Added BOOST_CONCEPT_USAGE for defining usage requirements: Using this

macro is better than simply defining a destructor because usage
patterns will be tested whenever associated types are accessed.


[SVN r35198]

include/boost/concept/usage.hpp

@@ -0,0 +1,32 @@
+// Copyright David Abrahams 2006. Distributed under the Boost
+// Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+#ifndef BOOST_CONCEPT_USAGE_DWA2006919_HPP
+# define BOOST_CONCEPT_USAGE_DWA2006919_HPP
+
+# include <boost/concept/assert.hpp>
+# include <boost/detail/workaround.hpp>
+
+namespace boost { namespace concept { 
+
+# if BOOST_WORKAROUND(__GNUC__, == 2)
+
+#  define BOOST_CONCEPT_USAGE(model) ~model()
+
+# else 
+
+template <class Model>
+struct usage_requirements
+{
+    ~usage_requirements() { ((Model*)0)->~Model(); }
+};
+
+#  define BOOST_CONCEPT_USAGE(model)                                    \
+    BOOST_CONCEPT_ASSERT((boost::concept::usage_requirements<model>));  \
+    ~model()
+
+# endif 
+
+}} // namespace boost::concept
+
+#endif // BOOST_CONCEPT_USAGE_DWA2006919_HPP