From: Jim Freeze <jfreeze@...>
Date: 2002-01-17T04:23:40+09:00
Subject: Re: Protecting a member hash

On Thu, Jan 17, 2002 at 04:09:34AM +0900, Jim Freeze wrote:
> On Thu, Jan 17, 2002 at 03:55:38AM +0900, Phil Tomson wrote:
> > In article <20020116131656.A93722@freebsdportal.com>,
> > Jim Freeze  <jfreeze@freebsdportal.com> wrote:
> > >Hi:
> > >
> > Why not just define a '[]' method, but not a '[]=' method in your class 
> > and don't use the attr_reader.  Like:
> > 
> > class Test
> >   def initialize
> >     #HasA hash:
> >     @entity = {'a'=>"fred"}
> >   end
> > 
> >   def [](key)
> >     @entity[key]
> >   end
> > 
> > end
> > 
> > t = Test.new
> > p t['a']       #this will work
> > t['b']="Error" #this will raise an error
> > 
> > So basically, you're hiding the hash from the outside world.
> 
> Well. that would work, but the real story is that I have
> multiple hashes in the class. So, the single Test#[] is
> limiting. I suppose I would write a class for each hash.

Well, after thinking about this a little longer and trying
to implement it, I don't think this will work either.

Assume I have the class:

class HashProtector
  def [](key)
    @hash[key]
  end

  def []=(key,val)
    @hash[key] = val
  end
  protected :[]=

  def initialize(a)
    @hash = a
  end
end

and create the Test class:

class Test
  attr_reader :h

  def initialize
    @h = HashProtector.new({'a'=>"fred"})
    @h2 = HashProtector.new({'a'=>"fred"})
  end

  def set_h(k,v)
    @h[k] = v
  end
end

Then set_h will not work because Test is not
derived from HashProtecter. What I need is 
some kind of friend mechanism.

-- 
Jim Freeze
Today is a fine day for Ruby programming.
www.freeze.org