From: Rick DeNatale <rick.denatale@...>
Date: 2009-02-17T00:46:45+09:00
Subject: Re: Inherit and override in the same time?

--00163630f2bfb08ba404630b16a1
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

On Mon, Feb 16, 2009 at 10:29 AM, Philippe Lang
<philippe.lang@attiksystem.ch> wrote:

> Hi Ruby gurus,
>
> Let's say we have a Parent class, and a Child1 class that inherits from
> the Parent class.
>
> I'd like to create a Child2 class that inherits most of the Child1
> class, except for one method, which I'd like to override completely. The
> problem is that I need to call the method in the Parent class, and
> apparently "super.super" is not supported:
>
> ----------------------------
> class Parent
>  def go
>   puts 'parent'
>  end
> end
>
> class Child1 < Parent
>  def go
>   super
>   puts 'child1'
>  end
>  def a_method_id_like_to_inherit
>  end
> end
>
> class Child2 < Child1
>  def go
>   super.super
>   puts 'child2'
>  end
> end
>
> Child2.new.go
> ----------------------------
> in `go': undefined method `super' for nil:NilClass (NoMethodError)
> ----------------------------
>
> I ended up doing this, which apprently works.
>
> ----------------------------
> class Parent
>  def go
>   puts 'parent'
>  end
> end
>
> class Child1 < Parent
>  def go
>   super
>   puts 'child1'
>  end
>  def a_method_id_like_to_inherit
>  end
> end
>
> class Child2 < Child1
>  def go
>   self.class.superclass.superclass.instance_method( :go ).bind( self
> ).call
>   puts 'child2'
>  end
> end
>
> Child2.new.go
> ----------------------------
> parent
> child2
> ----------------------------
>
> Isn't there anything simpler than that? Or is that kind of pattern
> considered as bad design maybe?
>
>
It's not clear WHY you want to do this but..

your self.class.superclass.superclass.instance_method(:go).bind(self).call

really doesn't do the same thing, in general as your desired super.super
(which I've not by the way seen in other dynamic OO languages either).

Consider what would happen if you added

class Child3 < Child2
end

Child3.new.go

I think that the right way to do this is to refactor the method to extract
just the behavior you want to control, so in this particular case something
like

class Parent
 def go
  puts 'parent'
 end
end

class Child1 < Parent

 def put_me
   puts 'child1'
 end

 def go
  super
  put_me
 end

end

class Child2 < Child1
 def put_me
  puts 'child2'
 end
end

class Child3 < Child2
end

Parent.new.go
Child1.new.go
Child2.new.go
Child3.new.go

HTH

-- 
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale

--00163630f2bfb08ba404630b16a1--