Design and Specification of Iterators Using the Swapping Paradigm

How should iterators be abstracted and encapsulated in modern imperative languages, e.g., Ada and C++? We consider the combined impact of several factors on this question: the need for a “common interface model” for userdefined iterator abstractions; the importance of formal methods in specifying such a model; and problems involved in modular correctness proofs of iterator implementations and clients. A series of iterator designs illustrates the advantages of the “swapping paradigm” over designs based on the traditional “copying paradigm.” Specifically, swapping-based designs admit more efficient implementations, while offering straightforward formal specifications and the potential for modular reasoning about program behavior. The final proposed design schema defines a common interface model for iterators for a wide variety of generic collection abstractions in languages such as Ada and C++. Index Terms — Formal specification, iterator, program verification, proof of correctness, swapping. Copyright © 1992 by the authors. All rights reserved. 1 Department of Computer and Information Science, The Ohio State University, 2036 Neil Ave. Mall, Columbus, OH 43210. Telephone: 614-292-1517. Internet: [email protected], [email protected]. 2 Department of Mathematics and Computer Science, Muskingum College, New Concord, OH 43762. Internet: [email protected]. 3 Department of Computing and Information Science, Queen’s University, Kingston, Ontario K7L 3N6, Canada. Internet: [email protected].