What is Operator Overloading in C++?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Pick the operator and form

   Decide whether to implement it as a member function or a non-member (often friend) function based on symmetry and access needs.

2. Step 2

   Name the function

   Define a function called operator followed by the symbol, e.g. operator+ or operator==.

3. Step 3

   Define the semantics

   Write the body so it produces a correct, intuitive result matching the built-in operator's meaning.

4. Step 4

   Return the right type

   Return a new value for arithmetic, a reference for stream and compound-assignment operators.

5. Step 5

   Keep it consistent

   Ensure related operators agree, such as == with != and < defining a total order.

Code Example

#include <iostream>

class Complex {
    double re, im;
public:
    Complex(double r = 0, double i = 0) : re(r), im(i) {}

    // Non-member-friendly via member here: returns a new value
    Complex operator+(const Complex& rhs) const {
        return Complex(re + rhs.re, im + rhs.im);
    }

    bool operator==(const Complex& rhs) const {
        return re == rhs.re && im == rhs.im;
    }

    // Stream operator must be a non-member; friend grants access
    friend std::ostream& operator<<(std::ostream& os, const Complex& c) {
        os << c.re << " + " << c.im << "i";
        return os;
    }
};

int main() {
    Complex a(1, 2), b(3, 4);
    Complex sum = a + b;         // uses operator+
    std::cout << sum << '\n';    // uses operator<<
    std::cout << (a == b) << '\n';
}

Follow-up Questions

• When should an operator be a member versus a non-member function?
• Why must operator<< and operator>> be non-member functions?
• Which operators cannot be overloaded in C++?
• How do you overload the assignment operator correctly?
• What is the rule of three/five and how does it relate to operators?