CODEWITHSUNDEEP
c
soxarered
soxarered

Reputation: 163

How to allocate an array of pointers to a struct

I am using the CvPoint2D32f struct of opencv, which is defined as 

typedef struct CvPoint2D32f
    {
        float x; /* x-coordinate, usually zero-based */
        float y; /* y-coordinate, usually zero-based */
    }
    CvPoint2D32f;

I am trying to allocate an array of pointers of these objects as follows, and it is not working. How is this to be accomplished?

CvPoint2D32f *cvpoints[8];
for (i = 0; i < 16; i++)
    {
        if (i % 2 == 0)
        {
            (*cvpoints)[i/2].x = float(points[i]); // points is an array of ints.  
        }
        else
        {
            (*cvpoints)[i/2].y = float(points[i]);
        }
    }

Upvotes: 2

Views: 2522

Answers (4)

Gavin H
Gavin H

Reputation: 10482

You have an array of pointers to CvPoint2D32f, not an array of the structs themselves. You have no memory allocated for the actual structures. You have three options:

  • Change your declaration to CvPoint2D32f cvpoints[8]; so you have all of the structures in an array on the stack.

  • After you allocate the array for the pointers, add a malloc statement to allocate memory. Example:

    CvPoint2D32f *cvpoints[8];
for (i = 0; i < 8; i++)
    {
    cvpoints[i] = (CvPoint2D32f *)malloc(sizeof(CvPoint2D32f));
    }

  • Use malloc to allocate size for all the structures. Example:

    CvPoint2D32f *cvpoints = (CvPoint2D32f *)malloc( 8 * sizeof(CvPoint2D32f));

Upvotes: 5

Brian Roach
Brian Roach

Reputation: 76898

CvPoint2D32f *cvpoints = (CvPoint2D32f*) malloc(sizeof(struct CvPoint2D32f) * 8);
memset(cvpoints, \0, sizeof(struct CvPoint2D32f) * 8);

for (i = 0; i < 16; i++)
{
    if (i % 2 == 0)
    {
        cvpoints[i/2].x = float(points[i]); // points is an array of ints.  
    }
    else
    {
        cvpoints[i/2].y = float(points[i]);
    }
}

And when you're done with them, you free the memory:

free(cvpoints);

Upvotes: 1

Vincenzo Pii
Vincenzo Pii

Reputation: 19805

You are just creating the pointers, but you need them to point to something in memory!

Example:

struct CvPoint2D32f *cvpoint = (struct CvPoint2D32f *)malloc(sizeof(struct CvPoint2D32f));
if (cvpoint == NULL) {
    //malloc error
}

Upvotes: 0

escargot agile
escargot agile

Reputation: 22379

Where do these pointers point to? I guess what you want to do is allocate dynamic memory for the structs, but I don't see any memory allocation in your code. Currently all pointers point to undefined memory, so of course this will fail.

Upvotes: 0

Related Questions