How to get a list of files in a folder in which the files are sorted with modified date time?

Most operating systems do not return directory entries in any particular order. If you want to sort them (you probably should if you are going to show the results to a human user), you need to do that in a separate pass. One way you could do that is to insert the entries into a std::multimap, something like so:

namespace fs = boost::filesystem;
fs::path someDir("/path/to/somewhere");
fs::directory_iterator end_iter;

typedef std::multimap<std::time_t, fs::path> result_set_t;
result_set_t result_set;

if ( fs::exists(someDir) && fs::is_directory(someDir))
{
  for( fs::directory_iterator dir_iter(someDir) ; dir_iter != end_iter ; ++dir_iter)
  {
    if (fs::is_regular_file(dir_iter->status()) )
    {
      result_set.insert(result_set_t::value_type(fs::last_write_time(dir_iter->path()), *dir_iter));
    }
  }
}

You can then iterate through result_set, and the mapped boost::filesystem::path entries will be in ascending order.