uires the tracked objects to be derived from the <code class="computeroutput">trackable</code>
      base class, which is not always practical when interacting
      with classes from 3rd party libraries.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.37.9.4"></a><code class="computeroutput">optional_last_value</code> as the Default Combiner</h3></div></div></div>
<p>
      The default combiner for Boost.Signals2 has changed from the <code class="co