Question

In the Learning OpenCV book:

.
.
CvCapture *capture = cvCreateFileCapture(argv[1]);
IplImage* frame;
while(1)
{
frame = cvQueryFrame(capture);
.
.
}

Here, we can see that *frame is a pointer. But, when we write the following frame = cvQueryFrame(capture);, we are not assigning an address to the pointer. What exactly will the values be that frame will hold in this case?

Thanks.

Answer 1

frame is pointer to an object of type IplImage. Presumably, the cvQueryFrame() function allocates an IplImage object and returns the address of this object. The statement

frame = cvQueryFrame(capture);

assigns the value of this returned pointer to frame. If indeed the function is allocating a new object you’ll probably be required to free this memory later by calling operator delete or some function like

void cvDestroyFrame( IplImage * );

Also the statement you make at the end of your question (“*frame is a pointer”) is not accurate – frame is a pointer; *frame means you’re de-referencing the pointer which makes its type IplImage.