Convert RGB image to gray scale using iterator in OpenCV C++

ImageOperator.cpp

void ImageOperator::to_gray_m1(const cv::Mat &input, cv::Mat &output) {
  
    unsigned char *data_out = (unsigned char*)(output.data);
    int ind = 0;
    auto end = input.end<cv::Vec3b>();
    cv::MatConstIterator_<cv::Vec3b> it = input.begin<cv::Vec3b>();
    for (; it != end; ++it) {
        const unsigned char &r = (*it)[2];
        const unsigned char &g = (*it)[1];
        const unsigned char &b = (*it)[0];
        data_out[ind] = 0.3*r+0.59*g+0.11*b;
        ind++;
    }

}