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

explicit sentry(basic_istream& 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.264

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& 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])).265

explicit operator bool() const;