Overloading the << Operator for Your Own Classes

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The latest version of this topic can be found at 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, 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;  
}  

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;  

Output Streams