From: Robert Klemme <shortcutter@...>
Date: 2010-01-09T05:15:14+09:00
Subject: Re: ruby constructor return value?

On 01/08/2010 12:39 PM, I V wrote:
> On Thu, 07 Jan 2010 15:13:30 -0500, Thom Wharton wrote:
>> Basically, I am trying to get new to return nil if construction of the
>> object fails.  I have a strong background in C++ coding, and one thing
>> that has always irked me about the language is that constructors cant
>> fail -- the object is always created regardless of whether or not it can
>> be properly constructed.
> 
> I'm not sure what you mean here. In both C++ and Ruby, constructors can 
> fail - they do so by raising an exception - and, if they do fail, no 
> object is created.

That's not true: the object is created and then will be GC'ed.  You can 
easily demonstrate it:

robert@fussel:~$ ruby19 f.rb
[83760838, false]
[83760376, true]
f.rb:9:in `initialize': Constructor error (RuntimeError)
	from f.rb:14:in `new'
	from f.rb:14:in `<main>'
83760376
83760838
robert@fussel:~$ cat f.rb
class X
   def self.g(x)
     ObjectSpace.define_finalizer(x) {|id| puts id}
   end

   def initialize(fail = false)
     p [object_id, fail]
     X.g(self)
     raise "Constructor error" if fail
   end
end

X.new
X.new true
robert@fussel:~$

There is no other alternative to this approach.  The instance is 
*always* created.  It's only that you typically do not leak self from 
#initialize and so in case of exceptions nobody has a reference to the 
object and hence it is unusable.

Kind regards

	robert


-- 
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