From: Robert Klemme <shortcutter@...>
Date: 2009-03-19T03:02:24+09:00
Subject: Re: Is it safe to override #hash and #eql? to enable hash key equality?

On 18.03.2009 18:59, James wrote:
> Hi,
> 
> I would like to use object instances as hash keys where two objects
> containing the same datum both have the same key value.  This does not
> work by default (as expected given that these are two different
> instances):
> 
> class Foo
>   def initialize(datum)
>     @datum = datum
>   end
> end
> 
> # Two hash keys for the same datum
> a = {Foo.new(:baz) => 1, Foo.new(:baz) => 2}
> p a
> 
> # => {#<Foo:0x2a95e68888 @datum=:baz>=>1, #<Foo:0x2a95e68860
> @datum=:baz>=>2}
> 
> However, this functionality can be obtained by overriding Object#hash
> and Object#eql?:
> 
> class Bar
> 
>   attr_reader :datum
> 
>   def initialize(datum)
>     @datum = datum
>   end
> 
>   def hash
>     @datum.hash
>   end
> 
>   def eql?(other)
>     @datum.eql? other.datum
>   end
> 
> end
> 
> # One hash key for the same datum
> b = {Bar.new(:baz) => 1, Bar.new(:baz) => 2}
> p b
> 
> #=> {#<Bar:0x2a95e684f0 @datum=:baz>=>2}
> 
> My simple question is if this is safe/normal/OK for my intentions.

Not only that: this is exactly how one should do it.

> Based on perusal of the online Ruby documentation it looks like #eql?
> has been overridden in a number of sub-classes for the same purpose.
> But I wanted to get feedback from the community - has anyone here
> tried this?  Are there any known side-effects?

No, your use is absolutely correct.  This is what this methods are meant 
for.  The only thing to remark is this: you can save yourself a lot 
typing by doing:

Bar = Struct.new :datum

If you need additional methods in that class you can even do

Bar = Struct.new :datum do
   def another_method
      123
   end
end

Kind regards

	robert