convert mat to iplimage

edit

cv::Mat mat_img;
IplImage ipl_img = mat_img.operator IplImage();

I have always had to do it myself in C:

void iplimage_from_cvmat(CvMat* input, IplImage * output)
{
  int x,y;
  for( x=0; x < output->width; x++ ){
    for( y=0; y < output->height; y++) {
      // note: CvMat is indexed (row, column) but IplImage is indexed (x,y)
      // so the indexes must be interchanged!
      cvSetReal2D( output, x, y, CV_MAT_ELEM(*input, uchar, y, x) );      
    }
  }
}