How are you supposed to use set_rdbuf()? - c++

Question

So I am creating a class that inherits from ifstream, and that gives me acces to the protected function set_rdbuf, and at first I just thought it allowed you to set your own filebuf that then would be used, but that does not seem to work for me. I wrote the following code:
class A : public std::fstream {
private:
std::filebuf m_file_buf;
public:
A() {
if (!m_file_buf.open("test_file.txt", ios_base::out | ios_base::in)) {
std::cout << "something went wrong";
}
// std::fstream::rdbuf()->swap(m_file_buf);
std::fstream::set_rdbuf(&m_file_buf);
std::cout << "m_ file buff is: " << m_file_buf.is_open() << "\n";
std::cout << "rd file buff is: " << std::fstream::rdbuf()->is_open() << "\n";
std::cout << "we are: " << std::fstream::is_open() << "\n";
}
};
int main() {
A a;
return 0;
}
This will log:
m_ file buff is: 1
rd file buff is: 0
we are: 0
I was under the assumption that when I called rdbuf() I would then get the one I had set it to. However, if I use the swap function that rdbuf() provides, then it prints as I expect it to
class A : public std::fstream {
private:
std::filebuf* m_file_buf;
public:
A() {
m_file_buf = new std::filebuf();
if (!m_file_buf->open("test_file.txt", ios_base::out | ios_base::in)) {
std::cout << "something went wrong";
}
std::fstream::rdbuf()->swap(*m_file_buf);
std::cout << "m_ file buff is: " << m_file_buf->is_open() << "\n";
std::cout << "rd file buff is: " << std::fstream::rdbuf()->is_open() << "\n";
std::cout << "we are: " << std::fstream::is_open() << "\n";
// Get it back again as we swapped it out
m_file_buf = std::fstream::rdbuf();
}
};
int main() {
A a;
return 0;
}
m_ file buff is: 0
rd file buff is: 1
we are: 1
//and when I at the end swap back, everything is 1/open
I swited to pointer in the swap code so that I could point to the return of rdbuf()
This was compiled on MSVC version 19.28 with arg /std:c++17
So am I wrong is how set_rdbuf is supposed to be used, or is there something else I am doing wrong?