C++ In a Nutshell

Name

basic_istream::sentry class — Sentry class for input streams

Synopsis

template <typename chT, typename traits=char_traits<chT> >
class basic_istream<chT,traits>::sentry{
  public: explicit sentry(basic_istream<chT,traits>& stream, 
                          bool noskipws = false);
  ~sentry(  );
  operator bool(  ) const;
private:
  sentry(const sentry&);            // Not defined
  sentry& operator=(const sentry&); // Not defined
};

A basic_istream object constructs a temporary sentry object prior to each input operation. The sentry object is destroyed when the input operation finishes and the function returns. The sentry manages tied streams and is responsible for skipping whitespace prior to a formatted read.

The stream passes itself and a flag to the sentry’s constructor. Formatted reads (operator>>) use false for the second argument; unformatted reads (get, getline, etc.) use true.

If stream.good( ) is true, the sentry first flushes any tied stream. That is, if stream.tie( ) is not null, the sentry calls stream.tie( )->flush( ). This ensures that prompts and similar output appears before the input is requested. Then, if the noskipws argument is false, and if the skipws bit is not set in the stream’s formatting flags ((ios_base::skipws & stream.fmtflags( )) == 0), the sentry’s constructor reads and discards whitespace characters. The sentry uses code similar to that shown in Example 13-17.

Example

Example 13-17. Skipping whitespace in an input sentry

const std::ctype<char_type>& ctype = std::use_facet<ctype<char_type> >(stream.getloc( ...