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Exploring Solution Spaces, Keith Ray's blog on Software development and other topics.
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2007.Sep.20 Thu
• Purity metric: the ratio of pure virtual methods over the total number of methods of a class. Purity of 1.0 is good for an abstract base class. Purity of 0.1 might be ok for a class implementing the Template Method Design Pattern. Purity of 0.5 is likely to be bad because the class is going to be confusing. It is likely to be an abuser of "inheritance for implementation". Note that Template Method relies on subclassing from a class with a partially-concrete implementation, and might be better replaced by the something like the Strategy Design Pattern, which has a concrete class parameterized by injection of pluggable strategy objects (whose parent class is purely abstract).
• Public Interface metric: the ratio of public methods over the total number of methods of a class. A measure of 1.0 is probably good: all public methods, and no private methods (at least it's testable). A Public Interface number of 0.1 is likely to be bad - the class is consisting of mostly private or protected methods, and thus hard to test. A low number is a sign of the Iceberg Class , often indicating that the class is too large and should be broken up into several smaller, collaborating classes (after the break-up, maybe only one class is still part of the public API, but the other classes who now have public methods would be testable.)
• Static Cling metric: the number of static variables in a class, including static variables in the class's implementation file, whether file-scope or function-scope.
• Anti-Polymorphic metric: number of static functions and non-virtual functions in a class, plus the number of file-local functions in the class's implementation file.