Which type of inheritance is more preferable?

Question

Is the only point in implementing the 2nd case is if I want to derive from a Collidable without being an Object? If that's the case, when is the 1st case ever favorable since the 2nd case offers more flexibility.

Both collidables have only a pure virtual function and Object is a base class for objects that can be drawn on the screen (in my case).

^Assuming I'm understanding the following code correctly (I'm not too sure TBH)

class Object
class CollidableObject : Object
class Actor : public CollidableObject

class Object
class Collidable
class Actor : public Object, public Collidable

Edit:

Based on Matt/Seth

class Object
class Collidable
class Clickable
class Trackable
class BlowUppable
class Actor : public Object, public Collidable, public Clickable, public Trackable,
              public BlowUppable

class SomeObjectThatIsTakenForGrantedThatEverythingElseIsInherited : public Actor

First example is the second case and second example is the first case. I guess thats the only use I see for the first case.

@Luchian
This is going to be a different question from the original since your reply was neither.

In this case, is there a difference changing an object from a is-a to a has-a relationship? In each case, to check for collisions, an object has to has to have a flag to know if collision should be checked. In your case, the member can be checked if its null or not, but in the derived case, the object itself tells whether it can collide or not. In an array/tree, I can either pass the derived object as a argument or pass hitbox as an argument using a get() method.

To be more in depth, I have another class - using the second case

class Hitbox : public Object, public Collidable

and the Actor class has it as a member

class Actor : public Object
{
     Hitbox *box;
};

Objects that have collision would have a hitbox instead and this represents your post accurately I think. But what's still getting me is that when I review your example again, does it mean that Hitbox should have a Collidable member instead?

class Hitbox
{
     Collidable *collision;
};

What I have:
An Actor holds a Hitbox which handles collision

What Hitbox should do:
Inherit Collidable or
Have Collidable as a member

Actor is already following your convention. Should Hitbox do the same?

Answer 1

I would do the second case, as Object and Collidable are cross-cutting concerns. If you go the CollidableObject route, you will probably end up with a combinatorial explosion of classes. It won't be long until you see a CollidableTrackableClickableBlowuppableObject.

Since Collidable is pure virtual, it is used in this case as an interface, so doesn't have many of the criticisms that argue against multiple inheritance. You are simply stating that an Actor implements the Collidable interface.

Answer 2

This is a perfect scenario for the strategy pattern. This is where the English language plays tricks on our minds and makes us think that CollidableObject is a valid object to have in this hierarchy. I say collision is more of a behavior than an object, so I'd go with neither of the two.

Prefer composition for this:

class Object
{
   CollidableBehavior* collisionBehavior;
}
class Actor : Object
{
   // collisionBehavior = new ActorCollision()
}

class AClassThatDoesntCollide
{
   // collisionBehavior = NULL
}