diff --git a/HISTORY b/HISTORY
index 16805a6..8cdd66c 100755
--- a/HISTORY
+++ b/HISTORY
@@ -1,5 +1,9 @@
 Dec. 24 2015
+Improved defining a class in C
+Added a pair of mb_get_class_userdata/mb_set_class_userdata functions
+Added a pair of mb_get_userdata/mb_set_userdata functions
 Refactored memory layout of the _routine_t struct
+Polished document
 
 Dec. 21 2015
 Fixed a class GC bug
diff --git a/MY-BASIC Quick Reference.pdf b/MY-BASIC Quick Reference.pdf
index 9a14e75..13c988e 100644
Binary files a/MY-BASIC Quick Reference.pdf and b/MY-BASIC Quick Reference.pdf differ
diff --git a/core/my_basic.c b/core/my_basic.c
index 973d110..7f4ba41 100755
--- a/core/my_basic.c
+++ b/core/my_basic.c
@@ -385,6 +385,7 @@ typedef struct _class_t {
 	char* name;
 	_ls_node_t* meta_list;
 	struct _running_context_t* scope;
+	void* userdata;
 } _class_t;
 #endif /* MB_ENABLE_CLASS */
 
@@ -8283,6 +8284,8 @@ int mb_end_class(struct mb_interpreter_t* s, void** l) {
 
 	_pop_scope(s);
 
+	s->last_instance = 0;
+
 	return result;
 #else /* MB_ENABLE_CLASS */
 	mb_unrefvar(s);
@@ -8292,6 +8295,60 @@ int mb_end_class(struct mb_interpreter_t* s, void** l) {
 #endif /* MB_ENABLE_CLASS */
 }
 
+int mb_get_class_userdata(struct mb_interpreter_t* s, void** l, void** d) {
+	/* Get the userdata of a class instance */
+#ifdef MB_ENABLE_CLASS
+	int result = MB_FUNC_OK;
+
+	mb_assert(s && l && d);
+
+	if(s && s->last_instance) {
+		if(d)
+			*d = s->last_instance->userdata;
+	} else if(s && s->last_routine && s->last_routine->instance) {
+		if(d)
+			*d = s->last_routine->instance->userdata;
+	} else {
+		if(d) *d = 0;
+
+		_handle_error_on_obj(s, SE_RN_CLASS_EXPECTED, 0, TON(l), MB_FUNC_ERR, _exit, result);
+	}
+
+_exit:
+	return result;
+#else /* MB_ENABLE_CLASS */
+	mb_unrefvar(s);
+	mb_unrefvar(l);
+	mb_unrefvar(d);
+
+	return MB_FUNC_ERR;
+#endif /* MB_ENABLE_CLASS */
+}
+
+int mb_set_class_userdata(struct mb_interpreter_t* s, void** l, void* d) {
+	/* Set the userdata of a class instance */
+#ifdef MB_ENABLE_CLASS
+	int result = MB_FUNC_OK;
+
+	mb_assert(s && l && d);
+
+	if(s && s->last_instance) {
+		s->last_instance->userdata = d;
+	} else {
+		_handle_error_on_obj(s, SE_RN_CLASS_EXPECTED, 0, TON(l), MB_FUNC_ERR, _exit, result);
+	}
+
+_exit:
+	return result;
+#else /* MB_ENABLE_CLASS */
+	mb_unrefvar(s);
+	mb_unrefvar(l);
+	mb_unrefvar(d);
+
+	return MB_FUNC_ERR;
+#endif /* MB_ENABLE_CLASS */
+}
+
 int mb_get_value_by_name(struct mb_interpreter_t* s, void** l, const char* n, mb_value_t* val) {
 	/* Get a value by its identifier name */
 	int result = MB_FUNC_OK;
@@ -9280,6 +9337,30 @@ int mb_set_inputer(struct mb_interpreter_t* s, mb_input_func_t p) {
 	return result;
 }
 
+int mb_get_userdata(struct mb_interpreter_t* s, void** d) {
+	/* Get the userdata of an interpreter instance */
+	int result = MB_FUNC_OK;
+
+	mb_assert(s && d);
+
+	if(s && d)
+		*d = s->userdata;
+
+	return result;
+}
+
+int mb_set_userdata(struct mb_interpreter_t* s, void* d) {
+	/* Set the userdata of an interpreter instance */
+	int result = MB_FUNC_OK;
+
+	mb_assert(s);
+
+	if(s)
+		s->userdata = d;
+
+	return result;
+}
+
 int mb_set_import_handler(struct mb_interpreter_t* s, mb_import_handler_t h) {
 	/* Set an import handler to an interpreter instance */
 	int result = MB_FUNC_OK;
diff --git a/core/my_basic.h b/core/my_basic.h
index 03bf600..1079e04 100755
--- a/core/my_basic.h
+++ b/core/my_basic.h
@@ -435,6 +435,8 @@ MBAPI int mb_push_value(struct mb_interpreter_t* s, void** l, mb_value_t val);
 
 MBAPI int mb_begin_class(struct mb_interpreter_t* s, void** l, const char* n, mb_value_t** meta, int c, mb_value_t* out);
 MBAPI int mb_end_class(struct mb_interpreter_t* s, void** l);
+MBAPI int mb_get_class_userdata(struct mb_interpreter_t* s, void** l, void** d);
+MBAPI int mb_set_class_userdata(struct mb_interpreter_t* s, void** l, void* d);
 
 MBAPI int mb_get_value_by_name(struct mb_interpreter_t* s, void** l, const char* n, mb_value_t* val);
 MBAPI int mb_add_var(struct mb_interpreter_t* s, void** l, const char* n, mb_value_t val, bool_t force);
@@ -471,16 +473,19 @@ MBAPI int mb_debug_set(struct mb_interpreter_t* s, const char* n, mb_value_t val
 MBAPI int mb_debug_set_stepped_handler(struct mb_interpreter_t* s, mb_debug_stepped_handler_t h);
 
 MBAPI const char* mb_get_type_string(mb_data_e t);
+
 MBAPI int mb_raise_error(struct mb_interpreter_t* s, void** l, mb_error_e err, int ret);
 MBAPI mb_error_e mb_get_last_error(struct mb_interpreter_t* s);
 MBAPI const char* mb_get_error_desc(mb_error_e err);
 MBAPI int mb_set_error_handler(struct mb_interpreter_t* s, mb_error_handler_t h);
+
 MBAPI int mb_set_printer(struct mb_interpreter_t* s, mb_print_func_t p);
 MBAPI int mb_set_inputer(struct mb_interpreter_t* s, mb_input_func_t p);
+
+MBAPI int mb_get_userdata(struct mb_interpreter_t* s, void** d);
+MBAPI int mb_set_userdata(struct mb_interpreter_t* s, void* d);
 MBAPI int mb_set_import_handler(struct mb_interpreter_t* s, mb_import_handler_t h);
-
 MBAPI int mb_gets(char* buf, int s);
-
 MBAPI char* mb_memdup(const char* val, unsigned size);
 MBAPI int mb_set_memory_manager(mb_memory_allocate_func_t a, mb_memory_free_func_t f);