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#include <iostream>
#include <cmath>   // std::sqrt

struct Vector2D {
    double x, y;

// Constructor
    Vector2D(double x = 0, double y = 0) : x(x), y(y) {}

// Member: unary negation
    Vector2D operator-() const { return {-x, -y}; }

// Member: compound assignment
    Vector2D& operator+=(const Vector2D& rhs) {
        x += rhs.x;  y += rhs.y;  return *this;
    }

double length() const { return std::sqrt(x*x + y*y); }
};

// Free: binary addition (symmetrical)
Vector2D operator+(Vector2D lhs, const Vector2D& rhs) {
    lhs += rhs;   // reuse compound assignment
    return lhs;
}

// Free: equality
bool operator==(const Vector2D& a, const Vector2D& b) {
    return a.x == b.x && a.y == b.y;
}

// Free: stream insertion (friend not needed if using public members)
std::ostream& operator<<(std::ostream& os, const Vector2D& v) {
    return os << "(" << v.x << ", " << v.y << ")";
}

int main() {
    Vector2D a{3.0, 0.0}, b{0.0, 4.0};
    Vector2D c = a + b;
    std::cout << "a + b = " << c << "
";          // (3, 4)
    std::cout << "length = " << c.length() << "
"; // 5
    std::cout << std::boolalpha << (a == b) << "
"; // false
    return 0;
}