[Rcpp-devel] class methods and R_RegisterCCallable - Rcpp-devel

Thu, Oct 24, 2019 9:23 AM #

On 24 October 2019 at 16:01, Serguei Sokol wrote:
| I'am trying to find a way to access class methods of one Rcpp package 
| (say pkgM) from C++ code in another one (say pkgA).
| If it were simple C++ routines and not class methods, the way is known 
| and even automatized. E.g. if I generate pkgM with
| 
| Rcpp::Rcpp.package.skeleton("pkgM", module=TRUE)
| Rcpp::compileAttributes("pkgM")
| 
| then in pkgM/src/RcppExports.cpp we can see
| 
| 
| static const R_CallMethodDef CallEntries[] = {
|      {"_pkgM_rcpp_hello_world", (DL_FUNC) &_pkgM_rcpp_hello_world, 0},
| ...
| 
| As far as I understand, it registers _pkgM_rcpp_hello_world for use with 
| R_GetCCallable from other packages. And it is a consequence of preceding
| rcpp_hello_world() with "// [[Rcpp::export]]".
| If I add "// [[Rcpp::interfaces(r, cpp)]]" in pkgM/src/rcpp_module.cpp 
| it produces pkgM/inst/include/{pkgM.h,pkgM_RcppExports.h} and
| 
| static const R_CallMethodDef CallEntries[] = {
| ...
|      {"_pkgM_RcppExport_registerCCallable", (DL_FUNC) 
| &_pkgM_RcppExport_registerCCallable, 0},
| ...
| but still nothing for methods and functions from RCPP_MODULE.
| 
| So my question, what is a canonical way (if defined) to expose class 
| methods (via RCPP_MODULE or any other mechanism) in such a way that they 
| become callable from C++ code of another package? Am I supposed to 
| register them manually in some way using 
| _pkgM_RcppExport_registerCCallable()? And if so, how can I retrieve them 
| in C++ code of another package?

From the top of my head I would say "No" for the simple reason that all R

does (and this is the same for all other foreign-function interfaces) has to
be C based.  Hence no class methods.  C++ signatures are compiler dependent,
and have been avoided for that reason for as long as we had them.

So I fear you need plain C wrappers for every member function you want to
expose. I could of course be wrong -- if someone else has counter-examples I
would be all ears!

Dirk

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org

I believe Dirk is correct in saying the traditional mechanism won't work 
for non-static member functions, but if the C++ code is fully contained 
in header files in 'inst/include', then it's possible to access that 
code from another package since there's no library that one needs to 
link against. I believe that's the mechanism that Rcpp itself uses.

Things get ugly when one tries to link to another package's library. 
I've tried a few experiments with Dirk's help 
(https://github.com/lsilvest/linktest), but haven't found a reliable way 
to make this portable (things break when packages are installed 
pre-compiled).

Leo

Serguei Sokol

Mon, Oct 28, 2019 3:39 AM #

Le 24/10/2019 Ã  18:23, Leonardo Silvestri a Ã©critÂ :

On 24 October 2019 at 16:01, Serguei Sokol wrote:
| I'am trying to find a way to access class methods of one Rcpp 
package | (say pkgM) from C++ code in another one (say pkgA).
| If it were simple C++ routines and not class methods, the way is 
known | and even automatized. E.g. if I generate pkgM with
| | Rcpp::Rcpp.package.skeleton("pkgM", module=TRUE)
| Rcpp::compileAttributes("pkgM")
| | then in pkgM/src/RcppExports.cpp we can see
| | | static const R_CallMethodDef CallEntries[] = {
|Â Â Â Â Â  {"_pkgM_rcpp_hello_world", (DL_FUNC) &_pkgM_rcpp_hello_world, 0},
| ...
| | As far as I understand, it registers _pkgM_rcpp_hello_world for 
use with | R_GetCCallable from other packages. And it is a consequence 
of preceding
| rcpp_hello_world() with "// [[Rcpp::export]]".
| If I add "// [[Rcpp::interfaces(r, cpp)]]" in 
pkgM/src/rcpp_module.cpp | it produces 
pkgM/inst/include/{pkgM.h,pkgM_RcppExports.h} and
| | static const R_CallMethodDef CallEntries[] = {
| ...
|Â Â Â Â Â  {"_pkgM_RcppExport_registerCCallable", (DL_FUNC) | 
&_pkgM_RcppExport_registerCCallable, 0},
| ...
| but still nothing for methods and functions from RCPP_MODULE.
| | So my question, what is a canonical way (if defined) to expose 
class | methods (via RCPP_MODULE or any other mechanism) in such a way 
that they | become callable from C++ code of another package? Am I 
supposed to | register them manually in some way using | 
_pkgM_RcppExport_registerCCallable()? And if so, how can I retrieve 
them | in C++ code of another package?

From the top of my head I would say "No" for the simple reason that 
all R

does (and this is the same for all other foreign-function interfaces) 
has to
be C based.Â  Hence no class methods.Â  C++ signatures are compiler 
dependent,
and have been avoided for that reason for as long as we had them.

So I fear you need plain C wrappers for every member function you want to
expose. I could of course be wrong -- if someone else has 
counter-examples I
would be all ears!

Dirk

-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org

I believe Dirk is correct in saying the traditional mechanism won't work 
for non-static member functions, but if the C++ code is fully contained 
in header files in 'inst/include', then it's possible to access that 
code from another package since there's no library that one needs to 
link against. I believe that's the mechanism that Rcpp itself uses.

Things get ugly when one tries to link to another package's library. 
I've tried a few experiments with Dirk's help 
(https://github.com/lsilvest/linktest), but haven't found a reliable way 
to make this portable (things break when packages are installed 
pre-compiled).

Thanks for sharing, it saved me some iterations of trying.

I tested a direct exposing of member function with
R_RegisterCCallable("pkgM", "Num__setx", (DL_FUNC) &Num::setX);

and then passing a call with a first argument as pointer to an object.
It worked on my Linux with gcc 8.3 with a warning at
(DL_FUNC) &Num::setX. Then I read here 
https://isocpp.org/wiki/faq/pointers-to-members that such kind of cast 
are explicetly prohibeted as inducing undefined behavior. So my working 
example was just a chance.

As Dirk suggested, wrapping a method in a plain C-line function worked.
Here, for the record, main steps:
0. put Num class definition in pkgM/inst/include/pkgM.h
1. in pkgM/Num.cpp

void wrap_Num__setx(Num *pobj, double val) {
     pobj->setX(val);
}

2. in pkgM/RcppExports.cpp add
     R_RegisterCCallable("pkgM", "Num__setx", (DL_FUNC) &wrap_Num__setx);
to R_init_pkgM(DllInfo *dll) { ...

3. in pkgA/DESCRIPTION
LinkingTo: Rcpp, pkgM
Depends: pkgM

4. in pkgA/rcpp_hello_world.cpp
// [[Rcpp::export]]
void a_setx(XPtr<Num> xp, double val)
{
   XPtr<Num> p(xp);
   static void (*fun)(Num*, double) = NULL;
   if (fun == NULL) fun = (void (*)(Num*, double)) 
R_GetCCallable("pkgM","Num__setx");
   fun(p, val);
}

5. in R session
library(pkgA) # loads pkgM too
num=Num$new() # created with pkgM infrastructure
a_setx(num at .xData$.pointer, 100)
num$x
#[1] 100

Best,
Serguei.