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

[Romain Francois]

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
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