Question

Is there an easy way to read points and segments in 2D, from the same file, using CGAL?

What should be the format for this file?

Answer 1

In CGAL operator<< is overloaded for kernel objects and streams. The format is, well, apparently undocumented but kind of obvious for simpler types.

#include <CGAL/basic.h>
#include <CGAL/Simple_cartesian.h>

typedef CGAL::Simple_cartesian<double> K;

int main()
{
  K::Point_2 p;
  std::stringstream ss;
  ss << "2.05 3.05";

  ss >> p; // read from a stream
  std::cout << p << std::endl; // write to a stream

  K::Segment_2 s;
  ss.clear();
  ss << "2.3 4.2 4.2 2.3";
  ss >> s; // read a segment from a stream
  std::cout << s << std::endl; // write a segment to a stream
  return 0;
}

Looking at the code a CGAL::Polygon_2 expects input like this:

"4 0 0 0 1 1 1 1 0"

where the first number is the number of Points following and after that the points.