Bug detected at Mat class constructor #4: Mat::Mat(int rows, int cols, int type, const Scalar& s); (Bug #4019)

    Mat M = Mat(10, 10, CV_64F, 0); 

the 0 is interpreted as a (null)pointer, not as a Scalar, so no memory was allocated.

http://docs.opencv.org/modules/core/doc/basic_structures.html#Mat::Mat(int%20rows,%20int%20cols,%20int%20type,%20void*%20data,%20size_t%20step)

you probably wanted:

    Mat M = Mat(10, 10, CV_64F, Scalar(0)); 

be rak wrote:

[...]

the 0 is interpreted as a (null)pointer, not as a Scalar, so no memory was allocated.

http://docs.opencv.org/modules/core/doc/basic_structures.html#Mat::Mat(int%20rows,%20int%20cols,%20int%20type,%20void*%20data,%20size_t%20step)

you probably wanted:

[...]

that is reasonable. thanks for mentioning that Scalar(), which is useful. but when a 1.0f or 1.f or any other value is sent as a scalar, it works just fine, so I think that when an 0 (or 0.0, 0.f) is sent, it should be translated as a scalar too, just like the other values do.
moreover, if a coder (such as me) want to declare a Mat object without allocating any memory, one would not sent in the 4th parameter, just for simplifing the code; if one sent in a value, then it supposed to be translated as a value, causing less confusion about value 0.0, 0.f, 0.0f and 0 itself.

further testing:

// the following code will crash at the marked line
int main(int argc, char** argv ) 
{
  printf("more test is assigned..");
  //Mat M = Mat(10, 10, CV_64F);// fine
  //Mat M = Mat(10, 10, CV_64F, 1.0);// fine
  //Mat M = Mat(10, 10, CV_64F, 1.f);// fine
  //Mat M = Mat(10, 10, CV_64F, 0.0);// fine
  //Mat M = Mat(10, 10, CV_64F, 0.f);// fine
  Mat M = Mat(10, 10, CV_64F, 0);// not fine. (declare a float matrix but send a type-unclear scalar)
  M.at<double>(0, 0) = 0.f;// [mark] fails at here! where a float is assigned to an entry of matrix. it seems that
                           //        refers to rak, the initial scalar 0 is miss translated to null pointer
  printf("Okay!");
  return 0;
}

my suggestion remained, and put it simply, if the matrix type is defined according to the 3rd parameter (int type) instead 4th, it should work just fine, if I am right.
hope helpful. --xianglin01

hi Alexander Alekhin, thanks for response and suggestions!

Alexander Karsakov wrote:

Hi wooden glider!

It's not a bug. But it can be useful, you may implement this functionality and send pull request to us (http://code.opencv.org/projects/opencv/wiki/How_to_contribute).

Hi Alexander Karsakov!
Thanks for your appreciation! I'll try my best to submit a pull request.