From: Charles Oliver Nutter <headius@...>
Date: 2009-09-04T00:54:08+09:00
Subject: Re: doubling numbers with Object#object_id

On Wed, Aug 26, 2009 at 5:21 PM, Shot (Piotr Szotkowski)<shot@hot.pl> wrote:
> EbFFA:
>
>> I was playing with Ruby and wrote an interesting method:
>
>>     def double n
>>       n.object_id - 1
>>     end
>
>> Anyone know why this works? Why Ruby was implemented this way?
>
> Note that other Ruby implementations (JRuby, Rubinius, IronRuby…) do
> not have to keep this contract; it’s best considered as MRI peculiarity.
> But yes, I do find this design beautiful – as well as Ruby’s method
> lookup: http://www.viddler.com/explore/CobyR/videos/1/

I don't know that it's particularly beautiful; it's just a side effect
of Fixnums not having any internal state and not actually being
objects in MRI. Because they are "immediates" they don't actually have
an object_id. As a result, their object_id is just based on their
actual value, 2n+1. All other objects then get stuffed into the
remaining even object_ids, and fixnum.object_id - 1 == fixnum * 2.

- Charlie