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cppdraft/ostream/sentry.md

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+[ostream.sentry]
+
+# 31 Input/output library [[input.output]](./#input.output)
+
+## 31.7 Formatting and manipulators [[iostream.format]](iostream.format#ostream.sentry)
+
+### 31.7.6 Output streams [[output.streams]](output.streams#ostream.sentry)
+
+#### 31.7.6.2 Class template basic_ostream [[ostream]](ostream#sentry)
+
+#### 31.7.6.2.4 Class basic_ostream​::​sentry [ostream.sentry]
+
+[🔗](#lib:basic_ostream::sentry)
+
+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; };}
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L6205)
+
+The classsentry defines a class that is responsible for doing exception safe prefix and suffix
+operations[.](#1.sentence-1)
+
+[🔗](#lib:basic_ostream::sentry,constructor)
+
+`explicit sentry(basic_ostream& os);
+`
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L6217)
+
+Ifos.good() is nonzero, prepares for formatted or unformatted output[.](#2.sentence-1)
+
+Ifos.tie() is not a null pointer, callsos.tie()->flush()[.](#2.sentence-2)[283](#footnote-283 "The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.")
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L6233)
+
+If, after any preparation is completed,os.good() istrue,*ok_* == true otherwise,*ok_* == false[.](#3.sentence-1)
+
+During preparation, the constructor may callsetstate(failbit) (which may throwios_base​::​​failure ([[iostate.flags]](iostate.flags "31.5.4.4 Flags functions")))[.](#3.sentence-2)[284](#footnote-284 "The sentry constructor and destructor can also perform additional implementation-dependent operations.")
+
+[🔗](#lib:basic_ostream::sentry,destructor)
+
+`~sentry();
+`
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L6261)
+
+If(os.flags() & ios_base​::​unitbuf) && !uncaught_exceptions()&& os.good() istrue,
+callsos.rdbuf()->pubsync()[.](#4.sentence-1)
+
+If that function returns −1 or
+exits via an exception, sets badbit inos.rdstate() without propagating an exception[.](#4.sentence-2)
+
+[🔗](#lib:operator_bool,basic_ostream::sentry)
+
+`explicit operator bool() const;
+`
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L6279)
+
+*Effects*: Returns*ok_*[.](#5.sentence-1)
+
+[283)](#footnote-283)[283)](#footnoteref-283)
+
+The callos.tie()->flush() does not necessarily occur if the function can determine that no
+synchronization is necessary[.](#footnote-283.sentence-1)
+
+[284)](#footnote-284)[284)](#footnoteref-284)
+
+Thesentry constructor and destructor
+can also perform additionalimplementation-dependent operations[.](#footnote-284.sentence-1)