std::basic_filebuf<CharT,Traits>::overflow (3) - Linux Man Pages
virtual int_type overflow ( int_type c = Traits::eof() );
Writes some data from the put area to the associated character sequence (to the file).
Behaves like the base class std::basic_streambuf::overflow, except that to write the data, first uses std::codecvt::out of the imbued locale to convert the characters into external (possibly multibyte) representation, stored in a temporary buffer (allocated as large as necessary), then uses file I/O to copy all fully-converted bytes into the file.
If std::codecvt::always_noconv is true, the call to std::codecvt::out may be skipped.
If the conversion fails with std::codecvt_base::error, returns Traits::eof() without attempting any output.
If the associated file is not open (is_open() == false), returns Traits::eof() before doing anything.
Traits::not_eof(c) to indicate success or Traits::eof() to indicate failure.
This section is incomplete
Reason: no example
overflow writes characters to the associated output sequence from the put area
underflow reads from the associated file