[Rcpp-devel] class methods and R_RegisterCCallable

[Dirk Eddelbuettel]

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

-- 
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