在什么实际情况下bool（std :: ifstream）！= std :: ifstream :: good（）？ [英] In what practical case bool(std::ifstream) != std::ifstream::good()?

问题描述

我想知道在什么情况下我们可以：

I would like to know in what case we can have :

bool(std::ifstream) != std::ifstream::good()

不同的是 bool ：ifstream）不测试 eof 位，而 std :: ifstream :: good（）测试它。但实际上，如果尝试在文件结尾之后读取某些内容，则会引发 eof 位。但是一旦你尝试这样做，我认为失败或坏位也被设置。

The difference is that bool(std::ifstream) does not test the eof bit whereas std::ifstream::good() tests it. But practically, the eof bit is raised if one try to read something after the end of the file. But as soon as you try to do this I think that either fail or bad bit is also set.

因此，在什么情况下，您只能提高 eof 位？

Consequently in what case you can only raise the eof bit ?

推荐答案

简单地说，只要遇到文件的结尾而不试图阅读。考虑一个只包含一个1字符的文件one.txt。

Simply put, whenever you encounter the end of a file without attempting to read behind it. Consider a file "one.txt" which contains exactly one single '1' character.

未格式化输入的示例：

#include <iostream>
#include <fstream>

int main()
{
    using namespace std;
    char chars[255] = {0};
    ifstream f("one.txt");
    f.getline(chars, 250, 'x');
    cout << f.good() << " != " << bool(f) << endl;
    return 0;
}

0！= 1

按任意键继续。 。

0 != 1
Press any key to continue . . .

格式化输入示例：

#include <iostream>
#include <fstream>

int main()
{
    using namespace std;
    ifstream f("one.txt");
    int i; f >> i;
    cout << f.good() << " != " << bool(f) << endl;
    return 0;
}

0！= 1

按任意键继续。 。 。

0 != 1
Press any key to continue . . .

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