30 Input/output library [input.output]

30.7 Formatting and manipulators [iostream.format]

30.7.5 Output streams [output.streams] Class template basic_­ostream [ostream] 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
    explicit sentry(basic_ostream<charT, traits>& os);
    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.

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

If os.good() is nonzero, prepares for formatted or unformatted output. If os.tie() is not a null pointer, calls os.tie()->flush().318

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]))319


If (os.flags() & ios_­base​::​unitbuf) && !uncaught_­exceptions() && 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.