31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

31.7.5 Input streams [input.streams]

31.7.5.2 Class template basic_istream [istream]

31.7.5.2.4 Class basic_istream​::​sentry [istream.sentry]

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namespace std { template<class charT, class traits> class basic_istream<charT, traits>::sentry { bool ok_; // exposition only public: explicit sentry(basic_istream& is, bool noskipws = false); ~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_istream& is, bool noskipws = false);
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Effects: If is.good() is false, calls is.setstate(failbit).
Otherwise, prepares for formatted or unformatted input.
First, if is.tie() is not a null pointer, the function calls is.tie()->flush() to synchronize the output sequence with any associated external C stream.
Except that this call can be suppressed if the put area of is.tie() is empty.
Further an implementation is allowed to defer the call to flush until a call of is.rdbuf()->underflow() occurs.
If no such call occurs before the sentry object is destroyed, the call to flush may be eliminated entirely.282
If noskipws is zero and is.flags() & ios_base​::​skipws is nonzero, the function extracts and discards each character as long as the next available input character c is a whitespace character.
If is.rdbuf()->sbumpc() or is.rdbuf()->sgetc() returns traits​::​eof(), the function calls setstate(failbit | eofbit) (which may throw ios_base​::​failure).
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Remarks: The constructor explicit sentry(basic_istream& is, bool noskipws = false) uses the currently imbued locale in is, to determine whether the next input character is whitespace or not.
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To decide if the character c is a whitespace character, the constructor performs as if it executes the following code fragment: const ctype<charT>& ctype = use_facet<ctype<charT>>(is.getloc()); if (ctype.is(ctype.space, c) != 0) // c is a whitespace character.
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If, after any preparation is completed, is.good() is true, ok_ != false otherwise, ok_ == false.
During preparation, the constructor may call setstate(failbit) (which may throw ios_base​::​​failure ([iostate.flags])).283
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~sentry();
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Effects: None.
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explicit operator bool() const;
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Returns: ok_.
282)282)
This will be possible only in functions that are part of the library.
The semantics of the constructor used in user code is as specified.
283)283)
The sentry constructor and destructor can also perform additional implementation-dependent operations.