From: Bob Hutchison <hutch@...>
Date: 2007-03-15T21:34:36+09:00
Subject: Re: General Ruby OOP Question - using inheritance or include for shared attributes


On 15-Mar-07, at 4:25 AM, John Wilger wrote:

> 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

module A
   def self.included(other)
       super
       other.extend(ClassMethods)
   end

   module ClassMethods
     def foo
       puts "bar"
     end
   end
end

Is a little cleaner I think. And you can do all kinds of other weird  
stuff in the self.included method.

Another consideration is that in a single inheritance language like  
Ruby, inheritance is valuable. I tend to use mixins until I'm forced  
to use inheritance. In a language like Ruby the 'type hierarchy'  
isn't all that important (duck typing makes it mostly irrelevant).

Cheers,
Bob

----
Bob Hutchison                  -- blogs at <http://www.recursive.ca/ 
hutch/>
Recursive Design Inc.          -- <http://www.recursive.ca/>
xampl for Ruby                 -- <http://rubyforge.org/projects/xampl/>