On the Qafoo blog today Kore Nordmann has a new post talking about code reuse through inheritance. He talks about base classes, sharing code and abstraction.

To me, inheritance has two properties: Defining an is-a relationship [and] making it possible to share code between classes by extending from a common base class. The is-a relationship between classes is one thing I like to use inheritance for. [...] The other thing you can use inheritance for is to share code between multiple classes. [...] I personally think that the latter is one of the worst things you can do in object oriented design. Bear with me for a moment and let me try to explain why I think that.

His example of doing it the wrong way is using the Active Record design pattern and how it's usually implemented - storage logic in the base class and business/table logic in the extending class. He then gets into an example that's a bit "smaller" creating diff display functionality and how the "code reuse by inheritance" creeps in a lot in helper methods. He also briefly looks at testing (or not testing) private methods and and the "Depth of Inheritance Tree" metric's recommended value.