From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2001-08-12T06:23:56+09:00
Subject: [ruby-talk:19547] Re: the way class variables work

In article <Pine.LNX.4.30.0108110959360.5875-100000@candle.superlink.net>,
David Alan Black  <dblack@candle.superlink.net> wrote:
>Hello --
>
>
>But I'm still baffled, maybe more than ever, by the "why", that is,
>why class variables exist at all in Ruby.  No one seems to have
>anything good to say about them.  They seem to operate as a kind of
>not-quite-global global.  They introduce weird, backwards
>dependencies.  They come to your house at night, ring the doorbell,
>and run away.  They....  [Cut to shot of David cowering in corner]


It is kind of troubling, however, as I thought about it, in practice it 
isn't generally an issue.

For example, the 'classic' use of a class variable is to keep track of how 
many objects of a certain class exist.  In cases like this, the superclass 
(or parent class, or whatever you want to call it) always exclusively 
manipulates the class variable.  For example:

class Vehicle

   @@number_of_Vehicles = 0
  
   def initialize
      @@number_of_Vehicles += 1 #increment number of vehicles
   end

end

class Bicycle < Vehicle
   
   @@number_of_Bycycles = 0

   def initialize
      super
      @@number_of_Bycycles += 1
   end
end 

class Car < Vehicle

   @@number_of_Cars = 0

   def initialize
      super
      @@number_of_Cars += 1
   end

end

(actually, I think there are other ways to determine how many objects of a 
given class exist in Ruby, so in reality one may not do this)
Of course this is just a convention that one follows,  you could still 
access @@number_of_Vehicles within Class Car, but you avoid doing so 
because "it wouldn't be prudent". 

Is there anyway to apply private access control to class variables? (if 
not, should there be a way?)  Should there be a way to say:

class Vehicle
   private @@number_of_Vehicles = 0
   public
      def initialize
         @@number_of_Vehicles += 1
      end
end

class Car < Vehicle
   def initialize
      super
      #now try to change @@number_of_Vehicles
      @@number_of_Vehicles += 10 #<-- this would generate an access error
   end
end

The proposed behavior would be that the line that trys to modify 
@@number_of_Vehicles in class Car would generate an access error.
(BTW: this class declaration for Vehicle with the private 
@@number_of_Vehicles actually 'compiles' and runs for me without any 
problems - but I can still change the value of @@number_of_Vehicles in the 
subclass car)

Is it worthwhile to add this kind of behavior?

Phil