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.
implementation test to see if showmanyc() is implemented for filebuf
showmanyc() returns 6626