r - Pass a c++ object as a pointer an reuse in another function in Rcpp

Question

Suppose a I have the following 2 functions in C++:

// [[Rcpp::export]]
SEXP foo() {

  int a = 1;

  Rcpp::XPtr<int> ptr(&a, true);

  return ptr;
}

// [[Rcpp::export]]
int bar(SEXP a){

  Rcpp::XPtr<int> x(a);
  int b = *x;

  return b;
}

I want to be able to call something like the following in R. Of course, in this example I could return an int to R in foo, but in my original code, a is a somewhat complex data structure and I wan't to return only the pointer to this, so it can be reused by another C++ function called bar

a <- foo()
bar(a)

In this example I expected that bar(a) returned 1 instead of 0. How can I fix this?

Answer 1

Your initial variable a goes out of scope as soon as the function finishes in foo. Instead you need to initialize your pointer first, assign it, then wrap it with the XPtr. The following shows an example of how to do this.

// [[Rcpp::export]]
SEXP foo() {

  int *a = new int(1);

  Rcpp::XPtr<int> ptr(a);

  return ptr;
}

R code

Rcpp::sourceCpp('test.cpp')

a <- foo()
bar(a)
[1] 1