From: "Gray, Jeff" <jeff.gray@...>
Date: 2002-07-03T07:37:40+09:00
Subject: Treatment of class variables in inherited classes

This question probably has more to do with the fundamentals of OOP than Ruby
specifically, but I'm looking for an explanation of why the behavior of
class variables in inherited classes is the way it is.  Consider:

---
class Box
  # @@stuff = {:e => 1}   # intentionally commented out
  def Box.add(k, v)
    @@stuff[k] = v
  end
  def Box.[](knobname)
    @@stuff[knobname]
  end
end

class Crate < Box
  @@stuff = {:e => 2}
end

class Suitcase < Box
  @@stuff = {:e => 3}
end

Crate.add(:d, 4)
Suitcase.add(:d, 5)
puts Crate[:d]        # -> 4 [would be 5 if line 2 were uncommented]
puts Suitcase[:d]     # -> 5
puts Crate[:e]        # -> 2 [would be 3 if line 2 were uncommented]
---

So when a class variable is defined in a parent class, all inherited classes
see the same object.  More specifically, the most recent definition of that
class variable is used, even if it occurred in a subclass.  Yet when a class
variable is not initialized in the parent class, the variable points to
different objects in each class.  This is sensible, but it feels like there
is lacking a "have your cake and eat it too" mechanism that allows for
inherited classes to override/redefine class variables without affecting the
ancestor and sibling classes.

So what are the guiding principles of inheritance that account for this
behavior?  

Thanks,

   - jeff