Challenge 1: A const Member Function, and What Happens Calling a Non-const One on a const Object — Possible Solution
====================================================================

#include <iostream>

class Circle {
public:
    double radius;

    Circle(double r) { radius = r; }

    double area() const {
        return 3.14159 * radius * radius;
    }

    void setRadius(double r) {
        radius = r;
    }
};

int main() {
    const Circle c(5.0);
    std::cout << c.area() << std::endl;  // fine -- area() is const

    c.setRadius(10.0);  // compile error

    return 0;
}

Compile error (representative message):

    error: passing 'const Circle' as 'this' argument discards
    qualifiers [-fpermissive]
        c.setRadius(10.0);
        ^

WHY THIS WORKS AS AN ANSWER
------------------------------
area() is marked const and only reads radius, so calling it on the
const Circle c compiles fine. setRadius() is NOT const and modifies
radius, so calling it on the same const object is rejected by the
compiler -- exactly the chapter's own rule that a const object can
only call const-qualified member functions on itself, enforced as a
genuine compile error rather than a runtime check.