29 Input/output library [input.output]

29.7 Formatting and manipulators [iostream.format]

29.7.4 Input streams [input.streams]

29.7.4.2 Class template basic_istream [istream]

29.7.4.2.4 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; }; }

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.304

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) // c is a whitespace character.