auto l = make_unique<loginfo>();//instantiate structure for user info
fstream in;
in.open(userDB, ios::in | ios::binary);//open the text file contains info

cout << "id:";
cin >> id;

while (in>>l->id>>l->pwd)//read user info from text file
{
   //to check id's correct or not
   if (l->id == id)//id match
   {
      pwd = setpass("password:”) ;//password masking
      if (l->pwd == pwd)//password match
         cout << "user verified!" << endl;
      else  //password mismatch
      {
         cout << "incorrect password." << endl;
         pwd = setpass("password:");//pwd reinput
      }
      break;
    }
    else
    {
        cout << "incorrect id" << endl;
    }
}
break;

auto l = make_unique<loginfo>();
    fstream in;
    in.open(userDB, ios::in | ios::binary);

    cout << "id:";
    cin >> id;

    while (in >> l->id >> l->pwd)
    {
        if (l->id == id)
            cout << l->id << " " << l->pwd << endl;
    }