c++ - seekp 和 seekg 不适用于 fstream

Question

我在 Debian x64 PC 上运行的 c++ 程序有一个奇怪的行为。

我无法设法先读取文件，然后写入另一个值，然后再读取这些值。我已经阅读了很多信息，包括关于 stackoverflow 的问题，发现(也是通过实验)我需要更改 seekp 和 seekg 并且我这样做了。一切正常……直到我从流中读到一些东西。在读取操作之后，如果我在文件的开头查找然后调用 tellg()、tellp()，它们都会返回“-1”。

测试代码:

void testFstreamSeekp() {
    fstream in("file", ios::in | ios::out);

    cout << "g: " << in.tellg() << endl;
    cout << "p: " << in.tellp() << endl;

    in.seekp(0, ios_base::end);

    cout << "endp g: " << in.tellg() << endl;
    cout << "endp p: " << in.tellp() << endl;

    in.seekp(0, ios_base::end);
    in.seekg(0, ios_base::end);

    cout << "end g: " << in.tellg() << endl;
    cout << "end p: " << in.tellp() << endl;

    in.seekp(0, ios_base::beg);
    in.seekg(0, ios_base::beg);

    cout << "beg g: " << in.tellg() << endl;
    cout << "beg p: " << in.tellp() << endl;

        // Everything is fine until here (that is tellp() == 0, tellg() == 0)
    int a, b;
    in >> a >> b;
    cout << "a: " << a << endl << "b: " << b << endl;

        // tellg() == -1, tellp() == -1 ?????????!!!!!!!!!!
    cout << "read g: " << in.tellg() << endl;
    cout << "read p: " << in.tellp() << endl;

    in.seekp(0, ios_base::beg);
    in.seekg(0, ios_base::beg);

        // tellg() == -1, tellp() == -1 ?????????!!!!!!!!!!
    cout << "beg g: " << in.tellg() << endl;
    cout << "beg p: " << in.tellp() << endl;
}

谁能告诉我发生了什么，我该怎么做才能解决这个问题？

Answer 1

对于 fstream (std::basic_filebuf)，单个文件位置由 seekp() 和 seekg( )

不可能独立地跟踪put 和get 位置。

类模板std::basic_filebuf 保存单个文件位置

§ 27.9.1.1

1. The class basic_filebuf associates both the input sequence and the output sequence with a file.

2. The restrictions on reading and writing a sequence controlled by an object of class basic_filebuf are the same as for reading and writing with the Standard C library FILEs.

3. In particular:

   • If the file is not open for reading the input sequence cannot be read.
   • If the file is not open for writing the output sequence cannot be written.
   • A joint file position is maintained for both the input sequence and the output sequence.