[Rd] R_PreserveObject, R_ReleaseObject : reference counting needed ?

[Romain Francois]

Hello,

We are currently making lots of changes to Rcpp (see the open Rcpp 
mailing list if interested [1] in the details).

We are now using [2] R_PreserveObject and R_ReleaseObject to manage 
garbage collection instead of the PROTECT/UNPROTECT dance. This seems to 
work well, but I was wondering if there was documentation about it.

In particular, if we preserve the same SEXP twice (or more), should we 
implement some sort of reference counting ?

Reading the source (below, from memory.c) I think not, but some 
confirmation would help.

void R_PreserveObject(SEXP object)
{
     R_PreciousList = CONS(object, R_PreciousList);
}

static SEXP RecursiveRelease(SEXP object, SEXP list)
{
     if (!isNull(list)) {
	if (object == CAR(list))
	    return CDR(list);
	else
	    CDR(list) = RecursiveRelease(object, CDR(list));
     }
     return list;
}

void R_ReleaseObject(SEXP object)
{
     R_PreciousList =  RecursiveRelease(object, R_PreciousList);
}


I'd also be interested if there is some ideas on the relative efficiency 
of the preserve/release mechanism compared to PROTECT/UNPROTECT.

Thanks,

Romain

[1] http://lists.r-forge.r-project.org/pipermail/rcpp-devel/
[2] 
http://r-forge.r-project.org/plugins/scmsvn/viewcvs.php/pkg/src/RObject.cpp?rev=255&root=rcpp&view=markup

-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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