Abstract

In a system where classes are treated as first class objects, classes are defined as instances of other classes called metaclasses. An important benefit of using metaclasses is the ability to assign properties to classes (e.g. being abstract, being final, tracing particular messages, supporting multiple inheritance), independently from the base-level code. However, when both inheritance and instantiation are explicitly and simultaneously involved, communication between classes and their instances raises the metaclass compatibility issue. Some languages (such as Smalltalk) address this issue but do not easily allow the assignment of specific properties to classes. In contrast, other languages (such as Clos) allow the assignment of specific properties to classes but do not tackle the compatibility issue well. In this paper, we describe a new model of metalevel organization, called the compatibility model, which overcomes this difficulty. It allows safe metaclass programming since it m...

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