State Based Ownership, Reentrance, and Encapsulation

A properly encapsulated data representation can be revised for refactoring or other purposes without affecting the correctness of client programs and extensions of a class. But encapsulation is difficult to achieve in object-oriented programs owing to heap based structures and reentrant callbacks. This paper shows that it is achieved by a discipline using assertions and auxiliary fields to manage invariants and transferrable ownership. The main result is representation independence: a rule for modular proof of equivalence of class implementations.