Nuby question on Class Variable and Class Instance Variable

ruby-talk
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

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

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.

···

On 1/15/07, Nasir Khan <rubylearner@gmail.com> wrote:

You had me at "I generally avoid class variables" :slight_smile:

Here's another one you'll like:

   @@avar = 1
   class A
     @@avar = "hello"
   end
   puts @@avar # => hello

   A.class_eval { puts @@avar } # => hello

3

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.

···

On 1/15/07, Nasir Khan <rubylearner@gmail.com> 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.