An Abstract Machine for the Old Value Retrieval

The evaluation of post-conditions requires the computation of old attribute values. Until recently, existing computation methods were not efficient in terms of time- and space-complexity. Moreover they were applicable only to a restricted form of post-conditions. Recently a new algorithm was proposed to overcome those deficiencies. In this paper, an abstract machine corresponding to this algorithm is defined. Its transitions simulate steps of object-oriented systems and preserve an invariant implying properties needed to compute old attribute values. The machine is based on a kind of structure called here sufficiently persistent, as opposed to persistent and partially persistent structures. A space-bound on the structure size is given. It is also demonstrated that methods which do not have post-conditions can be abstracted away.