cppdraft_translate/cppdraft/ostream/sentry.md

[ostream.sentry]

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 onlypublic: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 classsentry 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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Ifos.good() is nonzero, prepares for formatted or unformatted output.

Ifos.tie() is not a null pointer, callsos.tie()->flush().283

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If, after any preparation is completed,os.good() istrue,ok_ == true otherwise,ok_ == false.

During preparation, the constructor may callsetstate(failbit) (which may throwios_base​::​​failure ([iostate.flags])).284

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~sentry();

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If(os.flags() & ios_base​::​unitbuf) && !uncaught_exceptions()&& os.good() istrue, callsos.rdbuf()->pubsync().

If that function returns −1 or exits via an exception, sets badbit inos.rdstate() without propagating an exception.

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explicit operator bool() const;

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Effects: Returnsok_.

283)283)

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

284)284)

Thesentry constructor and destructor can also perform additionalimplementation-dependent operations.