R devel - May 2023 - Heap access across multiple calls from R to C++

On Wed, 24 May 2023 02:08:25 +0200
<pikappa.devel at gmail.com> wrote:
>   initialize = function() {
> 
>     .C("make_native_obj", self$native_obj)
> 
>   })
> extern "C" void make_native_obj(double *obj) {
> 
>   auto native_obj = new NativeObjStruct();
> 
>   memcpy(obj, &native_obj, sizeof(obj));  
> 
> }
> Is there a better way to do this?
The .Call() interface (where functions take an arbitrary number of
native R objects and return a native R object) combined with external
pointers is likely to be a better approach here:

https://cran.r-project.org/doc/manuals/R-exts.html#External-pointers-and-weak-references

On the R side, the returned SEXP will have class 'externalptr', with
not much to do about it programmatically. C code can additionally
register a finalizer on the object in order to release the memory
automatically after the it is discarded.

-- 
Best regards,
Ivan

On 24 May 2023 at 09:09, Ivan Krylov wrote:
| On Wed, 24 May 2023 02:08:25 +0200
| <pikappa.devel at gmail.com> wrote:
| > Is there a better way to do this?
| 
| The .Call() interface (where functions take an arbitrary number of
| native R objects and return a native R object) combined with external
| pointers is likely to be a better approach here:
| 
|
https://cran.r-project.org/doc/manuals/R-exts.html#External-pointers-and-weak-references

Yes. We had numerous threads here and on r-package-devel (better list for the
question IMHO, maybe follow-up there?) stating that new code really should
avoid .C() and stick with .Call().

| On the R side, the returned SEXP will have class 'externalptr', with
| not much to do about it programmatically. C code can additionally
| register a finalizer on the object in order to release the memory
| automatically after the it is discarded.

(And Rcpp::XPtr sets those for you if you define RCPP_USE_FINALIZE_ON_EXIT.)

A simpler approach is of course available too, and I used it too.  Define
your class as you would in C++.  Then define
 - a helper function to initialize a class object via `new`, assign it to a
   static variable or a global pointer
 - helper functions for setters and getters of the values you need
 - a teardown function you can call, you may even tie it to R's on.exit()

Now _you_ control lifetime and allocation. From R. If anything breaks, you
get to keep the pieces.

We can also help on the rcpp-devel list.

Cheers, Dirk

-- 
dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org