How to copy vector of Mat and write video ?

edit

I would like to duplicate the images of a video via a vector, There are the right number of frames in vector but when copying, they are empty when writing the file, so the file only has the first copy of the frames but not the rest.

I think the problem comes from this loop but I do not know why it does not work :

for (cv::Mat &trame_vid : video_vect) {
        video_vect.emplace_back(trame_vid.clone());
}

Clone makes a copy and it is sent to the vector via emplace_back(), if I understand correctly ?

the entire code :

 std::string filename = "chaplin.mp4";
  cv::VideoCapture capture(filename);

  if (!capture.isOpened())
    throw "Error when reading chaplin.mp4";
  double fps = capture.get(cv::CAP_PROP_FPS);
  int frame_width = capture.get(cv::CAP_PROP_FRAME_WIDTH);
  int frame_height = capture.get(cv::CAP_PROP_FRAME_HEIGHT);

  cv::Mat frame;
  std::vector<cv::Mat> video_vect;
  video_vect.reserve(capture.get(cv::CAP_PROP_FRAME_COUNT) * 2);
  while (1) {

    // Capture frame-by-frame
    capture >> frame;
    // If the frame is empty, break immediately
    if (frame.empty())
      break;
    // Copy current image to vector
    video_vect.emplace_back(frame.clone());
    imshow("Frame", frame);

    // ESC to stop
    char c = (char)cv::waitKey(20);
    if (c == 27)
      break;
  }

  int codec = cv::VideoWriter::fourcc('X', '2', '6', '4');
  cv::VideoWriter video("out.avi", codec, (int)fps,
                        cv::Size(frame_width, frame_height), true);

  std::cout << video_vect.size() << std::endl;
  //Copy Mat
  for (cv::Mat &trame_vid : video_vect) {
    video_vect.emplace_back(trame_vid.clone());
  }

  std::cout << video_vect.size() << std::endl;

  for (cv::Mat &trame_vid : video_vect) {
    video.write(trame_vid);
  }
  // Liberation des ressources
  capture.release();

  video.release();
  video_vect.clear();
  video_vect.shrink_to_fit();

  // Closes all the frames
  cv::destroyAllWindows();