tor of an class derived from
      <code class="computeroutput">boost::signals::trackable</code> will always be
      called before the destructor of the base <code class="computeroutput">boost::signals::trackable</code>
      class.  However, for thread-safety the connection between the signal and object
      needs to be disconnected before the object runs its destructors.
      Otherwise, if an object being destroyed
      in one thread is connected to a signal concurrently
      invoking in another thre