How to access/modify matrix element in OpenCV? Why at() is templatized?

Question

Should I know Mat element type for use at() correctly? For example, if I have

Mat rose = Mat(1,180, CV_64F, 0);

then can I call

rose.at<short>(i,j)++;

If not then which template argument should I use?

Why Mat::at is templatized while Mat itself is not?

UPDATE

This question contained sample code with another error, which is now here: How to fill Matrix with zeros in OpenCV?

Answer 1

As already properly pointed out by William, you should provide only the correct type as a template argument for at. I believe that cv::Mat itself is not made template only for simplification. However, OpenCV crew are trying to support C++ features, including templates. The interface became slightly heterogeneous in this way.

You could not deduce compiler type from a type variable at runtime for obvious reason. However, you can deduce it at compile time, if your type variable is known at this point, using a traits class:

template<int I>
struct CvType {};

template<>
struct CvType<CV_64F> { typedef double type_t; };
template<>
struct CvType<CV_32F> { typedef float type_t; };
template<>
struct CvType<CV_8U> { typedef unsigned char type_t; };
// Other types go here

void main()
{
  const int type = CV_64F;
  cv::Mat mat(10, 10, type);
  mat.at<CvType<type>::type_t>(1, 1);
}

In this case you can change the value of type and wouldn't need to change types manually for all the at or other methods calls.