std::map<ID,T>& data() { return _m; }
const std::map<ID,T>& data() const { return _m; }
std::size_t getHeapMemorySizeWithoutChildren() const;
std::map<ID,T>& data() { return _m; }
const std::map<ID,T>& data() const { return _m; }
std::size_t getHeapMemorySizeWithoutChildren() const;