From: James Coglan <jcoglan@...>
Date: 2008-07-23T01:40:11+09:00
Subject: Re: simple module for "count my instances" behaviour

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>
> The error message says that the @@counter class variable is not
> initialized in Countable::ClassMethods (so in the population method).
> Obviously, this @@counter variable is not the same as the one that is
> declared when including Countable, since User.new works as expected.
>
> Can someone explain me why and how to solve this?



I think what you really want is an instance variable in the including class,
since the counter is a property of the class. The instance variable must be
created anew in every including class, whereas I think your code will try to
reuse Countable's @@counter variable for all classes. This code works:

module Countable
  def initialize
    self.class.instance_eval do
      @counter ||= 0
      @counter += 1
    end
  end

  module ClassMethods
    def population
      return @counter || 0
    end
  end

  def self.included(base)
    base.extend(ClassMethods)
  end
end

class User
  include Countable
end

User.new
User.new
puts User.population

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