How to convert opencv Mat to png buffer?

     /* Insert an image. */
    cv::Mat img = cv::imread("logo.png");
    // encode image into jpg
    cv::vector<uchar> buf;
    cv::imencode(".png", img, buf, std::vector<int>() );
    // encoded image is now in buf (a vector)
    unsigned char *imageBuf = new unsigned char [buf.size()];
    memcpy(imageBuf, &buf[0], buf.size());
    //  size of imageBuf is buf.size();
    worksheet_insert_image_buffer(worksheet, 1, 2, imageBuf, buf.size());
    workbook_close(workbook);
    delete[] imageBuf;