Object-oriented database systems are an emerging. promising technology, underpinned by the integration of ideas from object-oriented languages alone with the specific needs of database applications. The fundamental reason for using such systems is that any real-world entity can be modeled by one object which matches its structure and behavior. To this end, the standard notion of object must be augmented so that it can model the fact that an entity may acquire new pieces of structure and behavior during its existence without changing its identity. To allow this extensibility in a statically typed system. a notion of context-dependent behavior (role playing) must be added to the basic features of object-oriented languages. This feature is also a useful modeling device. Languages with role mechanisms have already been proposed. However, their design is full of choices which cannot be easily justified. A strong foundation for the object-with-roles notion would be extremely helpful to justify these choices and to understand. and prove, the properties of such a mechanism. In this paper we describe such a foundation, building on the object model proposed by Abadi and Cardelli.
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