Metaclass Mechanisms: a CLOS perspective for Reifying Relationship

Since the recognition of the software crisis in the mid-70’s, a huge amount of work has been done to devise modeling tools that help analysts to develop systems of ever-increasing complexity. While “structured methods” were appropriate for describing systems only driven by data transformations, they were not appropriate to cope with more sophisticated technologies. These include OO database systems, World Wide Web and network applications, hypertext, multimedia and CAD/CAM systems, distributed computing or sophisticated man-machine interfaces. The object-oriented paradigm (software development methods [3, 11] and programming languages [1, 5, 12]) aim at remedying the situation. This led to a new way of thinking about problems with complementary models organized around real-world concepts: object models describe the structure of objects in a system; dynamic models represent aspects of a system concerned with time, control, and behavior. In these models, the fundamental construct is the object, organized in classes of similar objects (for instance, we have a class employee and a class department when modeling a company, which combines both data structure and behavior in a single entity). In object-oriented applications, objects and classes are connected and cooperate to perform collective tasks and maintain some invariants. From a structural point of view, such connections are generally modeled in object models by semantic relationships. Intuitively, a relationship describes a group of physical or conceptual connections between objects. For example, an employee works for a departement. All the connections in a relationship connect objects from the same classes. Relationships often appear as verbs