subclass to base class but not subclass pointer to base class pointer?

[John Colvin]

On Saturday, 20 April 2013 at 14:56:25 UTC, Namespace wrote:
> Why can D implicitly cast from the subclass to the base class, 
> but not implicitly from the subclasse pointer to the base class 
> pointer?
>
> This works: http://dpaste.1azy.net/30dd34a0
> This not: http://dpaste.1azy.net/ffacfd83
>
> Makes not much sense for me.

a pointer to class in D is actually a pointer to a pointer as 
classes are reference types. Polymorphism for pointers to 
pointers to objects is not allowed.
Consider this (ported from here 
https://groups.google.com/forum/?fromgroups=#!topic/comp.lang.c++/kNZ-ksORPwU):

class Base
{
     void doBaseThings();
}

class Dirv
{
     doDirvThings();
}

void AssignNewBaseTo(Base* basePtr)
{
         *basePtr = new Base;
}

void Oops()
{
         Dirv dirv;
         AssignNewBaseTo(&dirv);
         dirv.doDirvThings();         // runtime crash -- a Base 
object
                                      // being asked to do Dirv 
things.
}