32.6 Patterns for Extractors and Inserters of User-Defined Types

Here is the pattern for an extractor:

template<class charT, class Traits>
std::basic_istream<charT, Traits>& 
operator>> (std::basic_istream<charT,
            Traits>& is, UserDefinedType& x)
{
  std::ios_base::iostate err = 0;

  // Create a sentry object (may set error bits or throw failure)
  typename basic_istream<charT, Traits>::sentry ipfx(is);

  try {

    // Proceed with I/O only if sentry is okay.

    if(ipfx) {
      // Typically you access the stream's locale or buffer.
      // Don't call other stream I/O functions.

      // Add state bits to the err variable if necessary,
      // for example:
      // if (...) 
      //    err |= std::ios_base::failbit;
    }
  }
  catch(...) {
    bool flag = false;
    try { 
      // set failbit (may throw failure)
      is.setstate(std::ios_base::failbit);
    }
    catch(...) { flag = true; }

    // Re-throw original exception (thrown from facet or buffer)
    if (flag)
      throw;
  }

  // As the last step, set error bits (may throw failure)
  if (err)
    is.setstate(err);

  return is;
}

Similarly, the pattern for the inserter looks like this:

template<class charT, class Traits>
std::basic_ostream<charT, Traits>&
operator<<(std::basic_ostream<charT, Traits >& os,
            const UserDefinedType& x)
{
  std::ios_base::iostate err = 0;

  // Create a sentry object (may set error bits or throw failure)
  typename basic_ostream<charT, Traits>::sentry opfx(os);

  try {
    // Proceed with I/O only if sentry is okay
    if (opfx) {
      // Typically you access the stream's locale or buffer.
      // Don't call other stream I/O functions

      // Add state bits to the err variable if necessary,
      // for example:
      // if (...) 
      //    err |= std::ios_base::failbit;

      // Reset the field width after usage, that is,
      // os.width(0);
    }
  } //try
  catch(...) {
    bool flag = false;
    try { 
      // set failbit (may throw failure)
      os.setstate(std::ios_base::failbit);
    }
    catch(...) {
      // Catch exception, do not rethrow
      flag = true;
    }
    // Rethrow original exception (from facet or buffer)
    if (flag)
       throw;
  }

  // As the last step, set error bits (may throw failure)
  if (err)
    os.setstate(err);
 
  return os;
}