Weird access with external pointers - r

Question

I made a small reproducible example:
#include <Rcpp.h>
using namespace Rcpp;
class Index {
public:
Index(int i_) : i(i_) {}
int getI() { return i; }
private:
int i;
};
// [[Rcpp::export]]
SEXP getXPtrIndex(int i) {
Rcout << "getXPtrIndex: i = " << i << std::endl;
Index ind(i);
Rcout << "getXPtrIndex: ind.i = " << ind.getI() << std::endl;
return XPtr<Index>(&ind, true);
}
// [[Rcpp::export]]
void getXPtrIndexValue(SEXP ptr) {
XPtr<Index> ind_ptr(ptr);
Rcout << "getXPtrIndexValue: ind_ptr->i = " << ind_ptr->getI() << std::endl;
Index ind = *ind_ptr;
Rcout << "getXPtrIndexValue: ind.i = " << ind.getI() << std::endl;
}
Basically, I define a small class, along with a function to get an external pointer of an element of this class. The last function is used to print the weird accessor when returning the class element back to C++.
Results in R:
> (extptr <- getXPtrIndex(10))
getXPtrIndex: i = 10
getXPtrIndex: ind.i = 10
<pointer: 0x7ffeeec31b00>
> getXPtrIndexValue(extptr)
getXPtrIndexValue: ind_ptr->i = 33696400
getXPtrIndexValue: ind.i = 0
Why can't I access 10?
I'm using Rcpp version 0.12.12 (the latest I think).

Answer 1

It seems to have something to do with the temporary object---by the time your second function runs the "content" of the first is already gone.
So either just make
Index ind(10);
a global, and comment out the line in your first function. Then all is peachy (I changed the R invocation slightly):
R> extptr <- getXPtrIndex(10)
getXPtrIndex: i = 10
getXPtrIndex: ind.i = 10
R> getXPtrIndexValue(extptr)
getXPtrIndexValue: ind_ptr->i = 10
getXPtrIndexValue: ind.i = 10
R>
Or it also works the same way when you make you Index object static to ensure persistence. Corrected example below.
#include <Rcpp.h>
using namespace Rcpp;
class Index {
public:
Index(int i_) : i(i_) {}
int getI() { return i; }
private:
int i;
};
// [[Rcpp::export]]
SEXP getXPtrIndex(int i) {
Rcout << "getXPtrIndex: i = " << i << std::endl;
static Index ind(i);
Rcout << "getXPtrIndex: ind.i = " << ind.getI() << std::endl;
return XPtr<Index>(&ind, true);
}
// [[Rcpp::export]]
void getXPtrIndexValue(SEXP ptr) {
XPtr<Index> ind_ptr(ptr);
Rcout << "getXPtrIndexValue: ind_ptr->i = " << ind_ptr->getI() << std::endl;
Index ind = *ind_ptr;
Rcout << "getXPtrIndexValue: ind.i = " << ind.getI() << std::endl;
}
/*** R
extptr <- getXPtrIndex(10)
getXPtrIndexValue(extptr)
*/