the usage of "const sentry _OK(*this)"
I found class "sentry" was defined both in istream and ostream,the usage is like the photes,
what is the usage of "const sentry _OK(*this)" ?

[解决办法]
take a look the links bellow:

http://www.cplusplus.com/reference/istream/istream/sentry/
http://www.cplusplus.com/reference/ostream/ostream/sentry/

there are two classes, btw.