##### cppreference.com

##### Search

Create account

##### Namespaces

- Page
- Discussion

##### Variants

##### Views

- View
- Edit
- History

##### Actions

# std::basic_filebuf::pbackfail

From cppreference.com
< cpp‎ | io‎ | basic filebuf

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.

### [edit] Parameters

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

### [edit] Return value

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

Traits::eof() on failure.

### [edit] Example

| This section is incomplete Reason: no example

### [edit] 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`) [edit]
sungetc | moves the next pointer in the input sequence back by one (public member function of `std::basic_streambuf`) [edit]
sputbackc | puts one character back in the input sequence (public member function of `std::basic_streambuf`) [edit]
unget | unextracts a character (public member function of `std::basic_istream`) [edit]
putback | puts character into input stream (public member function of `std::basic_istream`) [edit]

Retrieved from "http://en.cppreference.com/mwiki/index.php?title=cpp/io/basic_filebuf/pbackfail&oldid=57575"
Category:

- Todo no example