It is used get pointer and the get pointer (gptr) is the internal pointer that points to the current location in the array with the buffered portion of the controlled input sequence.
Following is the declaration for std::basic_streambuf::gbump.
char_type* egptr() const;
n − it is a value by which to increase the get pointer.
Basic guarantee − if an exception is thrown, the stream buffer is in a valid state.
It modifies the stream buffer object.