Operator Overloading

You can add two ints with +. Wouldn't it be nice to add two of your own objects the same way? With operator overloading you can tell C++ what +, ==, << and friends mean for your class.

#include <iostream>
using namespace std;

struct Money {
    int dollars;
    int cents;

    Money operator+(const Money& other) const {
        int total = (dollars + other.dollars) * 100 + cents + other.cents;
        return {total / 100, total % 100};
    }

    bool operator==(const Money& other) const {
        return dollars == other.dollars && cents == other.cents;
    }
};

int main() {
    Money a = {5, 70};
    Money b = {2, 50};
    Money c = a + b;                  // calls a.operator+(b)
    cout << c.dollars << " dollars " << c.cents << " cents\n";
    cout << boolalpha << (c == Money{8, 20}) << "\n";
    return 0;
}

Output

8 dollars 20 cents
true

An operator is just a function with a special name: operator+, operator==, and so on. a + b is simply a nicer way of writing a.operator+(b).

Printing your objects with <<

To make cout << myObject work, write an operator<< function outside the class. It takes the stream and your object, and returns the stream so you can keep chaining:

#include <iostream>
using namespace std;

struct Point {
    int x;
    int y;
};

ostream& operator<<(ostream& out, const Point& p) {
    out << "(" << p.x << ", " << p.y << ")";
    return out;
}

int main() {
    Point a = {3, 4};
    Point b = {-1, 7};
    cout << "a is " << a << " and b is " << b << "\n";
    return 0;
}

Output

a is (3, 4) and b is (-1, 7)

operator< makes sorting easy

If your type has operator<, sort knows how to put objects in order:

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

struct Player {
    string name;
    int score;

    bool operator<(const Player& other) const {
        return score < other.score;
    }
};

int main() {
    vector<Player> players = {{"Ann", 30}, {"Bob", 10}, {"Cid", 20}};
    sort(players.begin(), players.end());
    for (const Player& p : players) {
        cout << p.name << " " << p.score << "\n";
    }
    return 0;
}

Output

Bob 10
Cid 20
Ann 30

Tip: Only overload operators when the meaning is obvious. + to add money: great. + to delete a file: confusing!