Question

  typedef pair<double, double> dd;     const double epsilon = 1e-6;    struct sort_by_polar_angle {      dd center;      // Constuctor of any type      // Just find and store the center      template<typename T> sort_by_polar_angle(T b, T e) {          int count = 0;         center = dd(0,0);          while(b != e) {                      center.first += b->first;                     center.second += b->second;                b++;              count++;         }                 double k = count ? (1.0/count) : 0;         center.first *= k;                center.second *= k;    }     // Compare two points, return true if the first one is earlier     // than the second one looking by polar angle     // Remember, that when writing comparator, you should     // override not ‘operator <’ but ‘operator ()’     bool operator () (const dd& a, const dd& b) const {          double p1 = atan2(a.second-center.second, a.first-center.first);          double p2 = atan2(b.second-center.second, b.first-center.first);          return p1 + epsilon < p2;     }    };   // ...   vector < dd >  points;   sort(all(points), sort_by_polar_angle(all(points)));  

When sort_by_polar_angle() is called, is it function as a construnctor? How the overloaded operator () correctly used?

Answer 1

When you call sort_by_polar_angle() in the sort() function, you are creating a temporary object of type sort_by_polar_angle (i.e. its constructor is called). Inside the sort algorithm, the functor object you passed is used something like functor() which will be translated into functor.operator().