Le 06/02/12 19:40, Chris DuBois a écrit :
Hi all,
I have a class that stores data and various statistics about my data. I
want to be able to work with it from R and from C++ functions, but I do
not want to have the entire data structure created as an R object. I
thought one might be able to return a pointer from the object to R, and
pass that pointer from R to a C++ function that needs to interact with
the object. This seems to work until garbage collection occurs, at
which point I get a segfault.
I have included an example below illustrating my current approach. I
altered the World example to return a pointer to the current object. I
have a C++ function that makes changes to the object.
I saw this post
http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2011-December/003214.html
but I wanted to use modules and I haven't been able to adapt that
solution to my situation.
Any help/advice is much appreciated,
Chris
library(inline)
library(Rcpp)
fx <- cxxfunction(,"",includes=
'
#include <Rcpp.h>
class World {
public:
World() : msg("hello") {}
void set(std::string msg) {
this->msg = msg;
}
std::string greet() {
return msg;
}
SEXP ptr() {
return wrap(XPtr<World>(this, true));
}
private:
std::string msg;
};
int fn(SEXP ptr_) {
World *s = XPtr<World>(ptr_);
s->set("c++ function has been here");
return 1;
}
RCPP_MODULE(example){
using namespace Rcpp ;
function("fn", &fn);
class_<World>( "World")
.constructor()
.method( "greet", &World::greet ,
"get the message")
.method( "set", &World::set ,
"set the message")
.method( "ptr", &World::ptr ,
"get a pointer");
}
', plugin="Rcpp")
example <- Module("example",getDynLib(fx))
s <- new(example$World)
# Interact with World object from R
s$greet()
s$set("hello from R")
s$greet()
# Grab pointer to this World object
s$ptr()
# Call a c++ function that uses World object
example$fn(s$ptr())
s$greet() # c++ function has altered s, as desired
# Causes segfault
gc()
That's subtle. When you call your ptr function, you create a new R
external pointer to encapsulate the same underlying C++ pointer. When
the R external pointer object you created goes out of scope, it becomes
a candidat for gabage collection. When GC occurs, the undelying pointer
is delete'd.
You could prevent the finalizer by using :
SEXP ptr() {
return XPtr<World>(this, false);
}
Also, you don't need wrap because XPtr has a SEXP operator inherited
from RObject.
Romain
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