From: Stefano Crocco <stefano.crocco@...>
Date: 2007-12-12T04:46:28+09:00
Subject: Re: Class variables per subclasses, defined in the superclass

Alle marted́ 11 dicembre 2007, Gareth Adams ha scritto:
> Hi guys,
>
> First the pastie, I got class_eval from #ruby-lang on freenode -
> http://pastie.caboo.se/127003
>
> I've commented out the Rails specific stuff so I'm sure this is a Ruby
> question.
>
> What I'm trying to do is have a per-subclass variable created by a class
> method on the super class (which is called from the subclass). Fun.
>
> When I load in the file above I get
>
>  -- Processor::Sponge.related_questions
>  => {:surname=>1, :email=>1, :location=>1, :phone_number=>1, :forename=>1}
>  -- Processor::CampaignMonitor.related_questions
>  => {:surname=>1, :email=>1, :location=>1, :phone_number=>1, :forename=>1}
>  -- Processor::Base.related_questions
>  => {:surname=>1, :email=>1, :location=>1, :phone_number=>1, :forename=>1}
>
> where the expected output is
>
>  -- Processor::Sponge.related_questions
>  => {:location=>1, :phone_number=>1}
>  -- Processor::CampaignMonitor.related_questions
>  => {:surname=>1, :email=>1, :forename=>1}
>  -- Processor::Base.related_questions
>  => {} # possibly nil, it's not too important as it shouldn't be called
> directly
>
> I'll be the first to admit I don't really understand class variables, is
> what I'm trying to do possible without having to add anything to the
> subclass?

I think you need class instance variable, not class variables. Class instance 
variable are instance variables of the class object itself. For example:

class A
  @var = 1
end

They follow the usual rules for instance variables. For example, to access 
them from outside self (and this includes accessing them from instance 
methods of the class), you need to provide suitable methods:

class A

  @var = 1
  
  def self.var
    @var
  end 

  def a_method
    puts self.class.va
  end

end

Unlike class variables, class instance variables arent shared between parent 
and children classes, so they should work for you (unless Rails, which I 
don't know, requires class variables, of course).

To use class instance variables, you should replace @@related_questions with 
@related_questions and remove the call to class_eval (they aren't needed 
anymore).

I hope this helps

Stefano