Generic programming: 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.

Example

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:

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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.

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// 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, const Date& dt); };   ostream& operator<<(ostream& os, const Date& dt) {     os << dt.mo << '/' << dt.da << '/' << dt.yr;     return os; }   int main() {     Date dt(5, 6, 92);     cout << dt; }

Output

5/6/92

Remarks

The overloaded operator returns a reference to the original ostream object, which means you can combine insertions:

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cout << "The date is" << dt << flush;