这当然会设置 eofbit 标志(和failbit)，但是，您将能够获得所需的字符.

This, of course, will set the eofbit flag (and the failbit too), however, you will be able to obtain the desired characters.

现在，假设您想再次读取文件(从头开始):

Now, suppose you want to read the file again (from the beginning):

in.seekg(0);        // Sets input position indicator. 
in.read(buf, 100);  // Try to read again.

这不起作用:因为如果你打电话:

This does not work: because if you call:

int count = in.gcount()  // Charecters readed from input.

你会注意到count == 0.这意味着它根本没有读取任何内容.

you will notice that count == 0. Meaning it has not read anything at all.

因此问题是:到达文件末尾后如何倒带文件?

Hence the question: How can you rewind the file after you get to the end of the file?

推荐答案

解决方案

使用 clear 在调用之前清除 ifstream 的状态 <代码>搜索.如果您以后不需要知道状态，请务必先检查.

Solution

Use clear for cleaning the state of the ifstream before call seekg. Be sure to check first if you don't will need to know the state later.

in.clear();
in.seekg(0);

说明

seekg 设置光标位置，但不会clear 状态位 failbit 所以，ifstream 实例thinks"还是有问题.

Explanation

seekg sets the cursor position, but doesn't clear state bit failbit so, the ifstream instance "thinks" there is something wrong yet.

来自标准规范:

std::basic_istream::seekg 表现为 UnformattedInputFunction，除了 gcount() 不受影响.

std::basic_istream::seekg behaves as UnformattedInputFunction, except that gcount() is not affected.

我们可以阅读UnformattedInputFunction:

以下标准库函数是UnformattedInputFunctions:

basic_istream::seekg，除了它首先清除 eofbit 并且不修改 gcount

basic_istream::seekg, except that it first clears eofbit and does not modify gcount

在问题示例中，如果您在 seekg 之前和之后打印状态:

In the question example if you print the state before and after the seekg you get:

cout << "State before seekg: " << in.rdstate() << endl;   // Prints 3 (11 in binary) failbit and eofbit activated.
in.seekg(0);
cout << "State after seekg: " << in.rdstate() << endl;    // Prints 2 (10 in binary) just failbit activated.

这就是为什么！！

seekg 不清除 failbit 并且由于某些实施原因，它不适用于激活这样的位.

seekg doesn't clear failbit and for some implementation reason, it doesn't works with such bit activated.