Challenge 1: Fixing cpp1-5's Own Leak With unique_ptr — Possible Solution
====================================================================

#include <iostream>
#include <memory>

class Circle {
public:
    double radius;
    Circle(double r) { radius = r; }
    ~Circle() { std::cout << "Circle destroyed" << std::endl; }
};

int main() {
    std::unique_ptr<Circle> c = std::make_unique<Circle>(5.0);
    // no explicit delete anywhere -- and none is needed
    return 0;
}

Output:

    Circle destroyed

WHY THIS WORKS AS AN ANSWER
------------------------------
Unlike cpp1-5's own Challenge 2 (a raw Circle* that was never deleted
and produced NO destructor output at all), this version produces
"Circle destroyed" even though delete was never written anywhere in
the code. c, a unique_ptr, genuinely owns the Circle object; when c
itself goes out of scope at the end of main, its own destructor runs
automatically (RAII, cpp1-5's own mechanism) and calls delete on the
Circle it owns -- closing exactly the gap that chapter's Challenge 2
demonstrated.