Expressive, Efficient Instance Variables

The decoupling of clients of abstractions from the implementations of those abstractions is a key benefit of objectoriented programming. However, many object-oriented languages provide instance variables in such a way that implementation-level representation decisions seep through interfaces, introducing coupling between clients and abstractions. As part of the Cecil language, we have developed a semantics for flexible instance variables based on the principle that a class should be decoupled from its clients, its subclasses, and its role extensions. We describe our design of instance variables and provide examples where their additional flexibility has been useful. However, implementing our semantics naively can impose significant performance penalties. To achieve reasonable performance, we have developed a number of optimizations that are effective at eliminating many of the costs of our flexible model. We discuss these optimizations and report on the results of experiments to assess the costs of our design under different optimization scenarios.