Handling for when userdata is not defined yet, but might be defined later

src/UserData/RetrievedUserData.hpp

@@ -0,0 +1,22 @@
+#ifndef PORYGONLANG_RETRIEVEDUSERDATA_HPP
+#define PORYGONLANG_RETRIEVEDUSERDATA_HPP
+
+#include "UserData.hpp"
+
+namespace Porygon::UserData{
+    class RetrievedUserData{
+        UserData* _ud;
+        uint32_t _key;
+    public:
+        explicit RetrievedUserData(UserData* ud) : _ud(ud), _key(0){}
+        explicit RetrievedUserData(uint32_t id) : _ud(nullptr), _key(id){}
+
+        UserData * Get();
+
+        inline uint32_t GetKey() const{
+            return _key;
+        }
+    };
+}
+
+#endif //PORYGONLANG_RETRIEVEDUSERDATA_HPP