# std::basic_stringbuf<CharT,Traits,Allocator>::pbackfail

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< cpp‎ | io‎ | basic stringbuf

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 or with Traits::eof() as the argument)
a) First, checks if there is a putback position, and if there really isn't, fails. (stringbuf has no external character source to re-read)
b) If the caller was wrong and the putback position is in fact available, simply 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, fails.
b) 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()`.
c) Otherwise, if the buffer is open for reading only, fails.
d) Otherwise, if the buffer is open for writing (mode && std::ios_base::out is nonzero), then decrements `basic_streambuf::gptr()` and writes `c` to the location pointed to gptr() after adjustment.

### 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.

### Example

| This section is incomplete Reason: no example

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

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