Using IndexableGetter function object - storing indexes of external container's elements

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#### Using IndexableGetter function object - storing indexes of external container's elements

    #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>

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

    template <typename Container>
    class my_indexable
    {
        typedef typename Container::size_type size_t;
        typedef typename Container::const_reference cref;
        Container const& container;

    public:
        typedef cref result_type;
        explicit my_indexable(Container const& c) : container(c) {}
        result_type operator()(size_t i) const { return container[i]; }
    };

    int main()
    {
        typedef bg::model::point<float, 2, bg::cs::cartesian> point;
        typedef bg::model::box<point> box;
        typedef std::vector<box>::size_type value;
        typedef bgi::rstar<16, 4> parameters;
        typedef my_indexable< std::vector<box> > indexable_getter;

        // boxes
        std::vector<box> boxes;

        // create some boxes
        for ( unsigned i = 0 ; i < 10 ; ++i )
        {
            // add a box
            boxes.push_back(box(point(i+0.0f, i+0.0f), point(i+0.5f, i+0.5f)));
        }

        // display boxes
        std::cout << "generated boxes:" << std::endl;
        BOOST_FOREACH(box const& b, boxes)
            std::cout << bg::wkt<box>(b) << std::endl;

        // create the rtree
        parameters params;
        indexable_getter ind(boxes);
        bgi::rtree<value, parameters, indexable_getter> rtree(params, ind);

        // fill the spatial index
        for ( size_t i = 0 ; i < boxes.size() ; ++i )
            rtree.insert(i);

        // 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 i, result_s)
            std::cout << bg::wkt<box>(boxes[i]) << 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 i, result_n)
            std::cout << bg::wkt<box>(boxes[i]) << std::endl;

        return 0;
    }

##### Expected results

    generated boxes:
    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))