A metric for evaluating class library interfaces and its application to library upgrades

The proposed metric for evaluating class library interfaces measures the balance between generalization and specialization. The key idea of the proposed metric is the fact that polymorphic operations satisfy Zipf's law, which is known as an approximating equation for the frequency of words in natural language. The metric shows the global property of an entire class library, independent of size (making it possible to compare class libraries of different sizes and different versions), and to avoid the problems associated with using the depth of the inheritance graph as the metric. The results of application of this metric to two upgrades of the InterViews C++ class library agree with the results of manual analysis on three points: the metric value for the groups whose interfaces are generalized are larger than the metric value of other groups which are not as generalized, the metric value increases when a class interface is generalized by creating abstract classes, and the metric value decreases when new specialized-functional extension classes are added to the class library.