C++---if(!cin)与if(!(cin >> xxx))

!cin，判断上一次的输入是否失败（fail）

!(cin >> xxx)判断当前输入是否失败（fail）

#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
//输入2b,打印出2;输入b2,打印出 "cin is fail"
int itest = 0;
if(!(cin >> itest)){
cerr << "cin is fail" << endl;
return EXIT_FAILURE;
}
cout << itest <<endl;

//不管输入什么，均能输出，因为都符合字符串
string str;
if(!(cin >> str)){
cerr << "cin is fail" << endl;
return EXIT_FAILURE;
}
cout << str << endl;

//跟itest一样
int itest1 = 0;
cin >> itest1;
if(!cin){
cerr << "cin is fail" << endl;
return EXIT_FAILURE;
}
cout << itest1 <<endl;

return EXIT_SUCCESS;
}