From: Mayank Kohaley <mayank.kohaley@...>
Date: 2011-03-08T15:10:19+09:00
Subject: Re: basic programming question, help please

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On Tue, Mar 8, 2011 at 11:13 AM, Gary Wright <gwtmp01@mac.com> wrote:

>
> On Mar 7, 2011, at 11:31 PM, Eric Christopherson wrote:
>
> > On Mon, Mar 7, 2011 at 5:31 PM, Gary Wright <gwtmp01@mac.com> wrote:
> >>
> >> class A
> >>  @@a = "defined in A"
> >> end
> >>
> >> class B
> >>  @@a = "defined in B"
> >>  A.send(:define_method, :show_a) { @@a }
> >>  A.send(:define_method, :show_x) { @@x }
> >> end
> >>
> >> class C < A
> >> end
> >>
> >> A.new.show_a   #  "defined in B"
> >>
> >> class Object
> >>  @@x = 'defined in Object'
> >> end
> >> A.new.show_x   # defined in Object"
> >>
> >> This shows that the resolution of class variables is based on the
> lexical
> >> scope and not the dynamic scope. The method #show_x also shows that
> >> the class hierarchy is also traversed when resolving a class variable
> (first B
> >> is considered and then Object because B is a subclass of Object)
> >
> > Interestingly, if you put
> >
> > @@x = "defined in B"
> >
> > in class B, A.new.show_x still shows "defined in Object".
> >
>
> You have to be very careful here if you are experimenting in irb, for
> example.
>
> Because class variables are actually unique within an entire subtree of the
> inheritance hierarchy the order and location in which the variable is
> defined is important.
>
> If you ran my sample code in irb you would have defined '@@x' in Object and
> effectively made it impossible to define any other '@@x' in any subclass of
> Object since the pre-existing '@@x' in Object would resolve for every
> subclass (because Object is the top of the hierarchy).
>
> On the other hand if you created '@@x' in B first then you could create
> another '@@x' in Object and they would be two different variables.  The
> '@@x' in B would resolve for B and any of its descendants and the '@@x' in
> Object would resolve for Object and any of its descendants *except* for the
> subtree with B as its root.
>
> If you think this is complicated and somewhat confusing you are right. Most
> programmers expect Ruby's class variables to behave like C++'s or Java's
> class variables but they are very different.
>
> Gary Wright
>
>
>
>
I am very new to Ruby and coming from C++ background, the class variables
act differently in Ruby vs C++ , Thanks for the explanation Gary

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