[Rcpp-devel] Making objects in the C++ "side" persistent

[Dirk Eddelbuettel]

On 12 June 2014 at 17:48, Martin Jakt wrote:
| Hi Fernando and list members,
| 
| > For my purposes, I think I could use S4 objects that make C++ copies of
| > themselves at creation time, accessible via an external pointer. All
| > methods would then make use of the C++ copies for speed, and at the end
| > of the session the S4 copies that live in the R side would be saved.
| 
| I'm wondering how you've gotten on with this. I'm struggling with a somewhat 
| similar problem in my own package. This uses the 'Rcpp modules' to provide an 
| interface to a couple of C++ functions. This works very well, but accessing 
| the objects (created by new()) after a save/reload cycle results in R 
| crashing.

It is a known shortcoming.
 
| I don't really mind to lose the information within the objects and am just 
| looking for some way to avoid R trying to access invalid pointers after a 
| reload. My thought was to somehow subvert the 'save' process such that the 
| pointer address is not written. Can this be done? I tried to set the 
| serialize() function for the class, but this seems not to be used by save().

I think this is the right route.  And we now have access to serialize() from
the C/C++ side (via the small package RApiSerialize which I used in RcppRedis).
It should not be too hard to move towards something like saveRDS/readRDS.
Maybe you can experiment with explicitly deserialization of the 'payload' of
your object, and then recreate it?
 
| The behaviour I would be happy with, is something similar to what you get with 
| RMySQL connections. These (understandably) get dropped between sessions, and 
| need to be recreated by calling the constructor.
| 
| I would appreciate any advice or pointers. I've tried searching the 
| documentation for answers, but have failed to find anything that I can find.

There were some old discussions when Modules was new and fresh, but nobody
has worked out a solution.  It should be doable, though, or at least
"approximatively".  But we can rely on just 'save' or 'saveImage'.  We need
to do the extra step.
  
| I'd also like to say that I _really_ appreciate Rcpp, especially the sourceCpp 
| function; it's makes R comfortable to use. Thanks to the developers.

Thanks for the kind words. Many of us use it too :)

Dirk
 
| Martin
| 
| -- 
| Martin Jakt
| Department of Systems Medicine
| Mitsunada Sakaguchi Laboratory
| Keio University School of Medicine
| _______________________________________________
| Rcpp-devel mailing list
| Rcpp-devel at lists.r-forge.r-project.org
| https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel

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