From: Niklas Frykholm <r2d2@...>
Date: 2001-08-12T16:09:37+09:00
Subject: [ruby-talk:19579] Re: @@variable scope

On Sun, Aug 12, 2001 at 06:48:03AM +0900, Tobias Reif wrote:
> Below is my current understanding of how families of classes share variables.
> I'm sure I got some things completely wrong, and would like to get these
> spots pointed out.
> 
> Class variables beginning with @@ are shared and scoped among whole families
> of classes: parent classes with descendant classes. If a cousin assigns a new
> value to a class variable, the value changes for it's cousin, ancestor, and
> descendant classes.  If all objects are of class Object, then all classes are
> descendant classes of class Object, and all objects of all classes share the
> farthest/oldest ancestor class Object.  Thus all classes are in the same
> family and thus @@ variables would be shared among all objects.
> 
> Where am I erring?

I think your view is too static. My understanding is that when a class variable
is encountered the class hierarchy is searched upward for a class variable with
that name. If a match is found the reference is bound to that variable,
otherwise a new class variable is created in the current class.

You can see the difference in this example.

	class A
		def set(x); @@a=x; end
		def get; @@a; end
	end

	class B<A
		def set(x); @@a=x; end
		def get; @@a; end
	end

	a = A.new; b = B.new
	a.set 4
	b.set 5
	puts a.get
	puts b.get

When b.set is run, @@a exists in the superclass, so B's @@a is bound to
this. Therefore the program will print 5 5.

Now switch the lines a.set 4 and b.set 5. Now, b.set will not find @@a in
A, so it will create its own variable. The program prints 4 5.

// Niklas