iostate

iostate is a type used in the C++ standard library to represent the state of an input/output stream. Defined within ios_base, it tracks whether a stream is in a good condition or has encountered errors. The type is used by standard stream classes such as std::istream, std::ostream, std::iostream, and their concrete implementations (for example, std::cin, std::cout, std::ifstream, std::ostringstream) to report the result of I/O operations.

The iostate type is a bitmask that combines four distinct flags: goodbit, eofbit, failbit, and badbit. goodbit indicates no error and is typically zero. eofbit signals end-of-file. failbit indicates a logical I/O error or that a formatted input operation failed. badbit indicates a more serious error that may reflect a loss of integrity of the stream. These bits can be combined; for instance, a stream may be at end-of-file and fail at the same time. While exact numeric values are implementation-dependent, they are designed as distinct bit patterns (commonly with nonzero values for eofbit, failbit, and badbit).

State of a stream can be checked with member functions such as good(), eof(), fail(), and bad(). good() returns true when no error bits are set (the stream is in a good state). eof() reports whether eofbit is set. fail() indicates either failbit or badbit is set, and bad() indicates badbit is set. Streams can also be tested in boolean contexts, which reflect their overall good or error state.

The iostate value may be manipulated with clear() calls to reset or modify specific bits, and it can be used with the exceptions mechanism via ios_base::exceptions to trigger exceptions (ios_base::failure) when certain bits become set after operations.

In summary, iostate provides a standardized way to represent and inspect the success or failure of I/O operations within the C++ standard library.