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#### Specializing index::indexable function object - storing shared pointers in the rtree

    #include <boost/geometry.hpp>
    #include <boost/geometry/geometries/point.hpp>
    #include <boost/geometry/geometries/box.hpp>

    #include <boost/geometry/index/rtree.hpp>

    #include <cmath>
    #include <vector>
    #include <iostream>
    #include <boost/foreach.hpp>
    #include <boost/shared_ptr.hpp>

    namespace bg = boost::geometry;
    namespace bgi = boost::geometry::index;

    namespace boost { namespace geometry { namespace index {

    template <typename Box>
    struct indexable< boost::shared_ptr<Box> >
    {
        typedef boost::shared_ptr<Box> V;

        typedef Box const& result_type;
        result_type operator()(V const& v) const { return *v; }
    };

    }}} // namespace boost::geometry::index

    int main(void)
    {
        typedef bg::model::point<float, 2, bg::cs::cartesian> point;
        typedef bg::model::box<point> box;
        typedef boost::shared_ptr<box> shp;
        typedef shp value;

        // create the rtree using default constructor
        bgi::rtree< value, bgi::linear<16, 4> > rtree;

        std::cout << "filling index with boxes shared pointers:" << std::endl;

        // fill the spatial index
        for ( unsigned i = 0 ; i < 10 ; ++i )
        {
            // create a box
            shp b(new box(point(i, i), point(i+0.5f, i+0.5f)));

            // display new box
            std::cout << bg::wkt<box>(*b) << std::endl;

            // insert new value
            rtree.insert(b);
        }

        // find values intersecting some area defined by a box
        box query_box(point(0, 0), point(5, 5));
        std::vector<value> result_s;
        rtree.query(bgi::intersects(query_box), std::back_inserter(result_s));

        // find 5 nearest values to a point
        std::vector<value> result_n;
        rtree.query(bgi::nearest(point(0, 0), 5), std::back_inserter(result_n));

        // display results
        std::cout << "spatial query box:" << std::endl;
        std::cout << bg::wkt<box>(query_box) << std::endl;
        std::cout << "spatial query result:" << std::endl;
        BOOST_FOREACH(value const& v, result_s)
            std::cout << bg::wkt<box>(*v) << std::endl;

        std::cout << "knn query point:" << std::endl;
        std::cout << bg::wkt<point>(point(0, 0)) << std::endl;
        std::cout << "knn query result:" << std::endl;
        BOOST_FOREACH(value const& v, result_n)
            std::cout << bg::wkt<box>(*v) << std::endl;

        return 0;
    }