basic_streambuf::pbackfail

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

The protected virtual member function endeavors to put back an element into the input stream, 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 T::to_char_type(c). The function can put back an element in various ways:

• If a putback position is available, it can store the element into the putback position and decrement the next pointer for the input buffer.
• It can make a putback position available by allocating new or additional storage for the input buffer.
• For a stream buffer with common input and output streams, it can make a putback position available by writing out, to some external destination, some or all of the elements between the beginning and next pointers for the output buffer.

If the function cannot succeed, it returns T::eof() or throws an exception. Otherwise, it returns some other value. The default behavior is to return T::eof().