Put character back.
  Puts back a character into the buffer and makes it available again as the next character. If c is EOF the character push back is automatically the same character extracted in that position, otherwise character c is written in the push back position.

Parameters.

c

Character value to be push back, or EOF.

Return Value.
  The function returns EOF in case of failure or any other value on success.

Basic template member declaration ( basic_stringbuf<charT,traits,Allocator> ):

typedef traits::int_type int_type; int_type pbackfail ( int_type c = traits::eof() );

See also.
  underflow
  stringbuf class