c ++ mysql ++如果查询是空的，代码永远不会执行，但它应该

Question

如果查询为空，则代码永远不会执行。我尝试了多种变体。这是简单的代码。

   mysqlpp::Query query = conn.query();
   query << "SELECT * FROM users WHERE username= " 
         << mysqlpp::quote_only << username 
         << "AND password= " 
         << mysqlpp::quote_only << password;

         mysqlpp::StoreQueryResult res = query.store();
         mysqlpp::StoreQueryResult::const_iterator it;
         for (it = res.begin(); it != res.end(); ++it)
         {
              mysqlpp::Row row = *it;
              if (!row.empty())
              {
              // user name and password match, log them in
              std::cout << "You are logged" << std::endl;
              // rest of code goes here   
              }

              else if (row.empty())  // even just 'else' doesnt get executed
              {
              // no username or password that matches with user input
              std::cout << "Wrong username or password" <<  std::endl; 
              // rest of code goes here 
              // this never get executed, and i have no idea why
              }   
         }

如何检查行是否为空？我几乎尝试了他在参考手册中找到的任何东西，但仍然没有。