The inheritance anomaly: ten years after Giuseppe Milicia Vladimiro Sassone Chi Spaces Technologies ltd. University of Sussex, UK Cambridge, UK
[email protected] [email protected] ABSTRACT Clearly we must make sure that no object is removed from an The term inheritance anomaly was coined in 1993 by Matsuoka empty buffer and that no object is inserted into a full buffer. In a and Yonezawa [15] to refer to the problems arising by the coexis- sequential setting, the burden of ensuring such constraints resides tence of inheritance and concurrency in concurrent object oriented with the buffer’s user. Indeed the buffer is created and used by languages (COOLs). The quirks arising by such combination have one thread only, which is responsible for the state of the object. been observed since the early eighties, when the first experimen- To facilitate usage, the buffer’s methods might return certain er- tal COOLs were designed [3]. In the nineties COOLs turned from ror codes in case of misuse. This approach is not feasible in a research topic to widely used tools in the everyday programming concurrent setting. The buffer will be used concurrently by mul- practice, see e.g. the Java [9] experience. This expository paper ex- tiple clients, leaving each of them no idea on the buffer’s current tends the survey presented in [15] to account for new and widely state. Although it is possible to envisage ad-hoc protocols among used COOLs, most notably Java and C] [19]. Specifically, we il- clients to keep track of the buffer’s state, such a solution is com- lustrate some innovative approaches to COOL design relying on plex and hardly feasible in practice.