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Showing content from https://timsong-cpp.github.io/cppwp/n4140/istream::sentry below:

[istream::sentry]

namespace std {
  template <class charT,class traits = char_traits<charT> >
  class basic_istream<charT,traits>::sentry {
    typedef traits traits_type;
    bool ok_;   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;
  };
}

The class sentry defines a class that is responsible for doing exception safe prefix and suffix operations.

explicit sentry(basic_istream<charT,traits>& is, bool noskipws = false);

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.312 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).

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.

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)
  

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]))313

~sentry();

explicit operator bool() const;

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