[FIXED] When is it OK for an abstract base class to have (non-static) data members?

Issue

I guess the question title sums it up. Is there a time when it would be considered good design for an ABC to have data members? I have been wondering if there is a situation where this is OK. The only ones I can come up with all are static, and even then it's kind of a stretch.

Solution

I don't see why an ABC couldn't properly have per-instance (aka non-static) data members, as needed to support the methods it supplies to subclasses. Take the common case in which an ABC exists to supply a Template Method DP (the hook methods being abstract) -- if part of the function of the organizing method is to update some instance variables (for example, a count of how many times the method was called), then obviously those variables should also be supplied by the ABC. Can you explain better why you think that's bad design?!

Answered By - Alex Martelli

Answer Checked By - Robin (PHPFixing Admin)