Output streams use the insertion (<<) operator for standard types. You can also overload the << operator for your own classes.
The write function example showed the use of a Date
structure. A date is an ideal candidate for a C++ class in which the data members (month, day, and year) are hidden from view. An output stream is the logical destination for displaying such a structure. This code displays a date using the cout object:
Date dt( 1, 2, 92 );
cout << dt;
To get cout to accept a Date
object after the insertion operator, overload the insertion operator to recognize an ostream object on the left and a Date
on the right. The overloaded << operator function must then be declared as a friend of class Date
so it can access the private data within a Date
object.
#include <iostream.h>
class Date
{
int mo, da, yr;
public:
Date( int m, int d, int y )
{
mo = m; da = d; yr = y;
}
friend ostream& operator<< ( ostream& os, Date& dt );
};
ostream& operator<< ( ostream& os, Date& dt )
{
os << dt.mo << '/' << dt.da << '/' << dt.yr;
return os;
}
void main()
{
Date dt( 5, 6, 92 );
cout << dt;
}
When you run this program, it prints the date:
5/6/92
The overloaded operator returns a reference to the original ostream object, which means you can combine insertions:
cout << "The date is" << dt << flush;