>&gt;</span> <span class="keyword">class</span> ActorT<span class="special">,</span> 
         <span class="keyword">typename</span> DefaultT<span class="special">&gt;</span> 
  <a class="link" href="result_of/extract_or_default.html" title="Struct template extract_or_default">result_of::extract_or_default</a><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">DescriptorT</span><span class="special">::</span><span class="identifier">value_type</span><span class="