From: sur max <sur.max@...>
Date: 2007-02-16T22:22:35+09:00
Subject: Re: class design issues

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Hi David,

Now this should not be available !!

lf1.name << "more stuffs"
this should generate error !! ... (i agree it is working)

but is name is an attr_reader then the manipulations with "<<" should not be
supported, as the case of "="
or it should be ?


On 2/16/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
>
> Hi --
>
> On Fri, 16 Feb 2007, Robert Klemme 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
>
> It does raise the danger of:
>
>    lf1.name << "more stuff"
>
> so it might be good to dup or freeze the mutable ones (though of
> course someone who really wants to can always pry in with
> instance_eval).
>
>
> David
>
> --
> Q. What is THE Ruby book for Rails developers?
> A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
>     (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
> Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
> A. Ruby Power and Light, LLC (http://www.rubypal.com)
>
>


-- 
sur
http://expressica.com

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