From: Paul Brannan <pbrannan@...>
Date: 2007-04-12T23:00:27+09:00
Subject: Re: A C++ reference in a Ruby extension

On Thu, Apr 12, 2007 at 10:07:52AM +0900, David Simas wrote:
> 
> 
> I'm trying to write an extension to access some C++ libraries from
> Ruby (1.8.4).  One of the C++ functions uses a reference to pass
> a value back to the caller as a side effect:
> 
>    ErrorHandle* DetectInvisibleWatermark(const char *inputfilename,
> 					const char *wmfilename,
> 					float horizOffset,
> 					float vertOffset,
> 					float lowPassFilterFactor,
> 			      	 	float &modulationStrength,
> 					const char *key, const char *seed,
> 					const char *outfilename);
> 
> (N.B.: "float &modulationStrength")
> 
> Any suggestions on how to handle this reference in an extension?
> I don't have the C++ sources, just libraries and header files.

In general, there are two options for handling out parameters:

1. Treat them as additional return values (so effectively you have
   "return a, b" instead of "return a") - this is less consistent with
   the C++ interface but is easy to use

2. Box the value so it can be modified (so for modulationStrength you'd
   pass in Box.new, then get modulationStrength out of the Box) - this
   is more consistent with the C++ interface but IMO is harder to use

But in this case you are using a return value for error handling.  I'd
raise an exception instead of returning error values.

As an aside, in general, returning errors in C++ is a bad idea, because
C++ doesn't provide a good mechanism for returning failure from a
constructor other than an exception.  It's usually better to be
consistent and use exceptions everywhere, rather than only where you
need them.

Paul