ublic members.
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="identifier">handler_ptr</span></code> is a move-only
          type, with <code class="computeroutput"><span class="identifier">unique_ptr</span></code> semantics.
          Actions required: user-defined composed operations using <code class="computeroutput"><span class="identifier">handler_ptr</span></code> to manage state can only
          be moved, not copied.
        </li>
<li class="listitem">
     