From: jos@... (Jos Backus)
Date: 2002-06-07T08:33:33+09:00
Subject: Re: Returning an instance from a different class from new

In article <3CFFC36E.DDC654F9@path.berkeley.edu>,
Joel VanderWerf  <vjoel@PATH.Berkeley.EDU> wrote:
>Jos Backus wrote:
>> 
>> [For some reason I am no longer subscribed to ruby-talk and I don't get any
>> mail back from the ctl address, which is why I have to use my provider's
>> crappy newsserver :-(]
>> 
>> How do I make the following return Bar instead of Foo?

[clever mods deleted]

Thanks, but this depends on the Bar and Baz initializers to not have other
arguments. What if I want to pass in extra arguments as in the example below?

The way I understand this works is that you override the Foo new class method.
Foo.new("1") causes Bar.new to be called with sel == nil, which causes super()
(i.e. Foo.new) to be called. super() returns and then Bar.new returns and then
Foo.new returns with the new object created by Bar.new. Does that make sense?

(Btw, the app is a script to {un,}subscribe myself from different mailing
lists run by different types of mailing list managers.)

class Foo
  def initialize(sel, f)
    @f = f
  end
  def self.new(sel=nil)
    case sel
      when "1"
        return Bar.new
      when "2"
        return Baz.new
      when nil
	super()
    end
  end
end

class Bar < Foo
end

class Baz < Foo
end

o = Foo.new("1", "field")
p o.type
lizzy:~/mail/setup% ruby k
k:23:in `new': wrong # of arguments(2 for 1) (ArgumentError)
        from k:23
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