Singleton example

class Customer

  # attr_accessor :name
  def name ; @name ; end
  def name=(newname) ; @name = newname ; end

  # accessor for the singleton

  def self.null_customer
    @@instance ||= Customer.new
    # put the singleton in its own class!
    class << @@instance
      def name ; "ANONYMOUS" ; end
      def name=(new) ; raise "Can't change name of null customer " ; end
      def logged_in? ; false ; end
    end
    @@instance
  end

end