From: Thom Wharton <thomas_wharton@...>
Date: 2010-01-08T05:13:30+09:00
Subject: Re: ruby constructor return value?

Thomas Hurst wrote:
> * 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.

Is this info still valid?  More specifically, would I have to create a 
new method for my class to have it return nil?  And would my class 
initializer method have to raise an exception or could it return nil if 
the construction fails?

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.

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?

Thanks,
Thom
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