CC_SYNTHESIZE with std::unordered_map

Hello, I have one question:

Why does “CC_SYNTHESIZE” not work with “std::unordered_map”?
with “std::list” works …

CC_SYNTHESIZE(std::unordered_map<Vec2, float>,      _fila_para_apagar1,      _Fila_Para_Apagar1);
ERROR

CC_SYNTHESIZE(std::list<Vec2>,                      _fila_para_apagar2,      _Fila_Para_Apagar2);
OK