std::basic_filebuf<CharT,Traits>:: pbackfail

From cppreference.com
protected :
virtual int_type pbackfail ( int_type c = Traits :: eof ( ) )

This protected virtual function is called by the public functions basic_streambuf::sungetc and basic_streambuf::sputbackc (which, in turn, are called by basic_istream::unget and basic_istream::putback ).

1) The caller is requesting that the get area is backed up by one character ( pbackfail() is called with no arguments), in which case, this function re-reads the file starting one byte earlier and decrements basic_streambuf::gptr() , e.g. by calling gbump ( - 1 ) .
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 which case
a) First, checks if there is a putback position, and if there isn't, backs up the get area by re-reading the file starting one byte earlier.
a) Then checks what character is in the putback position. If the character held there is already equal to c , as determined by Traits :: eq ( to_char_type ( c ) , gptr ( ) [ - 1 ] ) , then simply decrements basic_streambuf::gptr() .
b) Otherwise, if the buffer is allowed to modify its own get area, decrements basic_streambuf::gptr() and writes c to the location pointed to gptr() after adjustment.

This function never modifies the file, only the get area of the in-memory buffer.

If the file is not open ( is_open ( ) == false , this function returns Traits :: eof ( ) immediately.

Parameters

c - the character to put back, or Traits::eof() to indicate that backing up of the get area is requested

Return value

c on success except if c was Traits :: eof ( ) , in which case Traits :: not_eof ( c ) is returned.

Traits :: eof ( ) on failure.

Example

See also

[virtual]
puts a character back into the input sequence, possibly modifying the input sequence
(virtual protected member function of std::basic_streambuf<CharT,Traits> )
moves the next pointer in the input sequence back by one
(public member function of std::basic_streambuf<CharT,Traits> )
puts one character back in the input sequence
(public member function of std::basic_streambuf<CharT,Traits> )
unextracts a character
(public member function of std::basic_istream<CharT,Traits> )
puts a character into input stream
(public member function of std::basic_istream<CharT,Traits> )