boost::filesystem::path p(fromDirectory);
if (!boost::filesystem::exists(p) || !boost::filesystem::is_directory(p))
  return;

std::vector<boost::filesystem::path> paths;
std::copy(boost::filesystem::recursive_directory_iterator(p), boost::filesystem::recursive_directory_iterator(), std::back_inserter(paths));

auto pathsEnd = std::remove_if(paths.begin(), paths.end(),
  [](boost::filesystem::path& path) {
    return boost::filesystem::is_directory(path);
  }
);
paths.erase(pathsEnd, paths.end());
std::sort(paths.begin(), paths.end());