Two varieties of object-oriented systems exist: one based on classes as in Smalltalk and another based on exemplars (or prototypical objects) as in Act/1. By converting Smalltalk from a class based orientation to an exemplar base, independent instance hierarchies and class hierarchies can be provided. Decoupling the two hierarchies in this way enables the user's (logical) view of a data type to be separated from the implementer's (physical) view. It permits the instances of a class to have a representation totally different from the instances of a superclass. Additionally, it permits the notion of multiple representations to be provided without the need to introduce specialized classes for each representation. In the context of multiple inheritance, it leads to a novel view of inheritance (or-inheritance) that differentiates it from the more traditional multiple inheritance notions (and-inheritance). In general, we show that exemplar based systems are more powerful than class based systems. We also describe how an existing class based Smalltalk can be transformed into an exemplar-based Smalltalk and discuss possible approaches for the implementation of both and-inheritance and or-inheritance.

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Journal ACM SIGPLAN Notices
Lalonde, W, Thomas, D.A. (Dave A.), & Pugh, J.R. (John R.). (1986). An Exemplar Based Smalltalk. ACM SIGPLAN Notices, 21(11), 322–330. doi:10.1145/960112.28729