c – seekp和seekg不适用于fstream

2023年7月12日 286次阅读

我在Debian x64 PC上运行的程序中有一个奇怪的行为.

我无法首先读取文件,然后写入另一个值然后读取这些值.
我已经阅读了很多信息,包括有关stackoverflow的问题,发现(也通过实验)我需要更改seekp和seekg,我这样做.
一切正常……直到我从流中读到一些东西.在读取操作之后,如果我寻找文件的开头,然后调用tellg(),tellp(),它们都返回’-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;
}

有人能告诉我发生了什么,我该怎么做才能解决问题？

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

保持跟踪并独立获取头寸是不可能的.

类模板std :: basic_filebuf包含单个文件位置

§ 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.
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