John Lam wrote:
> This one puzzles me - I know there's something fundamental that I'm not
> understanding about Ruby's object model. Consider these classes:
> 
> class Base
>  def self.my_name=(value)
>    @my_name = value
>  end
>  def self.my_name
>    @my_name
>  end
> end
> 
> class Derived < Base; end
> 
> Base.my_name = 'John'
> puts Base.my_name
> John
> 
> Derived.my_name
> nil
> Derived.my_name = 'Mike'
> puts Derived.my_name
> Mike
> 
> So the class instance member @my_name isn't inherited by the Derived class.

That's right. The #my_name and #my_name= methods are inherited, but when
you call them on Derived, they operate on the instance variables of Derived.

> How do I share state between a derived class and its base class?

Well, there are "class variables", written @@my_name. But they can have
some surprising behaviors in current ruby(*), and in future ruby they
will not be shared up and down the inheritance hierarchy (they will only
be shared between a class and its instances).

An alternative:

class A
  class << self
    attr_accessor :__my_name

    def my_name; A.__my_name; end
    def my_name=(n); A.__my_name = n; end
  end
end

class B < A
end

B.my_name = "fred"

p A.my_name  # ==> "fred"

Or, if you don't want __my_name polluting the namespace, you could use this:

class A
  class << self
    def my_name; A.instance_variable_get(:@my_name); end
    def my_name=(n); A.instance_variable_set(:@my_name, n); end
  end
end

class B < A
end

B.my_name = "fred"

p A.my_name

The key difference between these two examples and your original attempt
is that, here, the "A" reference binds _statically_ to the class A,
whereas, in the original code, the "@my_name" reference binds
_dynamically_ to the instance on which my_name is called (which might
turn out to be A, or B or ...).

HTH.

(*) search the archives--briefly, if you assign to a subclass's @@x, and
then assign to the superclass's @x you get two different variables.
Reverse the order of assignment, and there is only one variable:

irb(main):014:0> class A; end
=> nil
irb(main):015:0> class B<A; end
=> nil
irb(main):016:0> class A; @@x=1; end
=> 1
irb(main):017:0> class B; p @@x; end
1
=> nil
irb(main):018:0> class B; @@y = 2; end
=> 2
irb(main):019:0> class A; p @@y; end
NameError: uninitialized class variable @@y in A
        from (irb):19
irb(main):020:0> class A; @@y = 3; end
=> 3
irb(main):021:0> class B; p @@y; end
2
=> nil

-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407