Calling object method passing superclass where subclass expected - c++

Question

For example i have three classes
class Shape
{
static bool collide(Box * a, Box * b)
static bool collide(Box * a, Sphere * b)
static bool collide(Sphere * a, Sphere * b)
};
class Box : public Shape{};
class Sphere : public Shape{};
and for example somewhere else i have this piece of code
Shape * a = new Box();
Shape * b = new Sphere();
Shape::collide(a, b);
The compiler raises and error.
How can i automatically downcast objects given as argument without adding too much logic?

Answer 1

The compiler can't cast the objects automatically, you will have to cast them manually, eg:
class Box;
class Sphere;
class Shape
{
virtual ~Shape(){}
static bool collide(Shape *a, Shape *b);
static bool collide(Box *a, Box *b);
static bool collide(Box *a, Sphere *b);
static bool collide(Sphere *a, Sphere *b);
};
class Box : public Shape{};
class Sphere : public Shape{};
bool Shape::collide(Shape *a, Shape *b)
{
if (Box *a_box = dynamic_cast<Box*>(a))
{
if (Box *b_box = dynamic_cast<Box*>(b))
return collide(a_box, b_box);
if (Sphere *b_sphere = dynamic_cast<Sphere*>(b))
return collide(a_box, b_sphere);
}
else if (Sphere *a_sphere = dynamic_cast<Sphere*>(a))
{
if (Sphere *b_sphere = dynamic_cast<Sphere*>(b))
return collide(a_sphere, b_sphere);
}
return false;
}
bool Shape::collide(Box *a, Box *b) { ... }
bool Shape::collide(Box *a, Sphere *b) { ... }
bool Shape::collide(Sphere *a, Sphere *b) { ... }
Shape * a = new Box();
Shape * b = new Sphere();
Shape::collide(a, b);
Needless to say, that can get a bit tedious as new shapes are added. You should instead add a single virtual method to Shape itself and let the derived classes override it to handle different types as needed, eg:
class Shape
{
virtual ~Shape(){}
virtual bool collide(Shape * b) { return false; };
};
class Box : public Shape{
bool collide(Shape * b) override {
if (Box *b_box = dynamic_cast<Box*>(b)) {
return ...;
}
if (Sphere *b_sphere = dynamic_cast<Sphere*>(b)) {
return ...;
}
return Shape::collide(b);
};
};
class Sphere : public Shape{
bool collide(Shape * b) override {
if (Sphere *b_sphere = dynamic_cast<Sphere*>(b)) {
return ...;
}
return Shape::collide(b);
};
};
Shape * a = new Box();
Shape * b = new Sphere();
a->collide(b);