From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2002-01-17T04:55:46+09:00
Subject: Re: Protecting a member hash

In article <20020116140956.A93861@freebsdportal.com>,
Jim Freeze  <jfreeze@freebsdportal.com> 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:
>> >
>> >I have a class that stores data in a hash.
>> >I would like the user to be able to retrieve
>> >values from the hash, but not modify it.
>> >
>> >class Test
>> >  attr_reader :entity
>> >  def initialize
>> >    @entity = {'a'=>"fred"}
>> >  end
>> >end
>> >
>> >t = Test.new
>> >p t.entity   #=> {"a"=>"fred"}
>> >t.entity['a'] = "changed"
>> >p t.entity   #-> {"a"=>"changed"}
>> >
>> >
>> >Other than freezing @entity, is it possible
>> >top protect it from being changed?
>> >
>> 
>> 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.
>Is there a cheaper way?
>

Jim,

What exactly are you trying to accomplish?  Maybe there's a different way 
to do it, perhaps with Structs or with  a hash of hashes?

Phil