What exactly is an OutputIterator and how to build one for use with CGAL Kd_tree::search?

Question

I'm using CGAL's Kd-tree implementation along with Fuzzy spheres as query objects to get the points enclosed in a sphere of radius r_max centered at a point. Here is this minimal working example:

    #include <CGAL/Simple_cartesian.h>
    #include <CGAL/Kd_tree.h>
    #include <CGAL/Search_traits_2.h>
    #include <CGAL/Fuzzy_sphere.h>
    #include <iostream>
    #include <fstream>

    typedef CGAL::Simple_cartesian<double>  K;
    typedef K::Point_2                      Point;
    typedef CGAL::Search_traits_2<K>        TreeTraits;
    typedef CGAL::Kd_tree<TreeTraits>       Kd_tree;
    typedef Kd_tree::Tree                   Tree;
    typedef CGAL::Fuzzy_sphere<TreeTraits>  Sphere;

    int main(int argc, char* argv[])
    {
        double r_max;
        Tree tree;

        /* ... fill the tree with points, set the value of r_max ...*/

        // Report indices for the neighbors within a sphere
        unsigned int   idc_query = tree.size()/2;           // test index
        Tree::iterator kti       = idc_query + tree.begin();                                                                                
        Sphere s_query(*kti, r_max);                            

        // Print points
        tree.search(std::ostream_iterator<Point>(std::cout, "\n"), s_query);

        return 0;
    }

I took and adapted the line below the comment "Print points" from the nearest_neighbor_searching.cpp file under the Spatial_searching folder of CGAL's examples (my version is 3.9).

The question is: Is there a way for me to set a different OutputIterator (rather than std::ostream_iterator) that stores a pointer/iterator/handle to the points resulting from the search in a container of sorts, instead of having the points' coordinates printed to the standard output? Thank you.

Answer 1

In the C++ standard library, there are five kinds of iterators:

• Input iterator
• Output iterator
• Forward iterator
• Bidirectional iterator
• Random access iterator

For more information, see cplusplus.com

In your case, you need an Output iterator, ie., an object it that can be incremented (++it) and de-referenced (*it) to get a non-const reference, that can be written to.

You can create an output iterator that inserts all items written to it at the end of a container using std::back_inserter:

#include <iterator>
#include <vector>

...

std::vector<Point> points;
tree.search(std::back_inserter(points), s_query);

Answer 2

Things have evolved in CGAL, that is you can store other things than just points. Have a look at the Examples for Using an Arbitrary Point Type with Point Property Maps in the User Manual.