From: Mayank Kohaley <mayank.kohaley@...>
Date: 2011-03-08T14:14:03+09:00
Subject: Re: basic programming question, help please

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On Tue, Mar 8, 2011 at 10:01 AM, Eric Christopherson <
echristopherson@gmail.com> wrote:

> On Mon, Mar 7, 2011 at 5:31 PM, Gary Wright <gwtmp01@mac.com> wrote:
> >
> > On Mar 7, 2011, at 7:03 AM, Mayank Kohaley wrote:
> >
> >> The class method in Ruby is represented using self.<method name> or
> <class
> >> name>.<method name>, A class method is used to modify the class
> variables(
> >> which has the same value in every object of that class, as you have in
> your
> >> program @@number_of_squares).
> >
> > This is only partially correct.  A class method is not 'used to modify
> > class variables'.  It can be used to do that but so can many other
> mechanisms
> > including code in an instance method, a block, or a module or class
> definition.
> >
> > It is misleading to say a class variable has 'the same value in every
> object
> > of the class' since it suggests that class variables are in some way
> associated
> > with instances (or 'self') but they aren't. They are associated with the
> innermost
> > lexical scope, which for most instance method definitions is just the
> class
> > definition block but that is more coincidence than design.  Consider the
> code
> > below where an instance method for A is defined within the lexical scope
> of B.
> >
> > 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".
>
>
After putting @@x = "defined in B" in class B it shows "defined in B" and
not "defined in Object", and it makes sense as it works according to lexical
scope and not the dynamic scope. This is how I changed the class B without
changing anything else

class B
 @@a = "defined in B"
 @@x = "defined in B"
 A.send(:define_method, :show_a) { puts @@a }
 A.send(:define_method, :show_x) { puts @@x }
end

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