--- a/src/ov-builtin.h
+++ b/src/ov-builtin.h
@@ -52,17 +52,21 @@
 
   octave_function *function_value (bool) { return this; }
 
+  bool is_builtin_function (void) const { return true; }
+
   octave_value_list do_index_op (int nargout, const octave_value_list& args);
 
+protected:
+
+  // A pointer to the actual function.
+  fcn f;
+
 private:
 
   octave_builtin (void);
 
   octave_builtin (const octave_builtin& m);
 
-  // A pointer to the actual function.
-  fcn f;
-
   DECLARE_OV_TYPEID_FUNCTIONS_AND_DATA
 
   DECLARE_OCTAVE_ALLOCATOR