所以我的理解就是:  if(cin >> x) 就等价于 if(!(cin >> x).fail()) 

这样正确吗? 

但是我测试之后有点小问题:

我的代码如下:

#include <iostream>

using namespace std;

void check_cin_state(istream & is)
{
	if (cin.eof())
		cout << "cin eof()" << endl;
	else
		cout << "cin not eof()" << endl;
	if (cin.fail())
		cout << "cin fail()" << endl;
	else
		cout << "cin not fail()" << endl;
	if (cin.bad())
		cout << "cin bad()" << endl;
	else
		cout << "cin not bad()" << endl;
	if (cin.good())
		cout << "cin good()" << endl;
	else
		cout << "cin not good()" << endl;
	cout << endl;
}

int main()
{
	int x;
	if (cin >> x)
		check_cin_state(cin);
	check_cin_state(cin);	
	
	return 0;
}

输出如下:

cin eof()

cin fail()

cin not bad()

cin not good()

那么问题来了, 为什么当我eofbit被置位的时候, fail()也返回true?