basic_stringbuf::pbackfail

virtual int_type pbackfail(int_type c = T::eof());

The protected virtual member function endeavors to put back an element into the input buffer, then make it the current element (pointed to by the next pointer). If c compares equal to T::eof(), the element to push back is effectively the one already in the stream before the current element. Otherwise, that element is replaced by x = T::to_char_type(c). The function can put back an element in various ways:

• If a putback position is available, and the element stored there compares equal to x, it can simply decrement the next pointer for the input buffer.
• If a putback position is available, and if the stringbuf mode permits the sequence to be altered (mode & ios_base::out is nonzero), it can store x into the putback position and decrement the next pointer for the input buffer.

If the function cannot succeed, it returns T::eof(). Otherwise, it returns T::not_eof(c).