From: Stefano Crocco <stefano.crocco@...>
Date: 2010-01-08T05:42:45+09:00
Subject: Re: ruby constructor return value?

On Thursday 07 January 2010, Thom Wharton wrote:

> |Is this info still valid?  More specifically, would I have to create a
> |new method for my class to have it return nil?

Yes, it's still valid. Class.new, which is the method you actually invoke when 
you write MyCls.new, always returns the new instance, regardless of the value 
returned by initialize (if it were not so, every initialize method should 
explicitly return self to have new return the new instance).

> |And would my class
> |initializer method have to raise an exception or could it return nil if
> |the construction fails?

If you override new, you can have it treat the value returned by initialize 
how you want. There's nothing special in the initialize method (aside from the 
fact that it's always private). If you want, you can decide that your 
initialize method will return nil if it failed and anything else if it 
succeeds. Or it may return 5 if it fails and 2 if it succeeds. Then in the new 
method you check by the return value of initialize and proceed accordingly.

> |Btw, I've noticed that the File class will return nil if you call the
> |new method with a filename that doesnt refer to an existing file.  Does
> |the File class have its own new/initialize methods?  If so, is that code
> |available online for examination?

It does, but they're written in C, so you'll have to understand a bit of ruby 
C api to understand them. You can download the ruby source from
www.ruby-lang.org. The initialize method is implemented in rb_file_initialize 
C function while I think (but I'm not sure) that the new method is implemented 
in rb_io_s_new. Both functions are defined in io.c.

However, implementing a new method is truly quite easy:

class MyClass
  
  def self.new *args
    inst = allocate
    res = inst.send :initialize, *args
    if res < 5 then "Hello"
    else res
    end
  end
  
  def initialize x
    x
  end
  
end

i1 = MyClass.new 3
p i1
i2 = MyClass.new 6
p i2

MyClass.new returns the new instance if the initialize method returns a number 
greater than 4 and the string "Hello" if initialize returns a number up to 
four.

I hope this helps

Stefano