From: Michael Schuerig <michael@...>
Date: 2007-12-17T05:14:06+09:00
Subject: Re: Mixins and class variables

On Sunday 16 December 2007, Brubix wrote:
> I can't figure out how to set class variables from class methods
> inherited from a module ?

Consider if class variables are really what you want.

$ irb
>> module M
>>   def set(x)
>>     @x = x
>>     end
>>   def get
>>     @x
>>     end
>>   end
=> nil
>> class C
>>   extend M
>>   end
=> C
>> C.get
=> nil
>> C.set('bla')
=> "bla"
>> C.get
=> "bla"
>> class D < C; end
=> nil
>> D.get
=> nil

@x in this case is not a class variable, it is an instance variable of 
the singleton class of class C. You'll find some information on that in 
the Pickaxe.

The notable difference compared to class variables proper is that class 
variables are shared in the inheritance hierarchy, whereas singleton 
class instance variables are not.

$ irb
>> class C
>>   def self.set(x)
>>     @@x = x
>>     end
>>   def self.get
>>     @@x
>>     end
>>   end
=> nil
>> C.get
NameError: uninitialized class variable @@x in C
        from (irb):6:in `get'
        from (irb):9
>> C.set('bla')
=> "bla"
>> C.get
=> "bla"
>> class D < C; end
=> nil
>> C.get
=> "bla"
>> D.set('foo')
=> "foo"
>> C.get
=> "foo"


Michael

-- 
Michael Schuerig
mailto:michael@schuerig.de
http://www.schuerig.de/michael/