From: Patrick Bennett <patrick.bennett@...>
Date: 2003-08-22T13:35:21+09:00
Subject: Re: Class variables - a surprising result



Yukihiro Matsumoto wrote:

>I fell sympathy for your confusion.  Unfortunately I can't satisfy
>everyone.  I'm trying anyway.
>
Well, keep it up, you've done a great job so far.  :)

>I'd like to know if you are a designer, how would you design class
>variables in Ruby?  And how useful they are in daily usage?
>I'm just curious.
>
Hard to say....  Unfortunately the languages I've used  (some for too 
long of a time) tend to cloud my judgement a bit.
I'd probably side more towards the c++ static equivalent where a 'class' 
variable (ie: static int x;) is basically a unique variable that's within
the namespace of the class but that follows standard visibility rules 
during inheritance.  If a derived class defines a static (as opposed to 
an 'instance' variable) then
unless explicitly scoped (class::variable) that class's variable will be 
referenced.   If a base class changes its static, only the base class 
(and in a way, any subclass that didn't define a variable with the same 
name) are affected. 
I guess the real issue is that in Ruby there's no way to explicitly 
'declare' a variable. Assigning to it either creates it, or assigns to 
the existing instance, so there's no way for derived classes to declare 
*their own* version of a variable with the same name and be able to 
refer to their version or the base class version at will. 

Talking through it now, and seeing what what the real source of my 
confusion was, I don't think it's an issue.  Ruby can't do something I 
can do in C++ (guys, please, I don't want to see a bunch of posts to 
simulate this), but honestly, it's not something you really want to do 
anyway.  'Hiding' base class variables is usually bad news.
The source of my confusion was that when I saw @@x = y, I equated that 
to a static declaration (somewhere) and it being assigned an initial 
value, not just another assignment to a variable scoped by a class 
hierarchy (as if it was effectively declared in the base class).

If it matters, this is is how I was seeing it all:
class A
    @@val = 5
     def print
        puts "A print - val:#{@@val}"
        @@val = 6
    end
end

class B < A
    @@val = 8
    def print
        super
        puts "B print - val:#{@@val}"
    end
end

foo = B.new
foo.print
-----
A print - val:8
B print - val:6
A print - val:6
B print - val:6

The equivalent code (at least in terms of how I was looking at it - thus 
the source of my confusion) in C++, C#, Delphi (and others) would print 
5, 8, 6, 8