From: Zakaria <zakaria@...>
Date: 2004-06-19T01:16:16+09:00
Subject: Re: How to make generic #== method?

On Fri, Jun 18, 2004 at 04:06:32PM +0900, Gavin Sinclair wrote:
> Zakaria wrote:

> > 2) It doesn't work if the class has hash attribute because
> >    {'a' => 1}.hash != {'a' => 1}.hash

> Now that is weird.  I've a faint recollection of it being mentioned on
> ruby-talk before though, so perhaps its not a bug.

Could someone explain why?

> #hash is used to generate a hash code, which is used to put an object into
> a hash (i.e. a Hash object).  If two objects have the same #hash value,
> then a Hash thinks they're probably the same, but it's not guaranteed, so
> it double-checks with #eql?.  However, if two objects have *different*
> #hash values, then a Hash considers them to be definitely different.

> Bottom line, if you (re)define #== in an object, you really really should
> (re)define #hash as well, to make sure they are consistent.

They only needed if I put the object as the hash key right?

> > PS: Is there any place where I could post this snippet,
> >     so others could use it?

> As it happens, I've already written a detailed consideration of Joel's
> code for the not-yet-formally-announced project 'addlib', so that people
> can reuse it without having to manage the code themselves.

I hope I could contribute.

> Until then, however, you might take a look at the snippet repository on
> RubyForge.

URL? I know I'm lazy :)

> Joel's code is better, however.  What yours adds is the implicit
> definition of #contents (in Joel's terminology).  So why not:

I think Robert Klemme version is better and I believe it's faster.
And I also like the name, MemberEquivalence.
OK going to sleep now

> Gavin

Wassallam,


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