Challenge 2: Fixing the Slicing Without Changing Circle's Construction — Possible Solution
====================================================================

#include <iostream>

class Shape {
public:
    virtual double area() { return 0; }
};

class Circle : public Shape {
public:
    double radius;
    Circle(double r) : radius(r) {}
    double area() override { return 3.14159 * radius * radius; }
};

int main() {
    Circle c(5.0);      // unchanged from Challenge 1
    Shape &s = c;        // reference instead of a by-value copy -- no slicing

    std::cout << s.area() << std::endl;

    return 0;
}

Output:

    78.5397

WHY THIS WORKS AS AN ANSWER
------------------------------
Circle c(5.0) is constructed identically to Challenge 1 -- the only
change is that s is declared as Shape& (a reference) rather than a
plain Shape (a value). Since a reference is just an alias for the
existing Circle object (cpp1-2's own material) rather than a new,
separately-copied Shape-sized object, s.area() correctly dispatches
through Circle's own vtable, exactly matching the chapter's own advice
to always use a pointer or reference for polymorphic objects rather
than passing them by value.