The Impact of Inheritance on Software Structure

Abstract

In most object-oriented languages an inheritance hierarchy is assumed to represent a type inclusion hierarchy. However, inheritance, viewed from an architectural level of abstraction, can be seen as a simple extension of the module/package inclusion mechanisms (referred to here as import/export mechanisms) found in object-based languages such as Modula-2 [Wirth, 1988] and Ada [Booch, 1987]. One way to express the notion that inheritance is an extension of import/export mechanisms is to say that it allows the client-server relation between an exporting module and an importing module to be transitive, so that a client can transparently pass on services which it imports (i. e. inherits from parent classes) to its own clients. However, this extension, while useful, is far short of the expressive power needed to guarantee that valid type inclusion relations are properly captured in a class hierarchy. This chapter demonstrates these observations by presenting formal models of inheritance hierarchies, and of module hierarchies with import/exports, and defining a translation function from the former to the latter.