The GNU C++ Iostream Library - States

Checking the state of a stream

Use this collection of methods to test for (or signal) errors and other exceptional conditions of streams:

Method ios::operator void* () const

You can do a quick check on the state of the most recent operation on a stream by examining a pointer to the stream itself. The pointer is arbitrary except for its truth value; it is true if no failures have occurred (ios::fail is not true). For example, you might ask for input on cin only if all prior output operations succeeded:

	if (cout)
	{
	  // Everything OK so far
	  cin >> new_value;
	  ...
	}

Method ios::operator ! () const

In case it is more convenient to check whether something has failed, the operator ! returns true if ios::fail is true (an operation has failed). For example, you might issue an error message if input failed:

	if (!cin)
	{
	  // Oops
	  cerr << "Eh?\n";
	}

Method iostate ios::rdstate () const

Return the state flags for this stream. The value is from the enumeration iostate . You can test for any combination of

goodbit

There are no indications of exceptional states on this stream.

eofbit

End of file.

failbit

An operation has failed on this stream; this usually indicates bad format of input.

badbit

The stream is unusable.

Method void ios::setstate (iostate state)

Set the state flag for this stream to state in addition to any state flags already set. Synonym (for upward compatibility): ios::set .

See ios::clear to set the stream state without regard to existing state flags. See ios::good , ios::eof , ios::fail , and ios::bad , to test the state.

Method int ios::good () const

Test the state flags associated with this stream; true if no error indicators are set.

Method int ios::bad () const

Test whether a stream is marked as unusable. (Whether ios::badbit is set.)

Method int ios::eof () const

True if end of file was reached on this stream. (If ios::eofbit is set.)

Method int ios::fail () const

Test for any kind of failure on this stream: either some operation failed, or the stream is marked as bad. (If either ios::failbit or ios::badbit is set.)

Method void ios::clear (iostate state)

Set the state indication for this stream to the argument state. You may call ios::clear with no argument, in which case the state is set to good (no errors pending).

See ios::good , ios::eof , ios::fail , and ios::bad , to test the state; see ios::set or ios::setstate for an alternative way of setting the state.