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ios类定义了这四个常量badbit, eofbit, failbit, goodbit
这四个标志常量就是取对应标志位的掩码，也即输入的四种异常情况！
以上四个常量对应的取值为：
ios::badbit 001 输入（输出）流出现致命错误，不可挽回
ios::eofbit 010 已经到达文件尾
ios::failbit 100 输入（输出）流出现非致命错误，可挽回
ios::goodbit 000 流状态完全正常, 各异常标志位都为0
就一般说来，badbit是一些系统底层或者硬件出错，比如文件系统错误，磁盘错误，网络错误等等。failbit就是其他软件错误，如试图从不能解析为整数的字符串里想要读一个整数等，顺便说下eof也会造成failbit被置位。

//以上转载于百度知道
首先建立 fish.text文件， 存入数据：18 19 18.5 13.5 14 16 19.5 20 18 12 18.5 17.5

	const int SIZE = 20;
	char filename[SIZE];
	ifstream inFile;
	cout << "enter the filename: ";
	cin.getline(filename, SIZE);
	inFile.open(filename);//输入fish.text

	if (!inFile.is_open())
	{
		cout << "can not open the file! " << endl;
		exit(EXIT_FAILURE);
	}

	double value;
	double sum = 0.0;
	int count = 0;

	inFile >> value;
	while (inFile.good())//while input good and not at EOF
	{
		++count;
		sum += value;
		cout << value << endl;
		inFile >> value;
	}
	if (inFile.eof())
		cout << "End of file reached.\n";
	else if (inFile.fail())
		cout << "input terminated by data mismatch.\n";
	else
		cout << "input terminated for unknown reason.\n";

	if (count == 0)
		cout << "No data processed.\n";
	else
	{
		cout << "items read:" << count << endl;
		cout << "sum: " << sum << endl;
		cout << "Average: " << sum / count << endl;
	}

	inFile.close();

本文地址：https://blog.csdn.net/qq_44886707/article/details/107204801