Replenish the input buffer with data obtained from the held streambuf, and return the next byte of input (or eof if no input is available). This function assumes that either the input buffer is empty or that the end of it has been reached. If this object is not in a valid state, eof will be returned.
Declared in <bdlde_utf8checkinginstreambufwrapper.h>
virtual
int_type
underflow() override;