Question

After reading this question, i saw the answer by Naveen containing a link to this page, which basically says, that casting from Derived** to Base** is forbidden since could change a pointer to an pointer to a Derived1 object point to a pointer to a Derived2 object (like: *derived1PtrPtr=derived2Ptr).

OK, i understand this is evil …

But when casting Derived** to Base*const* this is not even possible, so whats the reason that this is not allowed anyway ?

Answer 1

First thing is that, if you really need to, you can cast any pointer type to any other pointer type. For instance, you can cast to void* as an intermediate step.

Second, with pointers-to-pointers, it’s not so much that there’s a reason to make particular cases hard as that there are no special rules to make any particular cases easy.

Basically, you have a pointer to X – where X in your case happens to be another pointer. Some X cases get special treatment (e.g. derived classes can implicitly cast to bases) – but your X is not one of them. It’s not the base class – it’s a pointer. There are no implicit casts defined for derived**, other than to void* – you can’t even implicitly cast derived** to void**.

I don’t think the const has much to do with it in this case, though I could be missing something.