Matrix (dis)allocation

Question

I use opencv c++ API for computer vision application and I manage matrices with cv::Mat structure. Opencv use ref counting to release created object as cv::Mat. I use also libshogun for more specific machine learning algorithms. Shogun has its own matrices structure called SGMatrix. I initialize SGMatrix from a cv::Mat like this:

cv::Mat cvmat(100,100,CV_32FC1,cv::Scalar(0.0)); 
SGMatrix<float> sgmatrix((float*)cvmat.data, cvmat.rows, cvmat.cols);

My problem is when I use another object in shogun lib like:

CSimpleFeatures<float>* features = new CSimpleFeatures<float>(sgmatrix);

where shogun is supposed now to own the matrix created with features, I get an error, at runtime, when opencv try to release cvmat which has already been released by shogun. How can I handle this ? I don't want to clone my matrix.

Answer 1

Allocate the memory for the inside of the matrix outside OpenCV, either with Shogun or with new / malloc. Then, create the matrix by using one of the constructors that takes as argument a void*pointer to the matrix data (which you have allocated before). This creates just an opencv header for your data.

That way, opencv knows it does not own the data inside the matrix, and it will not try to free it when the cv::Mat object is released.

Answer 2

Shogun will de-allocate the matrix as soon as the SGMatrix and the simplefeature object are destroyed. To avoid that use

SGMatrix<float> sgmatrix((float*)cvmat.data, cvmat.rows, cvmat.cols, false);

This way you have to take care of memory deallocation!