Why Inheritance Anomaly Is Not Worth Solving

Modern computers improve their predecessors with additional parallelism but require concurrent software to exploit it. Object-orientation is instrumental in simplifying sequential programming, however, in a concurrent setting, programmers adding new methods in a subclass typically have to modify the code of the superclass, which inhibits reuse, a problem known as inheritance anomaly. There have been much efforts by researchers in the last two decades to solve the problem by deriving anomaly-free languages. Yet, these proposals have not ended up as practical solutions, thus one may ask why. In this article, we investigate from a theoretical perspective if a solution of the problem would introduce extra code complexity. We model object behavior as a regular language, and show that freedom from inheritance anomaly necessitates a language where ensuring Liskov-Wing substitutability becomes a language containment problem, which in our modeling is PSPACE hard. This indicates that we cannot expect programmers to manually ensure that subtyping holds in an anomaly-free language. Anomaly freedom thus predictably leads to software bugs and we doubt the value of providing it. From the practical perspective, the problem is already solved. Inheritance anomaly is part of the general fragile base class problem of object-oriented programming, that arises due to code coupling in implementation inheritance. In modern software practice, the fragile base class problem is circumvented by interface abstraction to avoid implementation inheritance, and opting for composition as means for reuse. We discuss concurrent programming issues with composition for reuse.