The Language of the Visitor Design Pattern

Abstract. Design patterns have been developed to cope with the vast space of possible different designs within object-oriented systems. One of those classic patterns is the Visitor Pattern that is used for representing an operation to be performed on the elements of an object structure. We present a mapping from the Visitor Pattern to a grammar that defines the set of visit sequences that can be performed by a given Visitor. The language defined by the grammar is the language of the Visitor Design Pattern. The declared purpose is the documentation of a Visitor. To allow the application of the presented approach to data structures in general, the formalism handles complications that occur when traversing arbitrary object structures, such as lack of a common base class, multiple inheritance, and inheritance from concrete classes. The mapping encodes knowledge about the class hierarchy and the implementation of the accept methods of a Visitor Design Pattern. 1