to_string fixes

[SVN r44164]

include/boost/exception/to_string.hpp

@@ -6,22 +6,62 @@
 #ifndef UUID_7E48761AD92811DC9011477D56D89593
 #define UUID_7E48761AD92811DC9011477D56D89593
 
+#include <boost/utility/enable_if.hpp>
+#include <boost/exception/detail/is_output_streamable.hpp>
 #include <sstream>
-#include <string>
 
 namespace
 boost
     {
-    namespace
-        {
+	namespace
+	exception_detail
+		{
+		template <bool>
+		struct
+		has_to_string_dispatch
+			{
+			enum e { value=1 };
+			};
+
+		template <>
+		struct
+		has_to_string_dispatch<false>
+			{
+			enum e { value=0 };
+			};
+
+		template <class T>
+		std::string
+		to_string( T const & x, typename enable_if< is_output_streamable<T> >::type * = 0 )
+			{
+			std::ostringstream out;
+			out << x;
+			return out.str();
+			}
+
         template <class T>
-        std::string
-        to_string( T const & x )
-            {
-            std::ostringstream out;
-            out << x;
-            return out.str();
-            }
+        char to_string( T const &, typename disable_if< is_output_streamable<T> >::type * = 0 );
+
+		template <class T>
+        struct
+		has_to_string_impl
+			{
+			enum e { value=has_to_string_dispatch<1!=sizeof(to_string(*(T*)0))>::value };
+			};
+		}
+
+	template <class T>
+    struct
+	has_to_string
+		{
+		enum e { value=exception_detail::has_to_string_impl<T>::value };
+		};
+
+	template <class T>
+    std::string
+	to_string( T const & x, typename enable_if< is_output_streamable<T> >::type * = 0 )
+        {
+		return exception_detail::to_string(x);
         }
     }