From: Eric Christopherson <echristopherson@...>
Date: 2011-03-08T13:31:12+09:00
Subject: Re: basic programming question, help please

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".