#include <iostream>
#include <list>
#include <cmath>

struct P { double x, y; };

class tri {
  double sum;
public:
  tri() { sum = 0.0; }
  double result() { return 0.5 * fabs(sum); }
  void operator()(P p1, P p2) { sum += p1.x * p2.y - p2.x * p1.y; }
};

void mytransform(std::list<P>::const_iterator s, std::list<P>::const_iterator e, tri &S) {
  std::list<P>::const_iterator p, q;
  if ((p = s) == e) return;
  if ((q = ++s) == e) return;
  while (q != e) {
    S(*p, *q);
    p++; q++;
  }
}

int main() {
  tri s;
  static P table[] = { {6.0, 6.0}, {5.0, 6.0}, {4.0, 4.0}, {5.0, 4.0}};
  
  std::list<P> points;
  for (unsigned int i = 0; i < sizeof(table) / sizeof(P); i++)
    points.push_back(table[i]);
  points.push_back(table[0]);
  mytransform(points.begin(), points.end(), s);
  std::cout << s.result() << std::endl;
  return 0;
}
/* end */

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