std::basic_streambuf<CharT,Traits>::pbackfail (3) - Linux Man Pages
virtual int_type pbackfail( int_type c = Traits::eof() );
This protected virtual function is called by the public functions sungetc() and sputbackc() (which, in turn, are called by basic_istream::unget and basic_istream::putback) when either:
1) There is no putback position in the get area (pbackfail() is called with no arguments). In this situation, the purpose of pbackfail() is to back up the get area by one character, if the associated character sequence allows this (e.g. a file-backed streambuf may reload the buffer from a file, starting one character earlier).
2) The caller attempts to putback a different character from the one retrieved earlier (pbackfail() is called with the character that needs to be put back). In this situation, the purpose of pbackfail() is to place the character c into the get area at the position just before basic_streambuf::gptr(), and, if possible, to modify the associated character sequence to reflect this change. This may involve backing up the get area as in the first variant.
The default base class version of this function does nothing and returns Traits::eof() in all situations. This function is overridden by the derived classes: basic_stringbuf::pbackfail, basic_filebuf::pbackfail, strstreambuf::pbackfail, and is expected to be overridden by user-defined and third-party library stream classes.
ch - character to put back or Traits::eof() if only back out is requested
Traits::eof() in case of failure, some other value to indicate success. The base class version always fails.
This section is incomplete
Reason: no example
pbackfail backs out the input sequence to unget a character, not affecting the associated file
pbackfail puts a character back into the input sequence
pbackfail backs out the input sequence to unget a character
sungetc (public member function)
sputbackc (public member function)