Challenge 1: A Rectangle Class With Private Members — Possible Solution
====================================================================

#include <iostream>

class Rectangle {
private:
    double width;
    double height;

public:
    Rectangle(double w, double h) {
        width = w;
        height = h;
    }

    double area() {
        return width * height;
    }
};

int main() {
    Rectangle r(4.0, 6.0);
    std::cout << r.area() << std::endl;
    return 0;
}

Output:

    24

WHY THIS WORKS AS AN ANSWER
------------------------------
width and height are declared private, so they're only reachable from
inside Rectangle's own member functions -- exactly matching the
chapter's account.balance example. The constructor runs automatically
when Rectangle r(4.0, 6.0) is created (no separate init call needed),
and area() computes and returns the product, called directly via
r.area() with no explicit object pointer passed by the caller.