Parameter passing

Question

In a program in the Learning OpenCV book:

void onTrackbarSlide(int pos)
{
cvSetCaptureProperty(g_capture,CV_CAP_PROP_POS_FRAMES,pos);
}

And, in another location:

if(frames!=0)
{
cvCreateTrackbar("Position","Example3",&g_slider_position,frames,onTrackbarSlide);
}

If you see onTrackbarSlide , there is no parameter passed. In this case, what value will be passed to the onTrackSlide(int pos) method?

Answer 1

You are passing the address of function

void onTrackbarSlide(int pos)

as the last parameter in the function cvCreateTrackbar

This doesn't call the onTrackbarSlide . The cvCreateTrackbar method must be storing the address of the function onTrackbarSlide and using it as an Callback to intimate of some asynchronous happening.