From: Farrel Lifson <farrel.lifson@...>
Date: 2006-09-14T00:11:28+09:00
Subject: Re: What does it take to make an object hashable, no really...

On 13/09/06, 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.
>     Secondly, if an objects works in a hash, is it guaranteed to work in a
> set?
>     Thank you...
>
>
>
>
>

As far as I know all objects respond to .hash and can therefore be
used as keys.
 irb(main):001:0> class HashableClass
 irb(main):002:1> end
 => nil
 irb(main):003:0> h1 = HashableClass.new
 => #<HashableClass:0x2dc4270>
 irb(main):004:0> h2 = HashableClass.new
 => #<HashableClass:0x2dc1c78>
 irb(main):005:0> h1.hash
 => 23994680
 irb(main):006:0> h2.hash
 => 23989820
 irb(main):007:0> hash = {h1=>"H1",h2=>"H2"}
 => {#<HashableClass:0x2dc4270>=>"H1", #<HashableClass:0x2dc1c78>=>"H2"}
 irb(main):008:0> hash[h1]
 => "H1"

If you undefine hash or have it return nil, then you can run into problems:
 irb(main):009:0> def h1.hash
 irb(main):010:1>   nil
 irb(main):011:1> end
 => nil
 irb(main):012:0> hash[h1]
 NoMethodError: undefined method `%' for nil:NilClass
         from (irb):12:in `[]'
         from (irb):12
 irb(main):013:0>

Farrel