Custom list.sort comparisons in C++ -

i have std::list i'm trying sort based on calculations. point2d struct int no, double x, , double y;

here's method contains list.sort code:

std::vector<point2d> grahamscan::getsortedpointset(std::vector<point2d> points) { point2d lowest = getlowestpoint(points);  std::list<point2d> list;  (int = 0; < (int)points.size(); i++) {     list.push_back(points[i]); }  list.sort(compare_points);  std::vector<point2d> temp; (int = 0; < (int)list.size(); i++) {     temp.push_back(list.front());     list.pop_front(); } return temp; } 

and here's compare_points method wrote:

bool grahamscan::compare_points(const point2d& a, const point2d& b) { if (a.x == b.x && a.y == b.y) {     return false; }  double thetaa = atan2((long)a.y - lowest.y, (long)a.x - lowest.x); double thetab = atan2((long)b.y - lowest.y, (long)b.x - lowest.x);  if (thetaa < thetab) {     return false; } else if (thetaa > thetab) {     return true; } else {     double distancea = sqrt((((long)lowest.x - a.x) * ((long)lowest.x - a.x)) +         (((long)lowest.y - a.y) * ((long)lowest.y - a.y)));     double distanceb = sqrt((((long)lowest.x - b.x) * ((long)lowest.x - b.x)) +         (((long)lowest.y - b.y) * ((long)lowest.y - b.y)));      if (distancea < distanceb) {         return false;     }     else {         return true;     } } } 

the error visual studio spitting out @ me "grahamscan::compare_points":non-standard syntax; use '&' create pointer member"

i don't have experience in c++, i'm trying convert java code uses treeset c++ , attempt.

any assistance appreciated.

if want keep compare_points in grahamscan namespace need make static:

static bool grahamscan::compare_points 

the reason compiler complains compare_points member function. needs grahamscan object applied on. behind curtains real function signature of compare_points bool compare_points(grahamscan *this, const point2d& a, const point2d& b). either make static or don't define member function.

once make compare_points static, lowest variable no longer accessible it. easier way work around make lowest static:

class grahamscan {     // declaration inside class     static point2d lowest; }  // definition outside class point2d grahamscan::lowest; 

and use this:

std::vector<point2d> grahamscan::getsortedpointset(std::vector<point2d> points) {     grahamscan::lowest = getlowestpoint(points);     //... }