Nuby question on Class Variable and Class Instance Variable

Ruby
1

class A
@my_var = 123
@@my_var = 234

def A.cv
@@my_var
end

def A.cv=(val)
@@my_var = val
end

class <<self
attr_accessor :my_var
end
end

Obviously @my_var and @@my_var are two separate entitites, but are
similar
in many ways. I have some questions around them, any help is
appreciated.

  1. In terms of the figure in the PickAxe book 2nd Ed (page 381, lucille,
    Guitar etc), where would these two variables ( @my_var and @@my_var )
    appear?
  2. If I have a subclass -
    class B < A
    end
    the @@my_var is shared between the parent and child but @my_var is a
    separate instance in each of them. Are there any other such differences?
  3. As I have written accessor for @my_var, it most likely becomes a
    method
    in the virtual class A’, is there any way to write accessor for @@my_var
    using Module’s attr_accessor method?
  4. As a general guideline where would I use one or the other?

Thanks
Nasir

2

On 1/15/07, Nasir K. [email protected] wrote:

@@my_var = val
  1. In terms of the figure in the PickAxe book 2nd Ed (page 381, lucille,
  2. As a general guideline where would I use one or the other?

Thanks
Nasir

Class variables are evil. A smarter man than me said something on the
subject of class variables on this list recently. So, hopefully that
answers number 4. I’ll leave it to the non noobs to answer the others.

3

On 1/15/07, Nasir K. [email protected] wrote:

Obviously @my_var and @@my_var are two separate entitites, but are similar
in many ways. I have some questions around them, any help is appreciated.

Hi. This isn’t a direct answer to your question, but I wrote a little
article around a recent discussion of this on the list.