std::basic_filebuf::pbackfail

 

C++

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Technical Specifications

 

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

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basic_syncbuf (C++20)

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basic_osyncstream (C++20)

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streamoff

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iostream_category (C++11)

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std::basic_filebuf

Public member functions

basic_filebuf::basic_filebuf

basic_filebuf::~basic_filebuf

basic_filebuf::operator= (C++11)

basic_filebuf::swap (C++11)

basic_filebuf::is_open

basic_filebuf::open

basic_filebuf::close

Protected member functions

basic_filebuf::showmanyc

basic_filebuf::underflow

basic_filebuf::uflow

basic_filebuf::pbackfail

basic_filebuf::overflow

basic_filebuf::setbuf

basic_filebuf::seekoff

basic_filebuf::seekpos

basic_filebuf::sync

basic_filebuf::imbue

Non-member functions

swap(std::basic_filebuf) (C++11)

 

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.

See also

pbackfail [virtual]	puts a character back into the input sequence, possibly modifying the input sequence (virtual protected member function of std::basic_streambuf)
sungetc	moves the next pointer in the input sequence back by one (public member function of std::basic_streambuf)
sputbackc	puts one character back in the input sequence (public member function of std::basic_streambuf)
unget	unextracts a character (public member function of std::basic_istream)
putback	puts character into input stream (public member function of std::basic_istream)