Compare function in a class for std::sort can't compile

Question

I'm writing a program to get a convex hull. I need to sort the points by polar angle, and I chose a base point before, so I write a member compare function ( notice that, for each object the base point is different ). But when I'm applying it to the std::sort, the program can't compile.

Here is my program:

class ConvexHull
{
  Point p[MAXN], base;
public:
  int size;
  void Create(Point (&)[MAXN], const int);
  bool cmp(const Point& a, const Point& b) const
  {
    static int tmp;
    return (tmp = CrossProduct(base, a, base, b)) < 0 || (tmp == 0 && Dist(base, a) < Dist(base, b));
  }
};
void ConvexHull::Create(Point (&a)[MAXN], const int n)
{
  base = a[0];
  for (int i = 1; i < n; ++i)
    if (a[i].x < base.x || (a[i].x == base.x && a[i].y < base.y))
      base = a[i];
  std::sort(a, a+n, cmp);
  p[0] = a[0], p[1] = a[1];
  size = 2;
  for (int i = 2; i < n; ++i)
  {
    while (size >= 2 && CrossProduct(a[i], p[size-1], a[i], p[size-2]) <= 0) --size;
    p[size++] = a[i];
  }
  p[size++] = p[0];
}

and here is the error:

poj1113.cc: In member function 'void ConvexHull::Create(Point (&)[1000], int)':
poj1113.cc:41:24: error: no matching function for call to 'sort(Point [1000], Point*, <unresolved overloaded function type>)'
poj1113.cc:41:24: note: candidates are:
In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from poj1113.cc:3:
/usr/include/c++/4.7/bits/stl_algo.h:5463:5: note: template<class _RAIter> void std::sort(_RAIter, _RAIter)
/usr/include/c++/4.7/bits/stl_algo.h:5463:5: note:   template argument deduction/substitution failed:
poj1113.cc:41:24: note:   candidate expects 2 arguments, 3 provided
In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from poj1113.cc:3:
/usr/include/c++/4.7/bits/stl_algo.h:5499:5: note: void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = Point*; _Compare = bool (ConvexHull::*)(const Point&, const Point&)const]
/usr/include/c++/4.7/bits/stl_algo.h:5499:5: note:   no known conversion for argument 3 from '<unresolved overloaded function type>' to 'bool (ConvexHull::*)(const Point&, const Point&)const'

How to fix it? And is this ( I mean make the base member ) a bad deisgn?

Answer 1

The problem is that your cmp method needs to be static. The reason is that non-static methods expect an invisible first argument, the this pointer. The std::sort function do not pass this extra argument.

Since you reference member variables you can't make the function static, but there are other ways to solve this. I recommend using a new C++11 standard functionality, std::bind:

std::sort(a, a+n, std::bind(&ConvexHull::cmp, this));

The std::bind call creates a callable object, setting the first parameter to this so it will be correct when being called.