Custom list.sort comparisons in C++ - c++

Question

I have an std::list that I'm trying to sort based on some calculations. Point2D is a struct with only int no, double x, and double y;
Here's the method that contains my list.sort code:
std::vector<Point2D> GrahamScan::getSortedPointSet(std::vector<Point2D> points) {
Point2D lowest = getLowestPoint(points);
std::list<Point2D> list;
for (int i = 0; i < (int)points.size(); i++) {
list.push_back(points[i]);
}
list.sort(compare_points);
std::vector<Point2D> temp;
for (int i = 0; i < (int)list.size(); i++) {
temp.push_back(list.front());
list.pop_front();
}
return temp;
}
And here's the compare_points method I 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 is spitting out at me is "GrahamScan::compare_points":non-standard syntax; use '&' to create a pointer to member"
I don't have much experience in C++, but I'm trying to convert some Java code that uses a TreeSet to C++ and this is my attempt.
Any assistance would be appreciated.

Answer 1

If you want to keep compare_points in GrahamScan namespace you need to make it static:
static bool GrahamScan::compare_points
The reason the compiler complains is that compare_points is a member function. It needs a GrahamScan object to be applied on. Behind the curtains the real function signature of compare_points is something like bool compare_points(GrahamScan *this, const Point2D& a, const Point2D& b). So either make it static or don't define it as a member function.
Once you make compare_points static, your lowest variable will no longer be accessible to it. Easier way to work around that is to make lowest also static:
class GrahamScan
{
// declaration is inside class
static Point2D lowest;
}
// definition is outside class
Point2D GrahamScan::lowest;
and use it like this:
std::vector<Point2D> GrahamScan::getSortedPointSet(std::vector<Point2D> points)
{
GrahamScan::lowest = getLowestPoint(points);
//...
}