Composition through Superimposition

The conventional object-oriented paradigm provides part-of, inheritance and delegation relations as means for composition of objects. Although inheritance and delegation represent a considerable improvement over the module-based third generation languages, we have experienced that certain types of composability cannot be expressed using the aforementioned techniques. In some cases, the software engineer needs to superimpose certain behaviour on an object in such a way that the complete functionality of the object is affected. In this paper, we present three examples of behaviour that cannot be composed conveniently with object specifications when using the conventional object model, i.e. client interface, design patterns and acquaintance communication patterns. Subsequently, solutions to this lack of expressiveness are presented in the context of the layered object model (LayOM), our extensible research language.