31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

31.7.6 Output streams [output.streams]

31.7.6.2 Class template basic_ostream [ostream]

31.7.6.2.4 Class basic_ostream​::​sentry [ostream.sentry]

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namespace std { template<class charT, class traits> class basic_ostream<charT, traits>::sentry { bool ok_; // exposition only public: explicit sentry(basic_ostream& os); ~sentry(); explicit operator bool() const { return ok_; } sentry(const sentry&) = delete; sentry& operator=(const sentry&) = delete; }; }
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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& os);
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If os.good() is nonzero, prepares for formatted or unformatted output.
If os.tie() is not a null pointer, calls os.tie()->flush().289
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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])).290
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~sentry();
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If (os.flags() & ios_base​::​unitbuf) && !uncaught_exceptions() && os.good() is true, calls os.rdbuf()->pubsync().
If that function returns , sets badbit in os.rdstate() without propagating an exception.
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explicit operator bool() const;
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Effects: Returns ok_.
289)289)
The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.
290)290)
The sentry constructor and destructor can also perform additional implementation-dependent operations.