What is operator overloading in C++?

Operator overloading defines custom behavior for operators when applied to user-defined types — making them work naturally like built-in types. class Vector2D { public: double x, y; Vector2D(double x = 0, double y = 0) : x(x), y(y) {} // Addition: Vector2D operator+(const Vector2D& other) const { return Vector2D(x + other.x, y + other.y); } // Compound assignment: Vector2D& operator+=(const Vector2D& other) { x += other.x; y += other.y; return *this; } // Equality: bool operator==(const Vector2D& other) const { return x == other.x && y == other.y; } bool operator!=(const Vector2D& other) const { return !(*this == other); } // Unary negation: Vector2D operator-() const { return Vector2D(-x, -y); } // Subscript: double& operator[](int i) { return i == 0 ? x : y; } // Comparison (C++20 -- spaceship operator): auto operator<=>(const Vector2D&) const = default; // Friend: output stream: friend std::ostream& operator<<(std::ostream& os, const Vector2D& v) { return os << "(" << v.x << ", " << v.y << ")"; } }; // Usage: Vector2D a(1, 2), b(3, 4); Vector2D c = a + b; // (4, 6) a += b; std::cout << c << "\n"; // (4, 6). Rules: can't overload: ::, ., .*, ?:; can't change arity or precedence; at least one operand must be a user-defined type. Member vs free function: use member for operators that modify the object (+=, [], (), ->); use free (friend) for symmetric operators (+, -, ==, <<) to allow commutativity. Spaceship operator (<=>, C++20): define once, get all six comparison operators automatically.