11.3 A Simple Extractor and Inserter for the Example

Following the read and write conventions of iostreams, we would insert and extract the date object like this:

date birthday(2,6,1952);
cout << birthday << '\n';

date aDate;

cout << '\n' << "Please, enter a date (day month year)" << '\n';
cin  >> aDate;
cout << aDate << '\n';

For the next step, we would implement shift operators as inserters and extractors for date objects. Here is an extractor for class date:

template<class charT, class Traits>
basic_istream<charT, Traits>&                                 //1
operator>> (basic_istream<charT,Traits>& is,                  //2
            date& dat)                                        //3
{
 is >> dat.tm_date.tm_mday;                                   //4
 is >> dat.tm_date.tm_mon;
 is >> dat.tm_date.tm_year;
 return is;                                                   //5
}

//1	The returned value for extractors (and inserters) is a reference to the stream, so that several extractions can be done in one expression.
//2	The first parameter usually is the stream from which the data shall be extracted.
//3	The second parameter is a reference, or alternatively a pointer, to an object of the user-defined type.
//4	In order to allow access to private data of the date class, the extractor must be declared as a friend function in class `date`.
//5	The return value is the stream from which the data was extracted.

As the extractor accesses private data members of class date, it must be declared as a friend function to class date. The same holds for the inserter. Here's a more complete version of class date:

class date {
public:
   date(int d, int m, int y);
   date(tm t);
   date();
   // more constructors and useful member functions

private: 
   tm tm_date;

template<class charT, Traits>
friend basic_istream<charT, Traits> &operator >> 
  (basic_istream<charT, Traits >& is, date& dat);
 
template<class charT, Traits>
friend basic_ostream<charT, Traits> &operator << 
  (basic_ostream<charT, Traits >& os, const date& dat);
};

The inserter can be built analogously, as shown below. The only difference is that you would hand over a constant reference to a date object, because the inserter is not supposed to modify the object it prints.

template<class charT, class Traits>
basic_ostream<charT, Traits>&
operator << (basic_ostream<charT, Traits >& os, const date& dat)
{
 os << dat.tm_date.tm_mon << '-';
 os << dat.tm_date.tm_mday << '-';
 os << dat.tm_date.tm_year ;
 return os;
}

OEM Release, June 1998