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But the output hist matrix is invalid with values (rows, cols) as (-1, -1).

a multi (more than 2d) mat can't express it's dimensions using rows & cols only, thus they're left empty(at -1) and you have to query the Mat::size member (note: Not the function !):

  cv::Mat hist; // MatND is an alias for Mat, since a very long time already ..

  cv::calcHist(&matImageHSV, nImages, channels, cv::Mat(), // do not use mask
           hist, dims, histSize, ranges,
           true, // the histogram is uniform
           false );

cout << hist.dims << endl;
cout << hist.size << endl;

3
36 x 25 x 25  // hbins, sbins, vbins, again !

But the output hist matrix is invalid with values (rows, cols) as (-1, -1).

a multi (more than 2d) mat can't express it's dimensions using rows & cols only, thus they're left empty(at -1) and you have to query the Mat::size member (note: Not the function !):

 cv::Mat hist; // MatND is an alias for Mat, since a very long time already ..

 cv::calcHist(&matImageHSV, nImages, channels, cv::Mat(), // do not use mask
           hist, dims, histSize, ranges,
           true, // the histogram is uniform
           false );

cout << hist.dims << endl;
cout << hist.size << endl;

3
36 x 25 x 25  // hbins, sbins, vbins, again !

But the output hist matrix is invalid with values (rows, cols) as (-1, -1).

a multi (more than 2d) dimensional mat can't express it's dimensions using rows & cols only, thus they're left empty(at -1) and you have to query the Mat::size member (note: Not the function !):

cv::Mat hist; // MatND is an alias for Mat, since a very long time already ..

cv::calcHist(&matImageHSV, nImages, channels, cv::Mat(), // do not use mask
           hist, dims, histSize, ranges,
           true, // the histogram is uniform
           false );

cout << hist.dims << endl;
cout << hist.size << endl;

3
36 x 25 x 25  // hbins, sbins, vbins, again !