ing">void handler(boost::system::error_code ec, result_type result);
</pre>
<p>
          the initiating function returns a <code class="computeroutput">std::future</code> templated on
          <code class="computeroutput">result_type</code>. In the above example, this is <code class="computeroutput">std::size_t</code>.
          If the asynchronous operation fails, the <code class="computeroutput">error_code</code> is converted
          into a <code class="computeroutput">system_error</code> exception an