From: Julien Thewys <jt@...>
Date: 2008-07-23T01:24:58+09:00
Subject: simple module for "count my instances" behaviour

I want to make a simple module that makes its including classes
self-countable.
Here is the code (that does not work, incidentally):

#---code
module Countable
  @@counter = 0

  def initialize
    @@counter += 1
  end

  module ClassMethods
    def population
      return @@counter
    end
  end

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

class User
  include Countable
end

User.new
User.new
puts User.population
#---code

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?
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