c++ - std::stringstream 是否有两个缓冲区，因此有两个标记？

Question

关于 C++ 入门第 17 章； I/O 流；据说类型为 std::iostream 的对象或派生类型，如 std::fstream , std::stringstream支持阅读和写作。这些类型也只有一个标记和一个缓冲区:

The fstream and stringstream types can read and write the same stream. In these types there is a single buffer that holds data to be read and written and a single marker denoting the current position in the buffer. The library maps both the g and p positions to this single marker.

所以为了了解更多我已经完成了这个程序:

int main(){


    std::fstream fio("data.txt");
    std::cout << fio.tellg() << '\t' << fio.tellp() << '\n';
    fio.seekg(0, std::ios::end);
    std::cout << fio.tellg() << '\t' << fio.tellp() << '\n';

    std::stringstream ss("abcd\nefg\nhi\nj");
    std::cout << ss.tellg() << '\t' << ss.tellp() << '\n';
    ss.seekg(0, std::ios::end);
    std::cout << ss.tellg() << '\t' << ss.tellp() << '\n';

}

输出:

 0      0
 14     14
 0      0
 13     0 

正如您在使用 std::fstream 时所见如果我使用 seekg() 移动读取标记然后是 tellg和 tellp返回相同的值。这是非常合乎逻辑的。

但是，当移动 std::stringstream 的读取标记时, tellg和 tellp返回不同的值所以写标记没有被移动？

这是否意味着 std::stringstream有两个缓冲区和两个标记？谢谢

Answer 1

是的，书错了。
是std::basic_filebuf它同步读取和写入位置，并且，

the class template basic_fstream implements high-level input/output operations on file based streams. It interfaces a file-based streambuffer (std::basic_filebuf) with the high-level interface of (std::basic_iostream).

然而，基类， std::basic_streambuf , 确实保持独立的输入和输出缓冲区。注意

the I/O stream objects std::basic_istream and std::basic_ostream, as well as all objects derived from them (std::ofstream, std::stringstream, etc), are implemented entirely in terms of std::basic_streambuf.