29 Input/output library [input.output]

29.7 Formatting and manipulators [iostream.format]

29.7.4 Input streams [input.streams]

29.7.4.1 Class template basic_­istream [istream]

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

namespace std {
  template<class charT, class traits = char_traits<charT>>
  class basic_istream<charT, traits>::sentry {
    bool ok_;                   // exposition only
  public:
    explicit sentry(basic_istream<charT, traits>& 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<charT, traits>& 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.301
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<charT, traits>& 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])).302
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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_­.
301)
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.
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302)
The sentry constructor and destructor can also perform additional implementation-dependent operations.
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