 Our goal is to ensure A dtor, at its start, blocks until B dtor has started executing; and to give their side the information necessary to do the same. That's <a class="el" href="classipc_1_1session_1_1Session__base_1_1Graceful__finisher.html#af441f45ce1ed7b890c3ab77d959f8830" title="The reason Graceful_finisher exists, this method must be called at the start of *_session_impl dtor; ...">on_dtor_start()</a>: the method that'll do the blocking; when it returns the dtor of subclass can proceed. So what's nee