=add polymorphic_pointer_downcast function template

include/boost/polymorphic_cast.hpp

@@ -1,6 +1,7 @@
 //  boost polymorphic_cast.hpp header file  ----------------------------------------------//
 
 //  (C) Copyright Kevlin Henney and Dave Abrahams 1999.
+//  (C) Copyright Boris Rasin 2014.
 //  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)
@@ -8,6 +9,7 @@
 //  See http://www.boost.org/libs/conversion for Documentation.
 
 //  Revision History
+//  08 Nov 14  Add polymorphic_pointer_downcast (Boris Rasin)
 //  09 Jun 14  "cast.hpp" was renamed to "polymorphic_cast.hpp" and
 //             inclusion of numeric_cast was removed (Antony Polukhin)
 //  23 Jun 05  numeric_cast removed and redirected to the new verion (Fernando Cacciola)
@@ -47,6 +49,9 @@
 
 # include <boost/config.hpp>
 # include <boost/assert.hpp>
+# include <boost/pointer_cast.hpp>
+# include <boost/utility/declval.hpp>
+# include <boost/typeof/typeof.hpp>
 # include <typeinfo>
 
 namespace boost
@@ -86,6 +91,41 @@ namespace boost
         return static_cast<Target>(x);
     }
 
+//  polymorphic_pointer_downcast  --------------------------------------------//
+
+    //  BOOST_ASSERT() checked polymorphic downcast.  Crosscasts prohibited.
+    //  Supports any type with static_pointer_cast/dynamic_pointer_cast functions:
+    //  built-in pointers, std::shared_ptr, boost::shared_ptr, boost::intrusive_ptr, etc.
+
+    //  WARNING: Because this cast uses BOOST_ASSERT(), it violates
+    //  the One Definition Rule if used in multiple translation units
+    //  where BOOST_DISABLE_ASSERTS, BOOST_ENABLE_ASSERT_HANDLER
+    //  NDEBUG are defined inconsistently.
+
+    //  Contributed by Boris Rasin
+
+    namespace detail
+    {
+        template <typename Target, typename Source>
+        struct static_pointer_cast_result
+        {
+#ifdef BOOST_NO_CXX11_DECLTYPE
+            BOOST_TYPEOF_NESTED_TYPEDEF_TPL(nested, static_pointer_cast<Target>(boost::declval<Source>()))
+            typedef typename nested::type type;
+#else
+            typedef decltype(static_pointer_cast<Target>(boost::declval<Source>())) type;
+#endif
+        };
+    }
+
+    template <typename Target, typename Source>
+    inline typename detail::static_pointer_cast_result<Target, Source>::type
+    polymorphic_pointer_downcast (const Source& x)
+    {
+        BOOST_ASSERT(dynamic_pointer_cast<Target> (x) == x);
+        return static_pointer_cast<Target> (x);
+    }
+
 } // namespace boost
 
 #endif  // BOOST_POLYMORPHIC_CAST_HPP