Cannot dereference pointer to Alpha_shape_3

[stla]

Hello,

I have a poor knowledge in C/C++ and I don't know whether I'm doing something bad.

I have a R pointer (Rcpp:Xptr) to an Alpha_shape_3. I can get the ordinary pointer to the Alpha_shape_3 from it, but when I try to dereference it, the code does not compile : it throws an error "is private within this context".

void optimalAlpha_cpp(Rcpp::XPtr<Alpha_shape_3> as_xptr, int nc) {
  Alpha_shape_3* as_ptr = as_xptr.get();
  Alpha_shape_3 as = *as_ptr; // error: is private within this context
  Alpha_iterator opt = as.find_optimal_alpha(nc);
  Rcpp::Rcout << *opt;
}

The following code compiles but it returns a wrong optimal alpha which makes no sense (a huge number):

void optimalAlpha_cpp(Rcpp::XPtr<Alpha_shape_3> as_xptr, int nc) {
  Alpha_shape_3* as_ptr = as_xptr.get();
  Alpha_iterator opt = (*as_ptr).find_optimal_alpha(nc);
  Rcpp::Rcout << *opt;
}

Why I can't dereference the pointer?

[sloriot]

You are making a copy. You need to take a reference like
Alpha_shape_3& as = *as_ptr;

[stla]

Thanks! This code compiles. But I still get the nonsensical optimal alpha (7.35214e+19).

[stla]

Here is the full code:

Rcpp::XPtr<Alpha_shape_3> AS_cpp(Rcpp::NumericMatrix pts) {
  // make list of points
  const int npoints = pts.ncol();
  std::list<Point3> points;
  for(int i = 0; i < npoints; i++) {
    const Rcpp::NumericVector pt_i = pts(Rcpp::_, i);
    points.push_back(Point3(pt_i(0), pt_i(1), pt_i(2)));
  }
  // compute alpha shape
  Alpha_shape_3 as(points.begin(), points.end());
  // returns R pointer
  Rcpp::XPtr<Alpha_shape_3> as_xptr(&as, false);
  return as_xptr;
}

void optimalAlpha_cpp(Rcpp::XPtr<Alpha_shape_3> as_xptr, int nc) {
  Alpha_shape_3& as = *(as_xptr.get());
  Alpha_iterator opt = as.find_optimal_alpha(nc);
  Rcpp::Rcout << *opt;
}

If I apply AS_cpp in R, I get the R pointer. Then I pass it to optimalAlpha_cpp, and it always prints 7.35214e+19.

[sloriot]

Hard to tell without the point cloud, the alpha and the input parameters.

[stla]

The result is always 7.35214e+19, whatever the point cloud (e.g. a cube or any polyhedron). What do you mean by the alpha and the input parameters?
If I compute the optimal alpha inside the AS_cpp function, it works.

[stla]

If I move the contents of the second function inside the first function, it works.

[sloriot]

as in the first function is a local variable that gets destroyed when you leave the function. Doing Alpha_shape_3* as_ptr = new Alpha_shape_3(points.begin(), points.end()); should fix the issue.

[stla]

Yes, it works! Thanks!