急。C++高手进！

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题目：

double num; //定义一个double型变量num；

........

cin>>num; //读取num值

........

if（...) {cout<<"fail!"<<endl;}

else cout<<"sucess"<<endl;

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要求：当用户只输入数字时，输出success，

当输入数字+非数字，如123abc时，输出fail

其他情况，同样输出fail.

请完善代码

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string s;

cin>>num;

if(cin.fail()) cout<<"fail"<<endl;

else{

cin>>s;

if(cin.good()) cout<<"success"<<endl;

else cout<<"fail"<<endl;

}

if(IsNumric(num)){cout<<"fail!"<<endl;}

else cout<<"sucess"<<endl;