></code>
          function object. Note that the formatter accepts the attribute value wrapped
          into the <a class="link" href="../detailed/utilities.html#log.detailed.utilities.value_ref" title="Value reference wrapper"><code class="computeroutput"><span class="identifier">value_ref</span></code></a> wrapper which can be
          empty if the value is not present.
        </p>
<p>
          Lastly, the call to <code class="computeroutput"><a class="link" href="../../boost/log/register_formatte_id