From: Robert Klemme <shortcutter@...>
Date: 2007-02-16T22:00:06+09:00
Subject: Re: class design issues

On 16.02.2007 13:51, Spitfire wrote:
>   I have a class which takes an input and produces an object. Let's say, 
> it takes inputs about specifications of a life-form, and then creates it 
> (instantiates say an object, 'LifeForm'). Let's call this factory class 
> 'Creator'. Now, my problem is how do I ensure that once 'Creator' 
> returns a 'LifeForm', any external/requestor class can only view the 
> properties of LifeForm, that were set during creation, and not be able 
> to modify them???
>   How do I design these imaginary classes 'Creator' and 'LifeForm'?

First, it is very hard to actually prevent changes of instance variables 
(if it is possible at all).  For your purposes it is probably sufficient 
to define attribute readers only.  Second, you do not necessarily need a 
second class - basically LifeForm is the factory for LifeForm instances. 
  So you could do

class LifeForm
   attr_reader :age, :name, :foo

   def initialize(age,name,foo)
     @age = age
     @name = name
     @foo = foo
   end
end

irb(main):010:0> lf1 = LifeForm.new 10, "bla", "buzz"
=> #<LifeForm:0x7ef6f5e4 @name="bla", @foo="buzz", @age=10>
irb(main):011:0> lf1.name
=> "bla"
irb(main):012:0> lf1.name = "ddd"
NoMethodError: undefined method `name=' for #<LifeForm:0x7ef6f5e4 
@name="bla", @foo="buzz", @age=10>
         from (irb):12
         from :0

HTH

	robert