From: Garance A Drosihn <drosih@...>
Date: 2003-08-21T21:19:09+09:00
Subject: Re: Class variables - a surprising result

At 6:33 PM +0900 8/21/03, Jason Williams wrote:
>     class Sup
>      @@x = "A"
>      def test
>       print @@x
>      end
>     end
>    
>     class Sub1 < Sup
>      @@x = "B"
>     end
>    
>     class Sub2 < Sup
>      @@x = "C"
>     end
>    
>     Sup.new.test
>     Sub1.new.test
>     Sub2.new.test
>
>I expect this to print "ABC", and yet it prints "CCC".
>Why is this?

That's what I would expect to happen, although I am not sure
I could give a good explanation as to why it is "obvious"
that it should work that way.

As to what exactly is happening, the assignments to @@x are
not inside any method.  The assignments are processed as they
are read in, not in the order that '.new' (or any other method)
is called on any of the classes.  Perhaps the following will
give you a better idea of why you ended up with 'CCC':

# - - - - start
class Sup
     @@x = "A"
     @@line = 0
     def Sup.Ctest
	@@line += 1
	@@x += @@x[0,1]
	print @@line, "-", @@x, "\n"
     end
end

Sup.Ctest		# first test
Sup.Ctest		# second test

class Sub1 < Sup	# define subclass
     @@x = "B"
end

Sub1.Ctest		# third test
Sup.Ctest		# fourth test
# - - - - end

This produces:
1-AA
2-AAA
3-BB
4-BBB

I am not sure if that really helps you with the question
you're asking, though.  Note that since you are only using
class variables for this example, we can simply use a class
method.  There is no need for creating any instances of
either of these classes, as far as the class variables are
concerned.

To get back to why there would be only one @@x for a class
and all it's subclasses, let me refer to a line from the
"Programming Ruby" (pickaxe) book:  "A class variable is
shared among all objects of a class, and is also accessible
to the class methods...".

Any object which is an instance of 'Sub1' is also, by
definition, an instance of 'Sup'.  If a class variable is
shared among *all* objects of class 'Sup', then the exact
same variable must also be shared with the objects which
happen to be from class 'Sub1'.

-- 
Garance Alistair Drosehn            =   gad@gilead.netel.rpi.edu
Senior Systems Programmer           or  gad@freebsd.org
Rensselaer Polytechnic Institute    or  drosih@rpi.edu