basic_filebuf::pbackfail

virtual int_type pbackfail(int_type c =
    traits_type::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 == traits_type::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 = traits_type::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 the function can make a putback position available, it can do so, set the next pointer to point at that position, and store x in that position.
• If the function can push back an element onto the input stream, it can do so, such as by calling ungetc for an element of type char.

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