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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()
    {
        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+0.0f, i+0.0f), 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));

        // note: in Boost.Geometry the WKT representation of a box is polygon

        // 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;
    }

##### Expected results

    filling index with boxes shared pointers:
    POLYGON((0 0,0 0.5,0.5 0.5,0.5 0,0 0))
    POLYGON((1 1,1 1.5,1.5 1.5,1.5 1,1 1))
    POLYGON((2 2,2 2.5,2.5 2.5,2.5 2,2 2))
    POLYGON((3 3,3 3.5,3.5 3.5,3.5 3,3 3))
    POLYGON((4 4,4 4.5,4.5 4.5,4.5 4,4 4))
    POLYGON((5 5,5 5.5,5.5 5.5,5.5 5,5 5))
    POLYGON((6 6,6 6.5,6.5 6.5,6.5 6,6 6))
    POLYGON((7 7,7 7.5,7.5 7.5,7.5 7,7 7))
    POLYGON((8 8,8 8.5,8.5 8.5,8.5 8,8 8))
    POLYGON((9 9,9 9.5,9.5 9.5,9.5 9,9 9))
    spatial query box:
    POLYGON((0 0,0 5,5 5,5 0,0 0))
    spatial query result:
    POLYGON((0 0,0 0.5,0.5 0.5,0.5 0,0 0))
    POLYGON((1 1,1 1.5,1.5 1.5,1.5 1,1 1))
    POLYGON((2 2,2 2.5,2.5 2.5,2.5 2,2 2))
    POLYGON((3 3,3 3.5,3.5 3.5,3.5 3,3 3))
    POLYGON((4 4,4 4.5,4.5 4.5,4.5 4,4 4))
    POLYGON((5 5,5 5.5,5.5 5.5,5.5 5,5 5))
    knn query point:
    POINT(0 0)
    knn query result:
    POLYGON((4 4,4 4.5,4.5 4.5,4.5 4,4 4))
    POLYGON((3 3,3 3.5,3.5 3.5,3.5 3,3 3))
    POLYGON((2 2,2 2.5,2.5 2.5,2.5 2,2 2))
    POLYGON((0 0,0 0.5,0.5 0.5,0.5 0,0 0))
    POLYGON((1 1,1 1.5,1.5 1.5,1.5 1,1 1))