/// Asynchronously write a data structure to the socket.
  template <typename T, typename Handler>
  void async_write(const T& t, Handler handler)
  {
    // Serialize the data first so we know how large it is.
    std::ostringstream archive_stream;
    boost::archive::text_oarchive archive(archive_stream);
    archive << t;
    outbound_data_ = archive_stream.str();

    // Format the header.
    std::ostringstream header_stream;
    header_stream << std::setw(header_length)
      << std::hex << outbound_data_.size();
    if (!header_stream || header_stream.str().size() != header_length)
    {
      // Something went wrong, inform the caller.
      boost::system::error_code error(boost::asio::error::invalid_argument);
      socket_.get_io_service().post(boost::bind(handler, error));
      return;
    }
    outbound_header_ = header_stream.str();

    // Write the serialized data to the socket. We use "gather-write" to send
    // both the header and the data in a single write operation.
    std::vector<boost::asio::const_buffer> buffers;
    buffers.push_back(boost::asio::buffer(outbound_header_));
    buffers.push_back(boost::asio::buffer(outbound_data_));
    boost::asio::async_write(socket_, buffers, handler);
  }