Problem using cv :: Imread

I am writing a code to read all the images in a folder and then write the information of the images in a binary all together. The problem is that when I use the function cv::imread the field .data of the cv::MAT always remains null, and when I'm going to write there is an error when accessing a null pointer. I am using absolute paths obtained from a file .txt in the following code:

void main(){
  const unsigned int SizeImg = 1024 * 768 * sizeof(char);
  std::ifstream file;
  file.open("files.txt", std::ios::out);
  std::string line;
  std::string files[50];
  std::string imagen;
  std::ofstream Binary("Images.bin", std::ios::out | std::ios::binary);
  int n = 0;
  while (std::getline(file,line))
  {
    std::cout << line << '\n';
    files[n] = line;
    n++;
  }
  file.close();
  for (int i = 0; i < n; i++){
    imagen = files[i];
    cv::Mat m = cv::imread(imagen, CV_LOAD_IMAGE_ANYCOLOR);
    //cv::imshow("ventana", m);
    cv::waitKey(0);
    Binary.write((char*)m.data, SizeImg);
  }
  Binary.close();
}

I'm starting with OpenCV so there may be other things that are not very good either.