std::basic_filebuf<CharT,Traits>::showmanyc (3) - Linux Manuals
virtual std::streamsize showmanyc()
If implemented, returns the number of characters left to read from the file.
The number of characters available for reading from the file, or -1 if the end of file was reached.
This function is optional. If not implemented, this function returns 0 (since the base class version std::basic_streambuf::showmanyc gets called)
Whether implemented or not, this function is normally called by std::basic_streambuf::in_avail if the get area is empty.
The name of this function stands for "stream: how many characters?", so it is pronounced "S how many C", rather than "show many C"
implementation test to see if showmanyc() is implemented for filebuf
// Run this code
in_avail (public member function of std::basic_streambuf<CharT,Traits>)
readsome (public member function of std::basic_istream<CharT,Traits>)