[Rcpp-devel] Registering a custom delete_finalizer for my XPtrs

[Steve Lianoglou]

Hi Romain,

Thanks for humoring my request!

I'll take a look at it within the next few days to see how it works ...

Cheers,
-steve

On Thu, Jul 7, 2011 at 1:48 PM, Romain Francois
<romain at r-enthusiasts.com> wrote:
> Hello,
>
> I have added (in rev 3120) some code that allows the developper to register
> his own finalizer instead of the default one.
>
> For example:
>
> require( Rcpp )
> require( inline )
>
> inc <- '
>
>    class Foo{
>        public:
>            Foo(double x_) :x(x_){}
>            double x;
>    } ;
>
>    void bla(Foo* x){
>        Rprintf( "hello\\n" ) ;
>    }
>
> '
> fx <- cxxfunction( , '
>    Foo* f = new Foo(2.0) ;
>    XPtr<Foo,bla> xp(f,true) ;
>    return xp ;
> ', inc = inc, plugin = "Rcpp" )
> fx()
> gc()
> gc()
>
> This is achieved by adding a second template parameter to XPtr, with a
> default value so that the original behaviour is retained.
>
> The class signature of XPtr hence becomes:
>
> template <typename T, void Finalizer(T*) = standard_delete_finalizer<T> >
> class XPtr : public RObject
>
> The second parameter is a function that takes a a single argument (a pointer
> to the target class).
>
> Hope this gives you what you need.
>
> Romain
>
>
> Le 04/07/11 01:02, Steve Lianoglou a écrit :
>>
>> Greetings,
>>
>> Preamble: My C++ is quite ... hmmm, does anybody know any good
>> euphemisms for "weak"?. So, sorry if this (or questions that will
>> likely follow) is too basic.
>>
>> I've been using Rcpp to help make a more R-like wrapper library to the
>> shogun-toolbox[1]. So far, it's been pretty fun, and (to my surprise
>> :-) I've been able to get basic things working, such as building a
>> handful of different types of SVMs. A testament to the quality of both
>> Rcpp and the shogun-toolbox code, since, as I said, my C++ isn't "the
>> best".
>>
>> I know I'm not making the most out of using Rcpp, and wanted to write
>> my code in a more Rcpp-inspired manner before I get too deep. (I
>> actually don't know if Rcpp-modules would work here, but maybe will
>> try that in the future as well).
>>
>> So, in my C++ code, I wire different objects together, and return a
>> pointer to the shogun object that I just made. Currently I've got a
>> simple function that wraps pointers to shogun objects into externalptr
>> SEXP's and registers my custom finalizer, which basically does what
>> you'd expect, eg:
>>
>> SEXP SG2SEXP(shogun::CSGObject *o) {
>>     SEXP xp = R_MakeExternalPtr(o, R_NilValue, R_NilValue);
>>     R_RegisterCFinalizer(xp, _shogun_ref_count_down);
>>     return xp;
>> }
>>
>> What I thought a more Rcpp-inspired thing to do is to instead
>> instantiate pointers to shogun objects using
>> Rcpp::XPtr<SomeShogunObject>and just rely on the auto-wrapping to make
>> things "more clean", maybe like:
>>
>> // ...
>> Rcpp::XPtr<SomeShogunObject>  so(new SomeShogunObject(what,ever), true);
>> so->do_something(special);
>> // ...
>> return so;
>>
>> But I don't want Rcpp's "vanilla" delete_finalizer to be invoked on my
>> object's R-side destruction -- I want whatever I've defined in my
>> `_shogun_ref_count_down` function to be used instead.
>>
>> (this is where my n00b-ness becomes self-evident):
>>
>> Since XPtr::setDeleteFinalizer doesn't take a parameter for the
>> finalizer function to use, I thought I the "expected" way to achieve
>> this is through template specialization of delete_finalizer in my
>> package. Since every object in the shogun-toolbox extends CSGObject, I
>> thought this would be easy enough and maybe do something like:
>>
>> template<>
>> void delete_finalizer<shogun::CSGObject>(SEXP p) { /* something special */
>> }
>>
>> But that doesn't work/compile -- I guess because this specialization
>> and Rcpp's delete_finalizer definition aren't in the same compilation
>> units, as described here:
>>
>> http://www.parashift.com/c++-faq-lite/templates.html#faq-35.12
>>
>> -- or maybe I have to stick it in the Rcpp namespace somehow, like the
>> custom wrap/as trick?
>>
>> So -- long story short ... is there a more Rcpp/spiffy way for me to
>> register my own delete_finalizer function, or should I just keep going
>> with my SG2SEXP function for now, and save the Rcpp-swagger for when I
>> see if Rccp-modules is a good fit (and just use its *.finalizer()
>> business)?
>>
>> Thanks,
>>
>> -steve
>>
>> [1] shogun-toolbox: http://www.shogun-toolbox.org/
>>
>
>
> --
> Romain Francois
> Professional R Enthusiast
> +33(0) 6 28 91 30 30
> http://romainfrancois.blog.free.fr
> http://romain-francois.com
> |- http://bit.ly/lJoWbH : Montpellier Comédie Club - Juin 2011
> |- http://bit.ly/kaSV6U : Stand up set at Up The Creek
> `- http://bit.ly/hdKhCy : Rcpp article in JSS
>
>
>



-- 
Steve Lianoglou
Graduate Student: Computational Systems Biology
 | Memorial Sloan-Kettering Cancer Center
 | Weill Medical College of Cornell University
Contact Info: http://cbio.mskcc.org/~lianos/contact