Using Ptr<IplImage> in OpenCv causes memory exception - opencv

Question

void hello(Mat original)
{
Mat a=original.clone();
Ptr<IplImage> b = &a.operator IplImage();
return;
}
This function will cause a memory exception. Beause I have to use an old funciton which parameter is IplImage*, I transform the Mat into Ptr and cause memory exception. The above function is just a simplified version.

Answer 1

From OpenCV doc
The operator creates the IplImage header for the matrix without copying the underlying data. You should make sure than the original matrix is not deallocated while the IplImage header is used.
You can pass the reference of the returned header to a function that accepts an IplImage*, likevoid foo(IplImage* img) in the code below):
#include <opencv2\opencv.hpp>
using namespace cv;
void foo(IplImage* img)
{
// Set to blue
cvSet(img, Scalar(255,0,0));
}
int main()
{
// Small green image
Mat3b img(10, 10, Vec3b(0, 255, 0));
{
// Doesn't modify original image
Mat mat = img.clone();
IplImage ipl = mat;
foo(&ipl);
// ipl and mat are blue now
// img is still green
}
{
// Modify original image
IplImage ipl = img;
foo(&ipl);
// ipl and img are blue now
}
// img is blue
return 0;
}

Answer 2

Ptr<T> is a template class thats performs automatic destruction of the wrapped object pointer (see the official documentation for more information about it).
In your code snippet, both a and b are local variables inside the function hello, so they will be destroyed when this function returns and in the reverse order of construction, so a will be destroyed after the destruction of b.
The memory exception could occur because both variables (a and b) refer to the same memory area: in fact, the function Mat::operator IplImage does not copy the underlying data of the Mat object, but creates the IplImage header for these matrix data.
So when the Mat destructor will try to destroy the object a, the portion of memory dedicated to the underlying data will already have been deallocated by b.