Assigned objects in child function not updated in parent function

Question

This is a cgal related question, but I think it is also a general C++ question, so I ask it here.

I am trying to use the Alpha_shape_2 class, and assign it to AlphaShapeCg class in a subroutine named GetAlphaShalCg. The problem is that some of the function in Alpha_shape_2 doesn't return correct result.

This is my code, it's really simple, but I don't quite know why there is a difference between assigning the Alpha_shape_2 to a wrapper in a subroutine, and then access the member in parent routine and accessing Alpha_shape_2 directly.

Here's the complete code that you can compile and play with, if you have CGAL installed.

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/algorithm.h>
#include <CGAL/Delaunay_triangulation_2.h>
#include <CGAL/Alpha_shape_2.h>

#include <iostream>
#include <fstream>
#include <vector>
#include <list>


typedef CGAL::Exact_predicates_inexact_constructions_kernel K;

typedef K::FT FT;

typedef K::Point_2  Point;
typedef K::Segment_2  Segment;


typedef CGAL::Alpha_shape_vertex_base_2<K> Vb;
typedef CGAL::Alpha_shape_face_base_2<K>  Fb;
typedef CGAL::Triangulation_data_structure_2<Vb,Fb> Tds;
typedef CGAL::Delaunay_triangulation_2<K,Tds> Triangulation_2;

typedef CGAL::Alpha_shape_2<Triangulation_2>  Alpha_shape_2;


template <class OutputIterator>
bool
file_input(OutputIterator out)
{
  std::ifstream is("./data/fin", std::ios::in);

  if(is.fail()){
    std::cerr << "unable to open file for input" << std::endl;
    return false;
  }

  int n;
  is >> n;
  std::cout << "Reading " << n << " points from file" << std::endl;
  CGAL::copy_n(std::istream_iterator<Point>(is), n, out);

  return true;
}

//------------------ main -------------------------------------------


struct AlphaShapeCg
{

    Alpha_shape_2 *AlphaShape;
};

void GetAlphaShalCg(AlphaShapeCg *ashape,  std::list<Point> points)
{

      Alpha_shape_2 A(points.begin(), points.end(),
          FT(100000),
          Alpha_shape_2::GENERAL);
    ashape->AlphaShape=&A;
}



int main()
{
  std::list<Point> points;
  if(! file_input(std::back_inserter(points))){
    return -1;
  }

   AlphaShapeCg ashape;


   GetAlphaShalCg(&ashape, points);

   Alpha_shape_2 *APtrs=(ashape.AlphaShape);
   int alphaEigenValue = APtrs->number_of_alphas(); // gives incorrect result; alphaEigenValue=0

  //Alpha_shape_2 A(points.begin(), points.end(),
  //  FT(100000),
  //  Alpha_shape_2::GENERAL);
  //   int alphaEigenValue = APtrs->number_of_alphas(); // gives correct result; alphaEigenValue!=0

}

Update: I tried to use

Alpha_shape_2 =new A(points.begin(), points.end(), FT(100000), Alpha_shape_2::GENERAL);

But this code simply won't compile because of this error:

error C2513: 'CGAL::Alpha_shape_2' : no variable declared before '='

Answer 1

You're assigning a pointer to a local variable that gets destroyed when you exit the function.

If you want to create the object in the function and return its address - you should use dynamic allocation (new it, don't forget to delete when you're done with it).