luaa: add luaA_toudata()

Convert a object to a udata if possible.

Signed-off-by: Julien Danjou <julien@danjou.info>

luaa.h

@@ -108,6 +108,30 @@ luaA_checkudata(lua_State *L, int ud, const char *tname)
     return NULL;
 }
 
+/** Convert a object to a udata if possible.
+ * \param L The Lua VM state.
+ * \param ud The index.
+ * \param tname The type name.
+ * \return A pointer to the object, NULL otherwise.
+ */
+static inline void *
+luaA_toudata(lua_State *L, int ud, const char *tname)
+{
+    void **p = lua_touserdata(L, ud);
+    if(p && *p) /* value is a userdata? */
+        if(lua_getmetatable(L, ud)) /* does it have a metatable? */
+        {
+            lua_getfield(L, LUA_REGISTRYINDEX, tname); /* get correct metatable */
+            if(lua_rawequal(L, -1, -2)) /* does it have the correct mt? */
+            {
+                lua_pop(L, 2); /* remove both metatables */
+                return p;
+            }
+            lua_pop(L, 2); /* remove both metatables */
+        }
+    return NULL;
+}
+
 static inline bool
 luaA_checkboolean(lua_State *L, int n)
 {