From: Thomas Hurst <tom.hurst@...>
Date: 2008-04-03T08:29:28+09:00
Subject: Re: ruby constructor return value?

* Brendan Stennett (brendan6@gmail.com) wrote:

> if o = Zip.new('00000')
>   //this block execute whether or not that zip code exists
> else
>   //never happens
> end

Normally you'd use an exception; e.g. make a ZipCodeNotFound class
inherited from StandardError and rescue it.  You could even do:

  o = Zip.new(..) rescue nil

And this will rescue the exception and return nil.  It will also eat any
other StandardError, so beware.  If you want users of the class to be
able to just check for nil/false, make a factory method:

  class Zip
    def self.find(zip)
      new(zip)
    rescue ZipCodeNotFound
      false
    end
  end

  o = Zip.find('..')

However, new is just a method like any other; you can override it if you
really want it to potentially return nil:

  class Zip
    def self.new(*args)
      o = allocate
      if o.__send__(:initialize, *args)
        return o
      else
        return nil
      end
    end
  end

Allocate will make a new object without calling the constructor, you can
then call initialize yourself (using __send__ as it's a private method)
and conditionally return your new object.

Since this may be surprising behavior for .new I don't really recommend
it, though.

-- 
Thomas 'Freaky' Hurst
    http://hur.st/