badbit

In the C++ standard library, badbit is a flag representing an irrecoverable error on a stream. It is part of the iostreams state set defined by std::ios_base::iostate and is used alongside eofbit and failbit to describe a stream's condition. The presence of badbit indicates that the stream has become unusable for further I/O operations, usually due to a hardware or serious library-level problem.

Badbit differs from eofbit (end-of-file) and failbit (a logical error in a stream operation) in that it signals a more severe condition that often cannot be recovered from by simple retry. In practice, many standard library implementations set both badbit and failbit in irrecoverable situations, and a stream in this state is generally not reliable.

Detection and handling: You can query with stream.bad() to test for a badbit condition, or use rdstate() to inspect the full iostate value, and retrieve a boolean with bool(stream). You can clear the flags with stream.clear(), typically passing std::ios_base::goodbit to reset the state, though this does not repair the underlying error. If exceptions are enabled for badbit, a bad state will throw std::ios_base::failure.

Causes: Badbit is set by the underlying stream buffer when an irrecoverable error occurs on the input or output device, such as a hardware failure, a lost connection, or a corrupted internal buffer. It can also occur if a library detects data integrity problems in the stream. Handling usually involves reporting the error to the user or attempting a recovery from a higher level.

Notes: Badbit is defined by the C++ standard library's ios_base::badbit and is one of the four iostate flags used to describe stream readiness. It is distinct from the end-of-file indicator and non-recoverable logical failures, and its presence signals that further I/O may be impossible.