Ruby Static Inheritance

Question

I am trying to use a class variable in ruby. But class variables change throughout the entire hierarchy, and thus are useless for this goal:

Assume I have 3 classes, each inherited, except the parent.

class A
end

class B < A
end

class C < B
end

How would I modify or create a static variable in the middle class so that class A does not have it but class C does.

B.num = 2

A.num # undefined or nil
C.num # 2

I should also specify that A.num should still be able to be used, without changing B.num or C.num, unless its inherited.

Why is a class variable "useless"? Your sentence does not logically follow. — sawa
– sawa, Commented Jan 7, 2014 at 22:51
@sawa I don't believe my sentence does not flow. A Class Variable on class B will change the class variable on class A as well. It will change parents and children. Please review this answer: stackoverflow.com/questions/1251352/… — Alex V
– Alex V, Commented Jan 7, 2014 at 23:01
You did not seem to understand my comment correctly. I am mentioning that the fact that class variables are shared among the hierarchy does not lead to your conclusion that class variables are useless. I am asking for the basis of your claim that class variables are useless. — sawa
– sawa, Commented Jan 7, 2014 at 23:03
Obviously class variables are not useless. They don't work as expected for my desired goal. — Alex V
– Alex V, Commented Jan 7, 2014 at 23:08

sawa · Accepted Answer · 2014-01-07 23:07:15Z

3

Edited since the OP changed the question

Use a class instance variable for A and B.

class A
  singleton_class.class_eval{attr_accessor :num}
end

class B < A
  singleton_class.class_eval{attr_accessor :num}
end

class C < B
  def self.num; superclass.num end
  def self.num= v; superclass.num = v end
end

B.num = 2
A.num # => nil
C.num # => 2

edited Jan 7, 2014 at 23:07

answered Jan 7, 2014 at 22:55

sawa

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1 Comment

David Grayson Over a year ago

To add on to Sawa's answer: If you later decided that B should not override num, then you have to copy the definition of self.num from C into B. Every class that has a num or delegates to its super class must still have a few lines of code for num.

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