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[<>]

typedef CGAL::Exact_predicates_inexact_constructions_kernel         K;
typedef K::Point_3 Point3;

        struct Point:public Point3
        {
                Point(double dx,double dy,double dz):Point_3(dx,dy,dz)
                {

                        bUsed = false;
                }
                AcDbObjectId id;

                bool bUsed;
        };

        struct Construct_coord_iterator
        {
                // Get an iterator for the approximate coordinates. 
                const Point::Cartesian_const_iterator operator() (const Point&
nnp) const
                {
                        return (nnp.cartesian_begin());
                }

                //Get a past-the-end iterator for the approximate 
coordinates. 
                const Point::Cartesian_const_iterator operator() (const Point&
nnp, int) const
                {
                        return (nnp.cartesian_end());
                }
        };

        typedef CGAL::Search_traits<K::FT, Point, const
double*,Construct_coord_iterator> Traits;
        typedef CGAL::Orthogonal_incremental_neighbor_search<Traits>
NN_incremental_search;
        typedef NN_incremental_search::iterator NN_iterator;
        typedef NN_incremental_search::Tree Tree;

        struct point_not_used{
                bool operator()(const NN_iterator& it)
                { 
                        return (((*it).first).bUsed);
                        //return true;
                }
        };
        // An iterator that only enumerates dD points with positive
x-coordinate
        typedef CGAL::Filter_iterator<NN_iterator,point_not_used>
NN_positive_x_iterator;

.... //init a Tree ojbect and do the below seach.

NN_incremental_search NN(tree,query);
                NN_positive_x_iterator
it(NN.end(),point_not_used(),NN.begin()),end(NN.end(),point_not_used());
                for (int j=0;(j<nK)&&(it!=end);++j,++it)
                {
                        const Point& p = (*it).first;
                        Point& pp = const_cast<Point&>(p);


                        pp.bUsed = true;// why can this line not change the
tree's point extra info "bUsed".

                        idArr_Res.append(p.id);
                        acutPrintf(_T("\nsquard
distance=%g"),CGAL::to_double((*it).second));
                }

   the code above can be compiled well, but "pp.bUsed = true" did nothing, why
can't change the tree's point extral info.