user, <code>is_holder&lt;T&gt;::type</code> is
<a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::true_</code></a>
if <code>T</code> is derived from <code>holder_marker</code>, and it is
<a href="../../../mpl/doc/refmanual/bool.html"><code>boost::mpl::false_</code></a>
otherwise.
</p>

<h3><a name="holder_construct">Class template <code>holder</code></a></h3>

<p>
<code>holder&lt;T&gt;</code> is a syntactic construct meant to indicate
that <code>T</code> is a holder specifier without resorting