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["Björn Steffen" <>]

thanks for the reply

here is my example code:





//! \file examples/Arrangement_2/ex_observer.cpp extended
// Using a simple arrangement observer.

#include <CGAL/Cartesian.h>
#include <CGAL/Quotient.h>
#include <CGAL/MP_Float.h>
#include <CGAL/Arr_segment_traits_2.h>
#include <CGAL/Arrangement_2.h>
#include <CGAL/Arr_observer.h>

typedef CGAL::Quotient<CGAL::MP_Float>                Number_type;
typedef CGAL::Cartesian<Number_type>                  Kernel;
typedef CGAL::Arr_segment_traits_2<Kernel>            Traits_2;
typedef Traits_2::Point_2                             Point_2;
typedef Traits_2::X_monotone_curve_2                  Segment_2;
typedef CGAL::Arrangement_2<Traits_2>                 Arrangement_2;

// An arrangement observer, used to receive notifications of face splits and
// face mergers.
class My_observer : public CGAL::Arr_observer<Arrangement_2>
{
public:

 My_observer (Arrangement_2& arr) :
   CGAL::Arr_observer<Arrangement_2> (arr)
 {}

 virtual void before_split_face (Face_handle,
                                 Halfedge_handle e)
 {
   std::cout << "-> The insertion of :  [ " << e->curve()
             << " ]  causes a face to split." << std::endl;
 }

 virtual void before_merge_face (Face_handle,
                                 Face_handle,
                                 Halfedge_handle e)
 {
   std::cout << "-> The removal of :  [ " << e->curve()
             << " ]  causes two faces to merge." << std::endl;
 }

 virtual void after_create_edge( Halfedge_handle e ) {
     std::cout << "-> after create edge: [ " << e->curve()
                            << " ]." << std::endl;

 }

 virtual void before_split_edge (       Halfedge_handle e,
                                                                        
Vertex_handle v,
                                                                        
X_monotone_curve_2 c1,
                                                                        
X_monotone_curve_2 c2) {
     std::cout << "-> before split edge: [ " << e->curve()
                                << " ] at " << v->point() << std::endl;
 }

 virtual void after_split_edge( Halfedge_handle e1, Halfedge_handle e2 ) {
     std::cout << "-> after split edge: [ " << e1->curve() << " ][ " <<
                e2->curve() << " ]" << std::endl;
 }

};

int main ()
{
 // Construct the arrangement containing one diamond-shaped face.
 Arrangement_2  arr;
 My_observer    obs (arr);

 Segment_2      s1 (Point_2(-1, 0), Point_2(0, 1));
 Segment_2      s2 (Point_2(0, 1), Point_2(1, 0));
 Segment_2      s3 (Point_2(1, 0), Point_2(0, -1));
 Segment_2      s4 (Point_2(0, -1), Point_2(-1, 0));

 insert_non_intersecting_curve (arr, s1);
 insert_non_intersecting_curve (arr, s2);
 insert_non_intersecting_curve (arr, s3);
 insert_non_intersecting_curve (arr, s4);

 // Insert a vertical segment dividing the diamond into two, and a
 // a horizontal segment further dividing the diamond into four:
 Segment_2      s_vert (Point_2(0, -1), Point_2(0, 1));
 Arrangement_2::Halfedge_handle
                e_vert = insert_non_intersecting_curve (arr, s_vert);

 Segment_2      s_horiz (Point_2(-1, 0), Point_2(1, 0));

 insert_curve (arr, s_horiz);

 std::cout << "The initial arrangement size:" << std::endl
           << "   V = " << arr.number_of_vertices()
           << ",  E = " << arr.number_of_edges()
           << ",  F = " << arr.number_of_faces() << std::endl;

 // Now remove a portion of the vertical segment.
 remove_edge (arr, e_vert);

 std::cout << "The final arrangement size:" << std::endl
           << "   V = " << arr.number_of_vertices()
           << ",  E = " << arr.number_of_edges()
           << ",  F = " << arr.number_of_faces() << std::endl;

 return (0);
}