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