A Unified Framework for Verification Techniques for Object Invariants ( Full Paper ) date : 30 th August 2007 please , check this site for updates and extensions of this paper

Verification of object-oriented programs relies on object invariants which express consistency criteria of objects. The semantics of object invariants is subtle, mainly because of call-backs, multiobject invariants, and subclassing. Several verification techniques for object invariants have been proposed. These techniques are complex and differ in restrictions on programs (e.g., which fields can be updated), restrictions on invariants (what an invariant may refer to), use of advanced type systems (such as Universe types or ownership), meaning of invariants (in which execution states are invariants assumed to hold), and proof obligations (when should an invariant be proven). As a result, it is difficult to understand whether/why these techniques are sound, whether/why they are modular, and to compare their expressiveness. This general lack of understanding also hampers the development of new approaches. In this paper, we develop and formalise a unified framework to describe verification techniques for object invariants. We distil seven parameters, which characterise a verification technique, and identify sufficient conditions on these parameters under which a verification technique is sound. We also define what it means for a technique to be modular. To illustrate the generality of our framework, we instantiate it with six verification techniques from the literature. We show how our framework facilitates the assessment and comparison of the soundness, modularity, and expressiveness of these techniques.