< cpp‎ | io‎ | basic filebuf
Revision as of 11:02, 9 December 2012 by Cubbi (Talk | contribs)

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.



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.


See also

Template:cpp/io/basic streambuf/dcl list pbackfailTemplate:cpp/io/basic streambuf/dcl list sungetcTemplate:cpp/io/basic streambuf/dcl list sputbackcTemplate:cpp/io/basic istream/dcl list ungetTemplate:cpp/io/basic istream/dcl list putback