C++参考文献6-yiteyi-C++库

以下C++程序的输出？

null

                     #include<iostream>                   
                     using                               namespace                               std;                   
                             
                     int                               main()                   
                     {                   
                                         int                               x = 10;                   
                                         int                               & ref = x;                   
                                         ref = 20;                   
                                         cout <<                               "x = "                               << x << endl ;                   
                                         x = 30;                   
                                         cout <<                               "ref = "                               << ref << endl;                   
                                         return                               0;                   
                     }                   