From: John Wilger <johnwilger@...>
Date: 2007-03-15T17:25:08+09:00
Subject: Re: General Ruby OOP Question - using inheritance or include for shared attributes

On Mar 15, 12:11 am, "Trans" <transf...@gmail.com> wrote:
> On Mar 15, 1:35 am, james.d.mast...@gmail.com wrote:
> > I have a generic Ruby OOP question.  Which is the more "correct" way
> > to share methods between two similar classes that do not necessarily
> > need to inherit from anything else: inheritance or include (as a mix-
> > in)?
>
> When it comes to Ruby there is a huge consideration here: Do you or
> will you ever wish to pass on class level methods? Or more technically
> speaking, do you want class singleton methods to be in the inheritance
> chain? If so then you have to use a class, since modules don't allow
> it (to my eternal dismay). For example:
>
>   class X; def self.x; "x"; end; end
>   class Y < X; end
>   Y.x  #=> x
>
>   module X; def self.x; "x"; end; end
>   class Y; include X; end
>   Y.x  #=> error

Modules may not allow it directly, but it seems that you can work
around it easily enough:

	module A
		def self.included( other )
			def other.foo
				puts 'bar'
			end
		end
	end

	class B
		include A
	end

	class C < B
	end

	#> B.foo
	#=> bar
	#
	#> C.foo
	#=> bar

Or am I missing something important here?

Really, the choice of whether to use inheritance or module inclusion
comes down to the question of whether you are sharing identity or just
behavior. For instance, do you want:

	class WalkingThing
		def walk
			puts 'walking'
		end
	end

	class Human < WalkingThing
	end

	class Robot < WalkingThing
	end

or:

	module WalkingThing
		def walk
			puts 'walking'
		end
	end

	class Human
		include WalkingThing
	end

	class Robot
		include WalkingThing
	end

A bit contrived, sure -- but hopefully makes the point.

--
Regards,

John Wilger
http://johnwilger.com