Overloading Binary Operators

A binary operator works with two operands.

Examples:

a + b
a - b
a == b
a < b

Why Member Binary Operators Take One Parameter

This is a very common viva question.

When a binary operator is overloaded as a member function, the left operand is the calling object.

c1 + c2

is treated like:

c1.operator+(c2);

So only the right operand is passed as an explicit parameter.

Complete Member Function Example

#include <iostream>
using namespace std;

class Complex {
private:
    double real;
    double imag;

public:
    Complex(double r = 0, double i = 0) : real(r), imag(i) {}

    Complex operator+(const Complex& other) const {
        return Complex(real + other.real, imag + other.imag);
    }

    bool operator==(const Complex& other) const {
        return real == other.real && imag == other.imag;
    }

    void display() const {
        cout << real << " + " << imag << "i" << endl;
    }
};

int main() {
    Complex a(2, 3);
    Complex b(4, 5);
    Complex c = a + b;

    c.display();

    cout << boolalpha << (a == b) << endl;
    return 0;
}

Output:

6 + 8i
false

Non-Member Binary Operator

When a binary operator is overloaded as a non-member function, both operands are passed explicitly.

operator+(a, b);

That means it takes two parameters.

#include <iostream>
using namespace std;

class Money {
private:
    int taka;

public:
    Money(int amount = 0) : taka(amount) {}

    friend Money operator+(const Money& left, const Money& right) {
        return Money(left.taka + right.taka);
    }

    void display() const {
        cout << taka << " taka" << endl;
    }
};

int main() {
    Money m1(100);
    Money m2(50);

    Money total = m1 + m2;
    total.display();

    return 0;
}

Member vs Friend

Use a member function when the left operand is naturally the object.

object + other

Use a non-member or friend function when the left operand is not your class, or when you want symmetric behavior.

Example:

5 + money

Here 5 cannot call a Money member function. A non-member operator may be needed.

Can Function Overloading Replace Operator Overloading?

Yes, sometimes.

Instead of:

Money total = m1 + m2;

you can write:

Money total = m1.add(m2);

The logic can be the same. But operator overloading gives natural syntax when the operation is obvious.

Viva Answer

For a member binary operator, only one parameter is passed because the left operand is the calling object and the right operand is the explicit argument. For a non-member binary operator, both operands must be passed as parameters.

Quick Check

In a + b, which object calls a member operator+?
Why does a member binary operator take one parameter?
Why might 5 + obj require a non-member operator?