Question

I’m using a CvPoint structure in OpenCV and I need to assign a value to x and y fields of the structure.

Here is my code:

CvPoint* P1;
P2[0].x=32;

But the programs always block while trying to set the value.

Any idea about how to set these values?

Answer 1

Well first of all P1 is a pointer to an object of type P1. In order to assign something to an object’s member via its pointer you need to use the -> operator. If this pointer points to the beginning of an array you use the operator[] to access individual elements. This operator returns a reference for the given index, in this case CvPoint& .

1. dynamic allocation of a single object

CvPoint* P1 = new CvPoint(); // default construction of an object of type CvPoint
P1->x = 32;

// do something with P1

// clean up 
delete P1;

2. dynamic allocation or an array

CvPoint* points = new CvPoint[2]; // array of two CvPoints
points[0].x = 32; // operator[] returns a reference to the CvPoint at the given index
points[1].x = 32;

// do something with points

// clean up
delete[] points;

Since in both examples the new operator has been used, it is mandatory to pair them with a matching call to delete or delete[] in case of an array.