select multiple points with mouse (opencv 2.4.9,vs2010,winforms)

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Hi,

I'm trying to select multiple points by mouse (with cv::EVENT_LBUTTONDOWN).

So I'm passing the std::vector<cv::point> to the Callback-Function:

//setting the roi manually
std::vector<cv::Point> points;   

   cv::Mat img = cv::imread("f:/open/me.jpg");

   cv::namedWindow("Original");

   cv::setMouseCallback("Original", mouse_call, (void*)&points);

   int X,Y;


   cv::imshow("Original",img);

And in Callback:

static void mouse_call(int event,int x,int y, int, void* param)
{
      if(event==cv::EVENT_LBUTTONDOWN){

        std::vector<cv::Point>* ptPtr = (std::vector<cv::Point>*)param;

            ptPtr->push_back(cv::Point(x,y));


    }

And, when I'm trying to access the vector-elements with:

points[0].x

points[0].y

It's compiling, but at runtime i get an error: "Debug Assertion Failed " "Expression: vector subscript out of range"

Any clue ? Thanks