Ticket #9799: cast.diff

cast.hpp

@@ -49 +49 @@
 # include <boost/type.hpp>
 # include <boost/limits.hpp>
 # include <boost/detail/select_type.hpp>
+# include <boost/mpl/if.hpp>
+# include <boost/type_traits/remove_pointer.hpp>
+# include <boost/type_traits/is_virtual_base_of.hpp>
 
 //  It has been demonstrated numerous times that MSVC 6.0 fails silently at link
 //  time if you use a template function which has template parameters that don't
@@ -82 +85 @@
 
 //  polymorphic_downcast  ----------------------------------------------------//
 
+    namespace detail
+    {
+
+        template<typename Out, typename In>
+        struct polymorphic_static_downcast
+        {
+            static Out eval(In* in)
+            { return static_cast<Out>( in ); }
+        };
+
+        template<typename Out, typename In>
+        struct polymorphic_dynamic_downcast
+        {
+            static Out eval(In* in)
+            { return dynamic_cast<Out>( in ); }
+        };
+
+    }
+
     //  BOOST_ASSERT() checked polymorphic downcast.  Crosscasts prohibited.
 
     //  WARNING: Because this cast uses BOOST_ASSERT(), it violates
@@ -95 +117 @@
     inline Target polymorphic_downcast(Source* x BOOST_EXPLICIT_DEFAULT_TARGET)
     {
         BOOST_ASSERT( dynamic_cast<Target>(x) == x );  // detect logic error
-        return static_cast<Target>(x);
+
+        typedef typename boost::remove_pointer<Out>::type out_class;
+        typedef boost::is_virtual_base_of<in_class, out_class> dyn;
+        typedef typename boost::mpl::if_<dyn
+        , polymorphic_dynamic_downcast<Out, In>
+        , polymorphic_static_downcast<Out, In>
+        >::type impl;
+
+        return impl::eval(x);
     }
 
 #  undef BOOST_EXPLICIT_DEFAULT_TARGET