OOP opencv trackbar - c++

Question

I am trying to make this OOP, so I made a class that implements the callback function for the trackbar.
This is my class
class Contours
{
public:
static Mat src_gray;
static int thresh;
Contours(Mat, int);
~Contours(void);
static void callback(int, void* );
private:
};
Contours::Contours(Mat src_gray,int thresh){
this->src_gray=src_gray;
this->thresh=thresh;
}
Contours::~Contours(void){}
void Contours::callback(int, void*)
{int largest_area=0;
int largest_contour_index=0;
Mat canny_output;
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
/// Detect edges using canny
Canny( src_gray, canny_output, thresh, thresh*2, 3 );
/// Find contours
findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
/// Draw contours
Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
cout<<contours.size()<<endl;
for( int i = 0; i< contours.size(); i++ )
{
double a=contourArea( contours[i],false); // Find the area of contour
if(a>largest_area){
largest_area=a;
largest_contour_index=i; } //Store the index of largest contour
//Scalar color = Scalar(255,255,255 );
//drawContours( drawing, contours, i, color, 2, 8, hierarchy, 0, Point() );
}
cout<<"cnt maxim: "<<largest_contour_index<<endl;
Scalar color = Scalar(255,255,255 );
drawContours( drawing, contours, largest_contour_index, color, 2, 8, hierarchy, 0, Point() );
/// Show in a window
namedWindow( "Contours", CV_WINDOW_AUTOSIZE );
imshow( "Contours", drawing );
//imwrite("sada.png",drawing);
}
And calling the function
createTrackbar( " Canny thresh:", "Source", &thresh, max_thresh, &Contours::callback );
I get
Error 5 error LNK2001: unresolved external symbol "public: static class cv::Mat Contours::src_gray" (?src_gray#Contours##2VMat#cv##A) D:\Licenta\Hand sign recognition\Algoritmi.obj Hand sign recognition
Error 6 error LNK2001: unresolved external symbol "public: static int Contours::thresh" (?thresh#Contours##2HA) D:\Licenta\Hand sign recognition\Algoritmi.obj Hand sign recognition
Any ideas why?

Answer 1

maybe use a different approach to the 'static callback' problem:
class Contours
{
public:
Mat src_gray; // non-static
int thresh;
Contours(Mat, int);
void callback(int, void* ); // non-static
};
void callback(int value, void*ptr) // static global
{
Contours* cnt = (Contours*)ptr; // cast 'this'
cnt->callback(value,0); // call non-static member
}
int main()
{
Mat gray = ...
Contours cnt(gray,17);
// now pass the address of cnt as additional void* Ptr:
createTrackbar( " Canny thresh:", "Source", &thresh, max_thresh, callback, &cnt );
...