[Rd] Heap access across multiple calls from R to C++

[pik@pp@@devei m@iii@g oii gm@ii@com]

Thanks a lot! Also, for suggesting using `.Call`. I will try all three and
follow up on the relevant list if I hit a dead-end.

Best wishes,
Pantelis

-----Original Message-----
From: Dirk Eddelbuettel <edd using debian.org> 
Sent: Wednesday, May 24, 2023 4:16 PM
To: Ivan Krylov <krylov.r00t using gmail.com>
Cc: pikappa.devel using gmail.com; r-devel using r-project.org
Subject: Re: [Rd] Heap access across multiple calls from R to C++


On 24 May 2023 at 09:09, Ivan Krylov wrote:
| On Wed, 24 May 2023 02:08:25 +0200
| <pikappa.devel using 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-a
| nd-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 using debian.org