From: "Christoph R." <chr_news@...>
Date: 2003-09-17T23:33:46+09:00
Subject: Re: Class variables, module inclusion and instance_eval

Tim Bates wrote:
....
> irb(main):001:0> module Mod
> irb(main):002:1> def self.append_features(aModule)
> irb(main):003:2> super
> irb(main):004:2> aModule.instance_eval do
> irb(main):005:3* @@foo = aModule.name
> irb(main):006:3> end
> irb(main):007:2> end
> irb(main):008:1> end
> => nil
> irb(main):009:0> class KlassA
> irb(main):010:1> include Mod 
> irb(main):011:1> end
> => KlassA
> irb(main):012:0> class KlassB 
> irb(main):013:1> include Mod
> irb(main):014:1> end
> => KlassB
> irb(main):015:0> class KlassA
> irb(main):016:1> puts @@foo
> irb(main):017:1> end
> KlassB
> => nil
> 
> Somehow, even though the class variable is defined within the context of
> the class (via instance_eval) it still gets shared, contrary to the
> example of Exhibit B. This is frustrating me, because I want my library
> to be able to define a class variable that belongs to only one class,
> and I can't find a way to do it. Currently, I'm doing this:

This has to do with Ruby's scoping rules - here is an example I changed
``append_feature'' to the more modern ``included''

---
module Mod
   def self.included(aModule)
     # the string version creates the expected
     #  module aModule  .. end
     # scope
     aModule.module_eval "@@foo = #{aModule.name}"
   end
end


class A
   include Mod
end


module Mod
   begin
     @@foo
   rescue NameError => mes
     puts mes
   end
end


class B
   include Mod

end

class A
   puts @@foo
end

## your way
module Nod
   def self.included(aModule)
      # scope of @@bar is module Nod ... end
     aModule.module_eval do @@bar = aModule.name end
   end
end


class A
   include Nod
end


module Nod
   puts "@@bar == #{@@bar} defined in #{self}"
end


class B
   include Nod
end

class A
   puts @@bar
end
---

results in

---
uninitialized class variable @@foo in Mod
A
@@bar == A defined in Nod
B
---


/Christoph