OpenCV - C++ - How do Convert a cv::Scalar to return a pointer in a C wrapper for cv::Scalar?

Question

I'm working with the Arjun Comar version of OpenCV 3.0 here https://github.com/arjuncomar/opencv ...Arjun Comar updated this version of OpenCV to auto generate C wrappers for all OpenCV functions so languages without a good c++ FFI could still create wrappers for OpenCV. The C wrapper for C++ is located in the files opencv_generated.cpp (must be built from the Arjun Comar OpenCV version linked above but I pasted my current version here http://www.heypasteit.com/clip/17SI and mat.cpp rect.cpp point.cpp and size.cpp located here https://github.com/arjuncomar/opencv/tree/master/modules/c/src

In that version he uses a Scalar pointer ('Scalar*') to stand in for all the function that require 'cv::Scalar(val1 val2 val3 val4)'. The only thing is he didn't make a create Scalar function or any alternate to create a Scalar*. my attempt is below and its not compiling

cpp

 Scalar* cv_create_Scalar(double val0, double val1, double val2, double val3)
 {
      //also tried cv:;Scalar - same error
     return Scalar(val0, val1, val2, val3);

 }

hpp

'Scalar* cv_create_Scalar(double val0, double val1, double val2, double val3);'

compile with this in cpp directory on Ubuntu Trusty

'g++ -Wall -shared -fPIC -o opencv-glue.so opencv-glue.cpp'

But I'm getting this error

 opencv-glue.cpp: In function ‘cv::Scalar* cv_create_Scalar
 (double, double, double, double)’:
 opencv-glue.cpp:28:41: error: cannot convert 
 ‘cv::Scalar’ to ‘cv::Scalar* {aka cv::Scalar_<double>*}’ in return
      return Scalar(val0, val1, val2, val3);
                                     ^
 opencv-glue.cpp:30:1: warning: control reaches end of 
 non-void function [-Wreturn-type]
  }

I know its not the compile function...its well tested but I need a Scalar* to pass to a function like this I created (which compiled right)

 Mat* cv_create_Mat_S(Size* size, int type, Scalar* s) {
      return new Mat(*size, type, *s);
  }

So I'll have a scalar to pass to the 'absdiff' function c wrapper Arjun Comar created here:

  void cv_absdiff(Mat* src1, Mat* src2, Mat* dst) {
        cv::absdiff(*src1, *src2, *dst);
  }

Any help is MUCH Appreciated=)

Answer 1

Obviously, it can't compile due to conversion, but can't you just create new Scalar dynamically? You're only have to remember that this memory has to be freed by delete.

 Scalar* cv_create_Scalar(double val0, double val1, double val2, double val3)
 {
      //also tried cv:;Scalar - same error
     return new Scalar(val0, val1, val2, val3);
 }

If you want to return a pointer to this object, is has to be done this way. You cannot return new object by creating a copy, you will have to change the method signature.