span class="special">;</span>
  <span class="keyword">mutable</span> <span class="identifier">string_type</span> <span class="identifier">destination_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
        This container is the base class template used to represent HTTP messages.
        It uses a "tag" type style specializations for a variety of trait
        classes, allowing for customization of the various parts of the message.
        For example, a user specializes <co