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std::basic_...f::pbackfail(3)  C++ Standard Libary std::basic_...f::pbackfail(3)

NAME
     std::basic_streambuf::pbackfail - std::basic_streambuf::pbackfail

Synopsis
	protected:
	virtual int_type pbackfail( int_type c = Traits::eof() );

	This  function	can  only be called if any of the following condition is
     satisfied:

	  * gptr() is null,
	  * gptr() == eback(), or
	  * traits::eq(traits::to_char_type(c), gptr()[-1]) returns false.

	This function is called by the	public	functions  sungetc()  and  sput-
     backc() (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 re-
     trieved
	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 possi-
     ble, 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  de-
     rived classes:
	basic_stringbuf::pbackfail,	basic_filebuf::pbackfail,     strstream-
     buf::pbackfail, and
	is expected to be overridden by  user-defined  and  third-party  library
     stream classes.

Parameters
	ch  -  character  to  put  back or Traits::eof() if only back out is re-
     quested

Return value
	Traits::eof() in case of failure, some other value to indicate	success.
     The base
	class version always fails.

Example
	 This section is incomplete
	 Reason: no example

	Defect reports

	The  following	behavior-changing  defect  reports were applied retroac-
     tively to
	previously published C++ standards.

	  DR	  Applied    to 		   Behavior	as     published
     Correct
										  be-
     havior
			  the		       calling		       condition
     corrected to
	LWG    32    C++98	    'traits::eq(*gptr(),traits::to_char_type(c))
     match
			  returns  false'  did	not  match  the  description  of
     the
			  sputbackc()
     description

See also
		  backs out the input sequence to unget a character, not affect-
     ing the
	pbackfail associated file
	[virtual]  (virtual  protected	member	function   of	std::basic_file-
     buf<CharT,Traits>)

	pbackfail puts a character back into the input sequence
	[virtual] (virtual protected member function of
		  std::basic_stringbuf<CharT,Traits,Allocator>)
	pbackfail backs out the input sequence to unget a character
	[virtual] (virtual protected member function of std::strstreambuf)
	sungetc   moves the next pointer in the input sequence back by one
		  (public member function)
	sputbackc puts one character back in the input sequence
		  (public member function)

Category:
	  * Todo no example

http://cppreference.com 	   2024.06.10	   std::basic_...f::pbackfail(3)
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