the error happens when i trying to do this line:

Math::Vector pre=edgeAroundVertexIter->middlePoint;

the error message also says:

const CGAL::HalfedgeDS_in_place_list_halfedge<CGAL::I_Polyhedron_halfedge<My_edge<CGAL::HalfedgeDS_list_types<CGAL::Cartesian<float>, CGAL::I_Polyhedron_derived_items_3<My_items>, std::allocator<int> > > > > &) @0x0: <error reading variable>
(gdb) p edgeAroundVertexIter->middlePoint

this is weird, because i'm pretty sure middlePoint is a member and there was no compiling error at all.