A Generalization and Solution to the Common Ancestor Dilemma Problem in Delegation-Based Object Systems

This paper studies the diamond problem in the context of delegation-based object systems. The diamond problem occurs when the same ancestor is inherited multiple times via different inheritance paths. The challenge is that replication and sharing of distinct attributes of the common ancestor must be simultaneously supported. We illustrate the relevance of the diamond problem by showing that it arises not only in multiple inheritance but also in other inheritance techniques, hence the more general term ’the common ancestor dilemma’. More specifically the hybrid approach that integrates object-based inheritance in a class-based model is also affected by the problem. We show that the hybrid approach provides an elegant solution for orthogonal expression of replication and sharing. Attributes that should be shared are modeled as part of the delegating objects, whereas attributes that should not be shared are modeled as part of subobjects of the delegating objects.