add 'is_placeholder'

[SVN r16451]

include/boost/mpl/is_placeholder.hpp

@@ -0,0 +1,64 @@
+//-----------------------------------------------------------------------------
+// boost mpl/is_placeholder.hpp header file
+// See http://www.boost.org for updates, documentation, and revision history.
+//-----------------------------------------------------------------------------
+//
+// Copyright (c) 2001-02
+// Aleksey Gurtovoy
+//
+// Permission to use, copy, modify, distribute and sell this software
+// and its documentation for any purpose is hereby granted without fee, 
+// provided that the above copyright notice appears in all copies and 
+// that both the copyright notice and this permission notice appear in 
+// supporting documentation. No representations are made about the 
+// suitability of this software for any purpose. It is provided "as is" 
+// without express or implied warranty.
+
+#ifndef BOOST_MPL_IS_PLACEHOLDER_HPP_INCLUDED
+#define BOOST_MPL_IS_PLACEHOLDER_HPP_INCLUDED
+
+#include "boost/mpl/arg_fwd.hpp"
+#include "boost/mpl/bool_c.hpp"
+#include "boost/mpl/aux_/yes_no.hpp"
+#include "boost/mpl/aux_/config/ctps.hpp"
+
+namespace boost { namespace mpl {
+
+#if !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
+
+template< typename T >
+struct is_placeholder
+    : bool_c<false>
+{
+};
+
+template< int N >
+struct is_placeholder< arg<N> >
+    : bool_c<true>
+{
+};
+
+#else
+
+namespace aux {
+
+aux::no_tag is_placeholder_helper(...);
+
+template< int N >
+aux::yes_tag is_placeholder_helper(arg<N>*);
+
+} // namespace aux
+
+template< typename T >
+struct is_placeholder
+{
+    BOOST_STATIC_CONSTANT(bool, value = 
+        sizeof(aux::is_placeholder_helper(static_cast<T*>(0))) == sizeof(aux::yes_tag)
+    );
+};
+
+#endif // BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
+
+}} // namespace boost::mpl
+
+#endif // BOOST_MPL_IS_PLACEHOLDER_HPP_INCLUDED