The sentry object takes as the constructor argument the stream strm, on which the pre- and postprocessing
should be done. The remaining processing then depends on the state of this object, which
indicates whether the stream is OK. This state can be checked using the conversion of the sentry
object to bool. For input streams, the sentry object can be constructed with an optional Boolean
value that indicates whether skipping of whitespace should be avoided even though the flag skipws
is set: