例13.5 通過測試cin的真值，判斷流對象是否處于正常狀態。
#include <iostream>
using namespace std;
int main( )
{float grade;
 cout<<″enter grade:″;
 while(cin>>grade)//能從cin流讀取數據
{if(grade>=85) cout<<grade<<″GOOD!″<<endl;
if(grade<60) cout<<grade<<″fail!″<<endl;
cout<<″enter grade:″;
}
 cout<<″The end.″<<endl;
 return 0;
}