From: Dave Howell <groups@...>
Date: 2005-10-20T05:25:25+09:00
Subject: Re: How do you write an equality method?


On Oct 17, 2005, at 16:24, David Corbin wrote:

> Without getting into the details of which equality method (eql?, 
> equal?, ==),
> I'm curious about how such a method should be written for class where 
> you're
> trying to keep the interface appropriatley private.
>
> Consider:
>
> class Foo
> 	def initialize(color)
> 		@color = color
> 	end
> 	.
> 	.
> 	.
> end
>
> If I do not wish to expose color via an attr_reader, how do I write an
> equality method for this class?

If Foo-class objects are the same if they have the same color . . .

	redOne = Foo.new(red)
	greenOne = Foo.new(green)
	pineOne = Foo.new(green)

	class Foo
		def ==(other)
			self.color == other.color
		end
	end

	Now, redOne == greenOne is false. greenOne == pineOne is true.

I did something similar when trying to sort iTunes music tracks:

	def <=>(other)
		answer = self.artist <=> other.artist
		answer = self.album <=> other.album if answer == 0
		answer = self.name <=> other.name if answer == 0
		answer = self.dbid <=> other.dbid if answer == 0
		return answer
	end