# A C++ reference in a Ruby extension

**URL:** <https://www.ruby-forum.com/t/a-c-reference-in-a-ruby-extension/91758>\
**Category:** Ruby\
**Created:** [April 12, 2007, 3:09am UTC](https://www.ruby-forum.com/t/a-c-reference-in-a-ruby-extension/91758 "2007-04-12T03:09:06Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![David\_Simas](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/d2c977/32.png) [@David\_Simas](https://www.ruby-forum.com/u/David_Simas)\
**Post date:** [April 12, 2007, 3:09am UTC](https://www.ruby-forum.com/t/a-c-reference-in-a-ruby-extension/91758/1 "2007-04-12T03:09:06Z")

</div>

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.

Thanks,

DGS

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<div class="post-metadata">

**Author:** ![David\_Simas](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/d2c977/32.png) [@David\_Simas](https://www.ruby-forum.com/u/David_Simas)\
**Post date:** [April 12, 2007, 5:37am UTC](https://www.ruby-forum.com/t/a-c-reference-in-a-ruby-extension/91758/2 "2007-04-12T05:37:15Z")

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On Thu, Apr 12, 2007 at 10:07:52AM +0900, David Simas wrote:

> ```
> float lowPassFilterFactor,
> float &modulationStrength,
> const char *key, const char *seed,
> const char *outfilename);
> 
> ```
> 
> (N.B.: “float &modulationStrength”)  
> Wrap it in a C function  
> void foo(float\* bar) {  
> baz(\*bar);  
> }

void baz(float& quux) {  
quux /= 1.5;  
}

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<div class="post-metadata">

**Author:** ![David\_Simas](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/d2c977/32.png) [@David\_Simas](https://www.ruby-forum.com/u/David_Simas)\
**Post date:** [April 12, 2007, 7:16am UTC](https://www.ruby-forum.com/t/a-c-reference-in-a-ruby-extension/91758/3 "2007-04-12T07:16:19Z")

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Logan C. wrote:

> > ```
> > float lowPassFilterFactor,
> > 
> > ```
> 
> void baz(float& quux) {  
> quux /= 1.5;  
> }

There’s still the question of how to get both return values back to the  
caller. It depends on how you want to use the function from ruby. You  
could return both the ErrorHandle and a float, as a two element array (a  
ruby array, that is). The caller can then unpack them like this:

err\_h, mod\_str = wrap\_DetectInvisibleWatermark(…)

Or you could return one and yield the other (a good choice if one is  
less likely to be used).

Or maybe return the float (as a ruby float, of course) and rb\_raise() if  
there is a nonzero ErrorHandle\* .

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<div class="post-metadata">

**Author:** ![David\_Simas](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/d2c977/32.png) [@David\_Simas](https://www.ruby-forum.com/u/David_Simas)\
**Post date:** [April 12, 2007, 4:01pm UTC](https://www.ruby-forum.com/t/a-c-reference-in-a-ruby-extension/91758/4 "2007-04-12T16:01:22Z")

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On Thu, Apr 12, 2007 at 10:07:52AM +0900, David Simas wrote:

> ```
> 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