tti::valid_member_metafunction, makes the process
      of passing our nested 'type' and our marker type a bit easier. Here the single
      template parameter is the invoked metafunction generated by BOOST_TTI_MEMBER_TYPE
      itself. It then picks out from the invoked metafunction both the return 'type'
      and the nested boost::tti::marker_type to do the correct calculation.
    </p>
<p>
      A simple example of this functionality would be:
    </p>
<pre class="programlisting"><span class="preprocess