-/* Initialize the field iterator structure to beginning. */
-void pb_field_iter_begin(pb_field_iter_t *iter, const pb_field_t *fields, void *dest_struct);
+/* Initialize the field iterator structure to beginning.
+ * Returns false if the message type is empty. */
+bool pb_field_iter_begin(pb_field_iter_t *iter, const pb_field_t *fields, void *dest_struct);