Add an example to explicit the behaviour of add_face in case of a wrong orientation.

Surface_mesh/examples/Surface_mesh/CMakeLists.txt

@@ -39,6 +39,7 @@ if ( CGAL_FOUND )
 
   create_single_source_cgal_program("draw_surface_mesh.cpp")
   create_single_source_cgal_program("sm_draw_small_faces.cpp")
+  create_single_source_cgal_program("sm_check_wrong_orientation.cpp")
   if(CGAL_Qt5_FOUND )
     target_link_libraries(draw_surface_mesh PUBLIC CGAL::CGAL_Qt5)
     target_link_libraries(sm_draw_small_faces PUBLIC CGAL::CGAL_Qt5)

Surface_mesh/examples/Surface_mesh/sm_check_wrong_orientation.cpp

@@ -0,0 +1,30 @@
+#include <vector>
+
+
+#include <CGAL/Simple_cartesian.h>
+#include <CGAL/Surface_mesh.h>
+
+typedef CGAL::Simple_cartesian<double> K;
+typedef CGAL::Surface_mesh<K::Point_3> Mesh;
+typedef Mesh::Vertex_index vertex_descriptor;
+typedef Mesh::Face_index face_descriptor;
+int main()
+{
+  Mesh m;
+
+  // Add the points as vertices
+  vertex_descriptor u = m.add_vertex(K::Point_3(0,1,0));
+  vertex_descriptor v = m.add_vertex(K::Point_3(0,0,0));
+  vertex_descriptor w = m.add_vertex(K::Point_3(1,1,0));
+  vertex_descriptor x = m.add_vertex(K::Point_3(1,0,0));
+
+  m.add_face(u,v,w);
+  face_descriptor f = m.add_face(u,v,x);
+  if(f == Mesh::null_face())
+  {
+    std::cerr<<"The face could not be added because of an orientation error."<<std::endl;
+    f = m.add_face(u,x,v);
+    assert(f != Mesh::null_face());
+  }
+  return 0;
+}