How does one obtain the area of a general polygon set in CGAL?

  c++, cgal

In the following CGAL example, drawn from here, I have a union of circles and rectangles. It looks like so:

Circles and rectangles union

How do I get the area of the resulting union?

// Compile with: clang++ -DBOOST_ALL_NO_LIB -DCGAL_USE_GMPXX=1 -O2 -g -DNDEBUG -Wall -Wextra -pedantic -march=native -frounding-math bob.cpp -lgmpxx -lmpfr -lgm

#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
#include <CGAL/Gps_circle_segment_traits_2.h>
#include <CGAL/General_polygon_set_2.h>
#include <CGAL/Lazy_exact_nt.h>
#include <list>
typedef CGAL::Exact_predicates_exact_constructions_kernel Kernel;
typedef Kernel::Point_2                                   Point_2;
typedef Kernel::Circle_2                                  Circle_2;
typedef CGAL::Gps_circle_segment_traits_2<Kernel>         Traits_2;
typedef CGAL::General_polygon_set_2<Traits_2>             Polygon_set_2;
typedef Traits_2::General_polygon_2                       Polygon_2;
typedef Traits_2::General_polygon_with_holes_2            Polygon_with_holes_2;
typedef Traits_2::Curve_2                                 Curve_2;
typedef Traits_2::X_monotone_curve_2                      X_monotone_curve_2;
// Construct a polygon from a circle.
Polygon_2 construct_polygon (const Circle_2& circle)
  // Subdivide the circle into two x-monotone arcs.
  Traits_2 traits;
  Curve_2 curve (circle);
  std::list<CGAL::Object>  objects;
  traits.make_x_monotone_2_object() (curve, std::back_inserter(objects));
  CGAL_assertion (objects.size() == 2);
  // Construct the polygon.
  Polygon_2 pgn;
  X_monotone_curve_2 arc;
  std::list<CGAL::Object>::iterator iter;
  for (iter = objects.begin(); iter != objects.end(); ++iter) {
    CGAL::assign (arc, *iter);
    pgn.push_back (arc);
  return pgn;
// Construct a polygon from a rectangle.
Polygon_2 construct_polygon (const Point_2& p1, const Point_2& p2,
                             const Point_2& p3, const Point_2& p4)
  Polygon_2 pgn;
  X_monotone_curve_2 s1(p1, p2);    pgn.push_back(s1);
  X_monotone_curve_2 s2(p2, p3);    pgn.push_back(s2);
  X_monotone_curve_2 s3(p3, p4);    pgn.push_back(s3);
  X_monotone_curve_2 s4(p4, p1);    pgn.push_back(s4);
  return pgn;
// The main program:
int main ()
  // Insert four non-intersecting circles.
  Polygon_set_2 S;
  Polygon_2 circ1, circ2, circ3, circ4;
  circ1 = construct_polygon(Circle_2(Point_2(1, 1), 1));  S.insert(circ1);
  circ2 = construct_polygon(Circle_2(Point_2(5, 1), 1));  S.insert(circ2);
  circ3 = construct_polygon(Circle_2(Point_2(5, 5), 1));  S.insert(circ3);
  circ4 = construct_polygon(Circle_2(Point_2(1, 5), 1));  S.insert(circ4);
  // Compute the union with four rectangles incrementally.
  Polygon_2 rect1, rect2, rect3, rect4;
  rect1 = construct_polygon(Point_2(1, 0), Point_2(5, 0),
                            Point_2(5, 2), Point_2(1, 2));
  S.join (rect1);
  rect2 = construct_polygon(Point_2(1, 4), Point_2(5, 4),
                            Point_2(5, 6), Point_2(1, 6));
  S.join (rect2);
  rect3 = construct_polygon(Point_2(0, 1), Point_2(2, 1),
                            Point_2(2, 5), Point_2(0, 5));
  S.join (rect3);
  rect4 = construct_polygon(Point_2(4, 1), Point_2(6, 1),
                            Point_2(6, 5), Point_2(4, 5));
  S.join (rect4);
  // Print the output.
  std::list<Polygon_with_holes_2> res;
  S.polygons_with_holes (std::back_inserter (res));
  std::copy (res.begin(), res.end(),
             std::ostream_iterator<Polygon_with_holes_2>(std::cout, "n"));
  std::cout << std::endl;
  return 0;

Source: Windows Questions C++