basic_filebuf<charT, traits>* rdbuf() const;
bool is_open() const;
void open(const char* s, ios_base::openmode mode = ios_base::in);
void open(const filesystem::path::value_type* s,
ios_base::openmode mode = ios_base::in); // wide systems only; see [fstream.syn]
void open(const string& s, ios_base::openmode mode = ios_base::in);
void open(const filesystem::path& s, ios_base::openmode mode = ios_base::in);
void close();