29 Input/output library [input.output]

29.7 Formatting and manipulators [iostream.format]

29.7.5 Output streams [output.streams]

29.7.5.1 Class template basic_­ostream [ostream]

29.7.5.1.3 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.
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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().314
3
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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])).315
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~sentry();
4
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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.
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explicit operator bool() const;
5
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Effects: Returns ok_­.
314)
The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.
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315)
The sentry constructor and destructor can also perform additional implementation-dependent operations.
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