From: Samuel Tesla <samuel@...>
Date: 2003-12-27T14:51:49+09:00
Subject: Re: array operations

Ferenc Engard <ferenc@engard.hu> writes:

> What does hash comparison mean? I have read the Object#hash and
> Object#eql? documentation, but they do not mention arrays. Moreover,
> there comes the question: what the default Object#hash do? It has a
> generic algorithm which computes something based on the object's
> attributes?

Hash comparison means that it compares the value of Object#hash for each
object.  This is where a quick look at the Ruby source is very enlightening.
Having had to do some battle with these methods recently myself, I have the
code fresh in my mind.  Object#hash just returns the object ID by default.
This is done in C code, so if you override Object#object_id, it will not return
what you expect.

> Anyway, I have to define both == and eql? in my classes to work
> properly? What is the semantical difference of this two method? And then
> I have to redefine hash also to ensure that a.eql?(b) implies
> a.hash==b.hash? 

You need to define #eql? and #hash for hash comparisons to work properly, and
you need to define #== for the == operator to work.  You need to define #=== if
you plan on using these in case statements (although it defaults to using #==),
and #equal? does not need to be redefined.

Hope this helps!

-- Samuel