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Overloading the << Operator for Your Own Classes

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.

// overload_date.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;

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;
}

int main( )
{
   Date dt( 5, 6, 92 );
   cout << dt;
}

Output

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;

See Also

Output Streams

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