From: walter@...
Date: 2004-03-18T01:41:55+09:00
Subject: Can subclass have superclass method return instance of subclass

Is it possible to have the Superclass method return an instance of 
the Subclass type.

Basically are there any hooks that would allow a Array methods return 
an instance of a sub-class of Array.

The partition method would need to return an array of Array 
Sublcasses.  

I know I can make an Array subclass (or another class altogether) 
that itself can be subclassed and would do what I wanted.  I just 
wondered if there is a much easier general solution that I am 
missing.  


example:

class Q
  attr_accessor :e, :r

  def initialize(e,r)
    @e,@r = e,r
  end
end

class QArray < Array
  def sum_e
    inject(0){|sum, item| sum+=item.e}
  end
end

qArray = QArray.new
qArray << Q.new(49, 'A')
qArray << Q.new(50, 'B')
qArray << Q.new(51, 'A')

puts qArray.class
puts qArray.sum_e
puts qArray.find_all{|item| item.e < 50}.sum_e  #fails here


Can find_all defined in Array be coerced into returning
a QArray and not a plain Array?



Thanks,

Walt
*****************************************************
Walter Szewelanczyk
IS Director
M.W. Sewall & CO.        email : walter@mwsewall.com     
259 Front St.            Phone : (207) 442-7994 x 128
Bath, ME 04530           Fax   : (207) 443-6284
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