#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point.hpp>
#include <boost/geometry/geometries/box.hpp>
#include <boost/geometry/index/rtree.hpp>
// just for output
#include <iostream>
namespace bg = boost::geometry;
namespace bgi = boost::geometry::index;
int main()
{
    typedef bg::model::point<double, 2, bg::cs::cartesian> point;
    typedef point value;
    typedef bgi::rtree< value, bgi::linear<16> > rtree_t;
    // create the rtree using default constructor
    rtree_t rtree;
    // create some values
    for ( double f = 0 ; f < 10 ; f += 1 )
    {
        // insert new value
        rtree.insert(point(f, f));
    }
    // query point
    point pt(5.1, 5.1);
    // iterate over nearest Values
    for ( rtree_t::const_query_iterator
            it = rtree.qbegin(bgi::nearest(pt, 100)) ;
            it != rtree.qend() ;
            ++it )
    {
        double d = bg::distance(pt, *it);
        std::cout << bg::wkt(*it) << ", distance= " << d << std::endl;
        // break if the distance is too big
        if ( d > 2 )
        {
            std::cout << "break!" << std::endl;
            break;
        }
    }
    return 0;
}

Expected results

POINT(5 5), distance= 0.141421
POINT(6 6), distance= 1.27279
POINT(4 4), distance= 1.55563
POINT(7 7), distance= 2.68701
break!