From: "David A. Black" <dblack@...>
Date: 2008-04-10T06:09:26+09:00
Subject: Re: My error or bug in "attr_reader" ?

Hi --

On Thu, 10 Apr 2008, Hexren wrote:

> Stefano Crocco wrote:
>> On Wednesday 09 April 2008, Hexren wrote:
>>> Hi guys.
>>> 
>>> The code posted below actually modifies the array in the object.
>>> Am I doing something wrong here ?
>>> If not, this should imho be included in the doc. It seems
>>> counterintuitive to me ;)
>>> 
>>> 
>>> Regards
>>> Oliver
>>> 
>>> ----------------------------------
>>> class AttrReaderErr
>>>    attr_reader :entries
>>>    def initialize()
>>>      @entries = [0,1,2]
>>>    end
>>> end
>>> 
>>> showcase = AttrReaderErr.new
>>> 
>>> puts showcase.entries
>>> #should this really work ?
>>> showcase.entries[1]="foo"
>>> #it does, but looks wrong
>>> puts showcase.entries
>>> ----------------------------------
>> 
>> It works correctly, once you understand exactly what's going on. 
>> attr_reader creates a method which returns the object stored in the 
>> instance variable @entries. If it were written by hand, it would be 
>> something like:
>> 
>> def entries
>>   @entries
>> end
>> 
>> Writing entries[1]="foo", you're calling the []= method of the object 
>> returned by entries, that is, the array stored in @entries. Since the []= 
>> method modifies the receiver, the content of @entries is modified. If you 
>> don't want this to happen, you need to create the entries method by hand 
>> and return a copy of the array:
>> 
>> def entries
>>   @entries.dup
>> end
>> 
>> I hope this helps
>> 
>> Stefano
>> 
>
>
> Yup, what you say makes sense, thanks :)
>
> But I maintain, that the doc is kind of confusing. The fingerprint:
> "attr(symbol, writable=false) => nil"
>
> Very strongly implies (at least to me) that the default is, for the instance 
> variable to be not writable. I am going to go, cry wolf on the doc mailing 
> list :)

A writable attribute is different from a mutable object. You can't set
the attribute to a new object, but the read operation does expose the
object behind the attribute -- so if that object is mutable, you can
mutate it.


David

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