std::ifstream 在读取文件中最后一项时设置 eofbit，但仅在读取数值类型时发生

std::ifstream sets the eofbit at the point when the last item in the file is read, but it only happens when reading numerical types

本文关键字：读取 eofbit 设置 类型 取数值 文件 ifstream 最后 std 一项      更新时间：2023-10-16

>假设给定一个包含数字列表的非空文本文件，

1 2 3 4 5

我们用std::ifstream将它们读成数字类型(int、double等(以及char

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                        
}
}

当读取为数值类型时，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

在读取文件最后一项时，设置了 eofbit。

但是在读入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                        
}
}

其中输出：

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

为什么会这样呢？

读取ints 时会EOF，因为int的流提取运算符会尝试读取，直到找到空格或不适合int的内容。因此，它将在读取5后尝试读取另一个char，遇到文件的末尾并设置eofbit。

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

相关文章：