From: Tim Pease <tim.pease@...>
Date: 2006-09-14T00:25:59+09:00
Subject: Re: What does it take to make an object hashable, no really...

On 9/13/06, Just Another Victim of the Ambient Morality
<ihatespam@rogers.com> wrote:
>     Suppose I want to define a class and be able to use this class as a key
> to a hash.  How does one do this?  ...And please test your response before
> posting it.

Chapter 22 of the Programming Ruby, The Pragmatic Programmer's Guide

A key must respond to "hash" and the value returned must not change.

What is not in the book is that you must also provide an eql? method
that evaluates to true when two objects are equivalent.

class C
  def hash() "C".hash end
end

h = {}
h[C.new] = 1
h[C.new]

=> nil

class C
  def eql?(other) true end
end

h[C.new]

=> 1



>     Secondly, if an objects works in a hash, is it guaranteed to work in a
> set?

No idea about this one.  Have not used the Ruby Set class thus far.

TwP