namespace std {
template <class charT,class traits = char_traits<charT> >
class basic_ostream<charT,traits>::sentry {
bool ok_; // exposition only
public:
explicit sentry(basic_ostream<charT,traits>& os);
~sentry();
explicit operator bool() const { return ok_; }
sentry(const sentry&) = delete;
sentry& operator=(const sentry&) = delete;
};
}
The class sentry defines a class that is responsible for doing exception safe prefix and suffix operations.
If os.good() is nonzero, prepares for formatted or unformatted output. If os.tie() is not a null pointer, calls os.tie()->flush().325
If, after any preparation is completed, os.good() is true, ok_ == true otherwise, ok_ == false. During preparation, the constructor may call setstate(failbit) (which may throw ios_base::failure ([iostate.flags]))326
If ((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.good()) is true, calls os.rdbuf()->pubsync(). If that function returns -1, sets badbit in os.rdstate() without propagating an exception.
explicit operator bool() const;
Effects: Returns ok_.
The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.
The sentry constructor and destructor can also perform additional implementation-dependent operations.