ON THE RELATIONSHIP BETWEEN CLASSES, OBJECTS, AND DATA ABSTRACTION

While most object-oriented programming is done in class-based language s, the trend in theoretical study has been to develop formal systems t hat are object-based, without classes and often without explicit inher itance mechanisms. This paper studies the correspondence between class constructs of the form found in C++, Eiffel, and Java and object prim itives of the form used in recent type-theoretic studies. One insight is that classes require both an extensible aggregate, to serve as the basis for inheritance, and a non-extensible form of object to support subtyping; typed object calculi without extensible objects or extensib le records do not seem adequate for conventional class-based programmi ng. We develop our analysis by comparing three approaches to class-bas ed programming, the first using records of object components called '' premethods'' and the latter two using an extensible form of object cal led a ''prototype.'' White the first approach is simplest, using fewer primitive operations on objects, it does not seem to accurately provi de several features of conventional class-based languages. In the latt er two approaches, we give more comprehensive treatments of classes by combining prototypes with standard abstraction mechanisms. AII three treatments of classes are based on typed translations into provably so und object calculi. (C) 1998 John Wiley & Sons, Inc.