Question

The more I research this problem, the more confused I seem to be getting. This is a homework problem which involves augmenting code given us by our professor. I know the problem has to do with the const keyword, and some very confusing new applications of it.

There is a generic class, Object, from which several subclasses (Sphere, Cone, Polygon) inherit. Here are the classes in Object:

public: // computational members                                                
    // return t for closest intersection with ray                               
    virtual float intersect(const Ray &ray) const = 0;

    // return color for intersection at t along ray r                           
    virtual const Vec3 appearance(const World &w,
                                  const Ray &r, float t) const = 0;

    //The following function is added by me
    virtual const Vec3 normal(Vec3 p);
};

I added the final function, normal.

So within, for instance, the Sphere class, this is implemented thusly:

const Vec3 Sphere::normal(Vec3 p)
{
  return (p - d_center).normalize();
}

When I `make’, I get the following error:

Appearance.cpp: In member function ‘const Vec3 Appearance::eval(const World&, const Vec3&, const Vec3&, Vec3, int) const’:
Appearance.cpp:46: error: passing ‘const Object’ as ‘this’ argument of ‘virtual const Vec3 Object::normal(Vec3)’ discards qualifiers
make: *** [Appearance.o] Error 1

Can you help me understand why this is happening? Thanks for your help.

Answer 1

When the error-message refers to the ‘this’ argument of a method, it means (a pointer to) the object that you’re calling the method on. For example, in shape->normal(v), shape is the this argument.

To specify that a given method doesn’t modify its own object — its this argument — you need to append const to its declaration. So, change this:

virtual const Vec3 normal(Vec3 p);

to this:

virtual const Vec3 normal(Vec3 p) const;

to indicate that it’s “safe” to call normal(...) on a const object.

Similarly, change this:

const Vec3 Sphere::normal(Vec3 p)

to this:

const Vec3 Sphere::normal(Vec3 p) const