title: std::streampos, std::streamoff and std::streamsize to long long int?
body: <p>To measure position/offsets/size of streams, the standard specify <code>std::streampos</code>, <code>std::streamoff</code> and <code>std::streamsize</code>, but they are implementation defined. </p>
  
  <p>How to convert these types to <code>long long int</code> in a secure and portable way ? (for example to measure a file size and inject it in a function that take a long long int as argument)</p>