From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-07-08T02:57:30+09:00
Subject: Re: Inheriting Array and slice() behaviour

On Fri, 8 Jul 2005, William Morgan wrote:

> Excerpts from Robert Klemme's mail of  6 Jul 2005 (EDT):
>> I wouldn't be so certain of this.  It's normal for operations like
>> #dup and #clone to behave that way.  Also, part of the smartness of
>> ruby is that you can simply do self.class.new to get a new instance of
>> the same class.  And often you need to create an instance of the same
>> class.  That's not too abnormal.
>
> Dup and clone work on objects that have already been initialized.
> Self.class.new is done explicitly, so that's fine too---if the
> constructor has required arguments, you supply them at that point.
>
> My problem with Array#slice is that the objects created will never have
> #initialize called. Indeed, they never *can* have it called, because who
> knows what arguments need to be supplied.

it's this simple:

   harp:~ > cat a.rb
   class Expression < ::Array
     def initialize(*a, &b)
       init(*a, &b)
     end
     def init(*a, &b)
       @a, @b = a, b
     end
     def inspect
       {'a' => @a, 'b' => @b}.inspect
     end
     inited_return_methods = %w(
       slice
     )
     inited_return_methods.each do |m|
       eval <<-definition
         def #{ m }(*a, &b)
           ret = super
           ret.init(*@a, &@b)
           ret
         end
       definition
     end
   end

   exp = Expression::new 'an arg', 'another arg'

   subexp = exp.slice 0,2

   p subexp


   harp:~ > ruby a.rb
   {"a"=>["an arg", "another arg"], "b"=>nil}


note that the subexp has the same values given to the ctor of the original exp.


this will not be exactly what you want, but the approach is generic:

   * remember what args the object was initialized with
   * make a public initializer
   * use meta-programming to re-construct the methods you want such that return
     values are, in fact, initialized.

if you remember what an object was initialized with it's always possible to
re-initialize it or to initialize another object with those values - you need
only decide the rules for doing so.

hth.

-a
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