c++ - std::ifstream 在读取文件中的最后一项时设置 eofbit，但它仅在读取数字类型时发生

Question

假设给定一个包含数字列表的非空文本文件，

1 2 3 4 5

然后我们用 std::ifstream 将它们读入数值类型(int、double 等)以及 字符

int main()
{
    ifstream fs {"textfile.txt"};
    int val {0}; //read into ints
    for (int i{0}; i < 5; ++i)
    {
        //read five times
        fs >> val;
        cout << val << " is read. Current state of file stream: "
            << bitset<3> (fs.rdstate()) << endl; //check stream state after each read                        
    }
}

当读取为数字类型时，程序输出:

1 is read. Current state of fs: 000
2 is read. Current state of fs: 000
3 is read. Current state of fs: 000
4 is read. Current state of fs: 000
5 is read. Current state of fs: 010

在读取文件的最后一项时，设置了 eofbit。

但是读入 char 时不会发生同样的事情。

int main()
{
    ifstream fs {"textfile.txt"};
    char ch {0}; //read into chars
    for (int i{0}; i < 5; ++i)
    {
        //read five times
        fs >> ch;
        cout << ch << " is read. Current state of file stream: "
            << bitset<3> (fs.rdstate()) << endl; //check stream state after each read                        
    }
}

输出:

1 is read. Current state of fs: 000
2 is read. Current state of fs: 000
3 is read. Current state of fs: 000
4 is read. Current state of fs: 000
5 is read. Current state of fs: 000

为什么会这样？

Answer 1

读取 int 时得到 EOF，因为 int 的流提取运算符会尝试读取，直到找到空格或不合适的内容对于 int。因此它将在读取 5 后尝试读取另一个 char，遇到文件末尾并且设置了 eofbit。

相比之下，char 的流提取运算符只会尝试读取一个 char，因此它不会遇到文件末尾。尝试从文件中读取 6 个 char，您也会遇到 EOF。