The Construction Principles of the Class Library Karla

Abstract

This article shows how to construct robust class libraries in the context of flexibility, implementation efficiency and extensibility as further design goals. A library is called robust if (1) no errors can be introduced into an application due to its use or inheritance from it, (2) error messages do not point deep into library code but into the user’s application, and (3) statically checkable incorrect uses of library classes are recognized by compilers. The principles derived from a theoretical analysis have been applied practically in Karla, a robust class library of data structures and algorithms the authors designed and implemented, and that is being used for research projects and student education.

The main focus of this article is on the construction of hierarchies of abstract classes. The analysis shows that it is not possible in general to achieve all of the above design goals for robust libraries at the same time. The only solution that ensures robustness is to drop one of these criteria. Redesigning an existing library for robustness may lead to an exponential growth in the number of classes in the hierarchy of abstract classes. However, it is shown that this class explosion can be controlled by automatically generating the required additional classes.