extern std::ostream& log;
extern std::ostream& null_log;
-extern class log log_error;
-extern class log log_warning;
-extern class log log_info;
-extern class log log_debug;
+extern class log& log_error;
+extern class log& log_warning;
+extern class log& log_info;
+extern class log& log_debug;
template <class T>