Opencv 2.1 Sobel detector passing error

Im Trying to perform a Sobel transform on an image however it gives this error:
error: invalid initialization of reference of type 'cv::Mat&' from expression of type 'IplImage*'|
cv.hpp|254|error: in passing argument 2 of 'void cv::Sobel(const cv::Mat&, cv::Mat&, int, int, int, int, double, double, int)'|
||=== Build finished: 2 errors, 0 warnings ===|

Below shows various ways people have used the Sobel function, I am unable to find any help on this particular issue

Yahoo! Groups
Sobel edge detection on OpenCV | About of Image Processing

I have done it different, but tried the same as above , to follow the manual below.
Is it a case of me misunderstanding what the manual is stating?
Image Filtering — opencv v2.1 documentation

Code:

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              IplImage* img=cvLoadImage("fight1.png");

              CvMat *mat = cvCreateMat(img->height,img->width,CV_32FC3 );
              //Get width and height
              int w=img->width;
              int h=img->height;
              //create Images
              IplImage* img2=cvCreateImage( cvSize( w, h ), IPL_DEPTH_8U, 1 );
              IplImage* img3=cvCreateImage( cvSize( w, h ), IPL_DEPTH_8U, 1 );
              //convert to Mat
              cvConvert(img,mat);

              //prepare stream reader
              ofstream ob;
              ob.open("object.txt");

                //Perform Sobel
              cvCvtColor( img,img2,CV_RGB2GRAY);
              Sobel(img2,img3,32 ,1,3,1.0,1.0,BORDER_DEFAULT);
                //cvLaplace( img, img2);

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