Transforming a boost::geometry polygon into an STL object

Question

How do I get a boost::geometry polygon into an STL object?

I am sure this must be simple because I cannot find examples anywhere in the documentation. Yet I have spent about 4 full work days trying to do this tiny thing. I am new to C++ (long time R programer), but these small data conversion things are driving me insane.

Yes there is a question whose title is much like mine: Getting the coordinates of points from a Boost Geometry polygon

But the code is so complex (and the poster kept changed it so many times) that I cannot make heads or tails of it, nor can I imagine that other C++ newbies will be able to.

This is a simple example that should translate to some of the other boost::geometry data types, so hopefully anyone can follow it.

 #include <iostream>

 #include <boost/geometry.hpp>
 #include <boost/geometry/geometries/polygon.hpp>
 #include <boost/geometry/geometries/adapted/boost_tuple.hpp>

 BOOST_GEOMETRY_REGISTER_BOOST_TUPLE_CS(cs::cartesian)

 //One thing I tried is a function to use with `for_each_point()` so I set that up first.

  template <typename Point>
  void get_coordinates(Point const& p)
  {
  using boost::geometry::get;
  std::cout << get<0>(p) get<1>(p) << std::endl;
  }

  int main()
  {
  typedef boost::tuple<double, double> point;
  typedef boost::geometry::model::polygon<point> polygon;

  polygon poly;
  boost::geometry::read_wkt("polygon((2.0 1.3, 2.4 1.7, 2.8 1.8, 3.4 1.2, 3.7 1.6, 3.4 2.0, 4.1 3.0, 5.3 2.6, 5.4 1.2, 4.9 0.8, 2.9 0.7, 2.0 1.3))", poly);

   polygon hull;
   boost::geometry::convex_hull(poly, hull);

 // Now I know I can `dsv()` and print to the screen like this:

  using boost::geometry::dsv;
  std::cout
    << "hull: " << dsv(hull) << std::endl;

  // And/Or I can use my function with for_each_point()



  boost::geometry::for_each_point(hull, get_coordinates<point>);

return 0;
}

How do I get these coordinates into an STL container? I would prefer two std::vector one for x and one for y, but anything will do.

Answer 1

Polygons are already in a STL container format, the boost::geometry::polygon is has its exterior ring and interior rings stored a std::vector, by default.

What you might want (considering your comments) is:

  polygon hull;
  boost::geometry::convex_hull(poly, hull);
  boost::geometry::for_each_point(boost::geometry::exterior_ring(hull), get_coordinates<point>);

This will work if you correct your get_coordinates function to (note the << usage):

  template <typename Point>
  void get_coordinates(Point const& p)
  {
      using boost::geometry::get;
      std::cout << get<0>(p) << ", " << get<1>(p) << std::endl;
  }

And change your comment indicators to // ;-)