and a class is a true object fully specified at the meta-level. A metaclass is a true class inheriting from another one. Because . ObjVlisp is built on a reflective architecture; the user has uniform access and .control to all the levels of the language: class level, metaclass level. .. He can add new levels and the metaclass links can be created indefinitely. Finally, we suggest to clarify the Smalltalk terminology with a new definition for class variables: the instance variables of a class - treated as an object - become the class variables of this class, explicitly expressed at the metaclass level.