From: ahoward <ahoward@...>
Date: 2002-11-09T00:52:24+09:00
Subject: return MyClass.new vs self.type.send :new


Rubyists-


I've seen a few threads relating to this subject, so i decided to start one...

What do people think is better style

  def Class
    def method
      Class.new
    end
  end

VS

  def Class
    def method
      self.type.send :new
    end
  end

the first is safer, but the second can be very nice for derived types since

  def SubClass < Class
  end

will result in this behavior

  sub = SubClass.new

  sub.method.type	>> SubClass


A recent example of why this might be a good design pattern is that
subclasses of Array must re-write all the 'usefull' methods like select, etc.
to return the SubClass'd type since these methods return an Array.

I can hear the arguments against :

  "what if the subclassed methods do something perverse with the methods"

  "what if the subclassed methods simply will not work if that is done"

etc.

I would say

  Sub-classing should always be reserved to _extend_ objects, old behavior
  should always remain - but may be added to.

  Agregation should be used in most cases.

  If your class can be broken using this methodology, then perhaps it should
  not be allowed to be subclassed at all without redesign.

  Subclassing is abused, and generally much harder to do right than generally
  realized.

The idiom of returning the 'current class'.new is used in perl alot, and it
_can_ be very usefull.  (probably did not help my argument to say the 'p' word
here...)

Any thoughts on this?

-a

-- 

 ====================================
 | Ara Howard
 | NOAA Forecast Systems Laboratory
 | Information and Technology Services
 | Data Systems Group
 | R/FST 325 Broadway
 | Boulder, CO 80305-3328
 | Email: ahoward@fsl.noaa.gov
 | Phone:  303-497-7238
 | Fax:    303-497-7259
 ====================================