thanks for Joaquín M López Muñoz 's reply. i have tried your code, it works as i need exactly. for(auto it=data.get<1>().begin(),it_end=data.get<1>().end(); it!=it_end; it=data.get<1>().upper_bound(it->m2)){ std::cout<<it->m2<<"\n"; // or whatever you need to do with the key } i'll use this method in reality, thanks! Bill