From: sur max <sur.max@...>
Date: 2007-02-16T22:09:04+09:00
Subject: Re: class design issues

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I think just declaring it as attr_accessor is not that much a protection !!

its fine that

lf1.name  # => error, undefined method `name=' f

BUT, it can easily be broken as....

lf1.instance_variable_set("@name", "new broken name")

lf1.name  # => "new broken name"




On 2/16/07, Robert Klemme <shortcutter@googlemail.com> wrote:
>
> 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
>
>


-- 
sur
http://expressica.com

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