27 Input/output library [input.output]

27.7 Formatting and manipulators [iostream.format]

27.7.3 Output streams [output.streams]

27.7.3.4 Class basic_ostream::sentry [ostream::sentry]

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;
  };
}
1

The class sentry defines a class that is responsible for doing exception safe prefix and suffix operations.

explicit sentry(basic_ostream<charT,traits>& os);

2

If os.good() is nonzero, prepares for formatted or unformatted output. If os.tie() is not a null pointer, calls os.tie()->flush().325

3

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

~sentry();

4

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;

5

Effects: Returns ok_.

324)

The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.

325)

The sentry constructor and destructor can also perform additional implementation-dependent operations.