From: Rein Henrichs <reinh@...>
Date: 2010-06-11T15:15:07+09:00
Subject: Re: comparing objects

On 2010-06-10 22:52:14 -0700, Robert Dober said:

> On Thu, Jun 10, 2010 at 6:48 PM, Mark Abramov <markizko@gmail.com> wrote:
>> Robert Dober wrote:
>>> On Thu, Jun 10, 2010 at 5:30 PM, Rein Henrichs <reinh@reinh.com> wrote:
>>> overwriting #hash and #eql? breaks �Hash!
>> That's not true, I think.
> Judge for yourself
> 
> require "forwardable"
> 
> def count klass
>   ObjectSpace.each_object( klass ).to_a.size
> end
> class N
>   extend Forwardable
>   attr_reader :n
>   def_delegators :n,  :hash
>   def eql? otha
>     n == otha.n
>   end
>   private
>   def initialize n
>     @n = n
>   end
> end # class N
> 
> 
> h = { N.new( 42 ) => true }
> h[ N.new( 42 ) ] = 42
> p h
> GC.start
> p count(N)
> 
> Cheers
> R.

This breaks Hash? Quite the opposite!

This is precisely what is meant by "defining the semantics" of a class 
for use by hashes and the very behavior you want when you define #eql? 
and #hash in the first place!

You wouldn't say that defining #<=> breaks Array#sort, so why would you 
say that this "breaks Hash"? This doesn't break Hash. If anything, it 
fixes it when using N objects as keys!
-- 
Rein Henrichs
http://puppetlabs.com
http://reinh.com