From: Gary Wright <gwtmp01@...>
Date: 2011-03-08T08:31:51+09:00
Subject: Re: basic programming question, help please


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)

Gary Wright