C++ dynamic memory allocation issue

Question

The following code gives me seg.fault (see the comment bellow) depending how I define newImagesData. If it's array everything is OK but if defined as a pointer - fault :(

//Mat newImagesData[FIFO_SIZE]; // Mat is a class from opencv
Mat* newImagesData;

void func1( )
{
    ...
    newImagesData = (Mat*)malloc(FIFO_SIZE * sizeof(Mat)); //if *newImagesData
    fifoInit(&newImagesFB, newImagesData, FIFO_SIZE);
    ...
}

void fifoInit(FIFOBuffer* fifo, Mat* imageData, int size)
{
    fifo->data = imageData;
    fifo->end = 0;
}

void fifoWrite(FIFOBuffer* fifo, Mat* frame)
{
    fifo->data[fifo->end] = *frame; // seg fault with *newImagesData ; OK with newImagesData[]
    memcpy(&fifo->data[fifo->end], frame, sizeof(Mat)); // OK with *newImagesData
    ...
}

If I use memcpy() it works fine. I can understand what I'm doing wrong. Any help?

juanchopanza · Accepted Answer · 2013-06-01 14:08:29Z

7

You should not dynamically allocate cv::Mat objects or arrays using malloc, since this only allocates space for the objects, but does not call their constructor (which must be called in order to instantiate a type such as cv:Mat). Correspondingly, free will not call the required destructor.

You should either use new (coupled with delete) or, preferably, use standard library structures such as std::vector (or 'std::dynarray' in C++14). For example,

std::vector<cv::Mat> newImagesData(FIFO_SIZE); 

std::dynarray<cv::Mat> newInagesData(FIFO_SIZE); // C++14 only

You can then use newImageData as you would use a FIFO_SIZE-length array of cv::Mat objects.

edited Jun 1, 2013 at 14:08

answered Jun 1, 2013 at 13:53

juanchopanza

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2 Comments

Kourosh Over a year ago

True. Using new and delete operators guarantees the constructor and destructor get called, which will not happen if you use malloc to allocate memory for a class object.

juanchopanza Over a year ago

@user2443309 probably just by pure luck. It looks like undefined behaviour.

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