#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point.hpp>
#include <boost/geometry/geometries/box.hpp>
#include <boost/geometry/index/rtree.hpp>
// to store queries results
#include <vector>
// just for output
#include <iostream>
#include <boost/foreach.hpp>
namespace bg = boost::geometry;
namespace bgi = boost::geometry::index;
int main()
{
    typedef bg::model::point<float, 2, bg::cs::cartesian> point;
    typedef bg::model::box<point> box;
    typedef std::pair<box, unsigned> value;
    // create the rtree using default constructor
    bgi::rtree< value, bgi::quadratic<16> > rtree;
    // create some values
    for ( unsigned i = 0 ; i < 10 ; ++i )
    {
        // create a box
        box b(point(i + 0.0f, i + 0.0f), point(i + 0.5f, i + 0.5f));
        // insert new value
        rtree.insert(std::make_pair(b, 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 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.first) << " - " << v.second << 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.first) << " - " << v.second << std::endl;
    return 0;
}

Expected results

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)) - 0
POLYGON((1 1,1 1.5,1.5 1.5,1.5 1,1 1)) - 1
POLYGON((2 2,2 2.5,2.5 2.5,2.5 2,2 2)) - 2
POLYGON((3 3,3 3.5,3.5 3.5,3.5 3,3 3)) - 3
POLYGON((4 4,4 4.5,4.5 4.5,4.5 4,4 4)) - 4
POLYGON((5 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)) - 4
POLYGON((3 3,3 3.5,3.5 3.5,3.5 3,3 3)) - 3
POLYGON((2 2,2 2.5,2.5 2.5,2.5 2,2 2)) - 2
POLYGON((0 0,0 0.5,0.5 0.5,0.5 0,0 0)) - 0
POLYGON((1 1,1 1.5,1.5 1.5,1.5 1,1 1)) - 1