====== Class Instance Variables vs Class Variables ======

Class Variables in Ruby are unlike class variables in many other programming variables. If you change the value of a class variable, all class variables in the inheritance chain are changed to this new value too. This is explained by Martin Fowler, [[http://www.martinfowler.com/bliki/ClassInstanceVariable.html|right here]].

The "solution" is to use class instance variables, as is explained [[http://railstips.org/2006/11/18/class-and-instance-variables-in-ruby|here]], by John Nunemaker. In his post, John also provides a module to have the class instance value carry over as a kind of constant to a subclass.

My own solution to have inheritable default values for class instance variables is this:

<code>
class Polygon
  class << self
    attr_writer :sides
    
    def sides
      @sides || 8
    end    
  end  
end
</code>

All subclasses of ''Polygon'' will share the default value of 8 sides, but each subclass can also easily be given its own number of sides.

<code>
class Triangle < Polygon
  @sides = 3
end

class Octogon < Polygon; end
</code>

This does what you'd expect:

<code>
>> Polygon.sides
=> 8
>> Triangle.sides
=> 3
>> Octogon.sides
=> 8
</code>

The new value for class ''Triangle'' is not carried down the inheritance chain, though. Let's say you have a class ''FantasyShape'':

<code>
class FantasyShape < Triangle; end
</code>

What happens now is this:

<code>
>> Triangle.sides
=> 3
>> FantasyShape.sides
=> 8
</code>

To fix this, you'd have to override the class method ''sides'' for ''FantasyShape'':

<code>
class FantasyShape < Triangle
  def self.sides
    @sides || superclass.sides
  end    
end
</code>

Or better yet, start the superclass lookup in the top of the inheritance chain:

<code>
class Polygon
  class << self
    attr_writer :sides
    
    def sides
      @sides || ((superclass.respond_to?('sides')) ? superclass.sides : 8)
    end    
  end  
end
</code>

Now we can simplify the code for class ''FantasyShape'' back to:

<code>
class FantasyShape < Triangle; end
</code>

And voilà:

<code>
>> Triangle.sides
=> 3
>> FantasyShape.sides
=> 3
</code>


Ruby rocks!