From: Brian Candler <B.Candler@...>
Date: 2003-04-14T23:22:59+09:00
Subject: Re: [Q] A class, that knows about it's instances + Sets

On Mon, Apr 14, 2003 at 10:44:57PM +0900, Chris Pine wrote:
>     def initialize fromCreateKey
>       raise 'Illegal object creation.' if fromCreateKey !=
> :afdifw489r3k4jge98

FWIW, I believe the normal way of doing this is simply to make 'new'
private (unless I'm missing some reason why this can't be done) - or in this
case protected because there are multiple classes involved:

  class Parent
    class <<self
      protected :new
    end
    def self.create(*params)
      obj = self.new(*params)

      #  Set constant in child class.
      begin
        const_get(:INSTANCES) << obj
      rescue NameError
        const_set(:INSTANCES,[obj])
      end
    end
  end

  class Child < Parent
    def initialize(foo="foo")
      @foo = foo
    end
  end

  Child.create
  Child.create("bar")
  p Child::INSTANCES

But it should be possible to refactor this as a module which you include
into a class to keep track of instances. For example, stealing shamelessly
from singleton.rb:

  module Tracker
    def Tracker.append_features(klass)
      klass.private_class_method(:new)
      @instances = nil
      klass.instance_eval %{
        @__instances__ = []
        def instances
          @__instances__
        end
        def create(*params)
          obj = new(*params)
          @__instances__ << obj
          obj
        end
      }
    end
  end

  class Thingy
    include Tracker
    def initialize(foo="foo")
      @foo = foo
    end
  end

  Thingy.create
  Thingy.create("bar")
  p Thingy.instances

Regards,

Brian.