Why does this extremely simple operation not work?

[monarch_dodra]

On Tuesday, 12 February 2013 at 16:58:24 UTC, William wrote:
> This is an absurdly noobish question, but here goes.  I'm 
> learning D (I'm already reasonably comfortable with C and 
> Objective-C, so compiled languages are not new to me), and I 
> can't figure out why this super simple operation doesn't work.
>
> I have a parent and a child class, and while implicit casting 
> from child to parent works (function which takes parent will 
> accept instance of child), it does not work with pointers (and 
> yes, I understand that because objects are reference types a 
> MyObject* is really a pointer to a pointer since a MyObject is 
> a pointer).  A function that takes a Parent* as an argument 
> will not accept &myChild in its place without an explicit 
> cast(Parent*)&myChild.
>
> I feel like there's some fundamental property of the D 
> implementation that I'm not getting.  I was under the 
> impression an subtype's instance could *always always always* 
> be put in place of an instance of the super type.  Why are 
> pointers an exception?
>
> class Parent {}
> class Child : Parent {}
>
> void myFunc(Parent* obj) {
> 	writeln("got ", obj);
> }
>
> void main() {
> 	Child myChild = new Child();
> 	myFunc(&myChild);
> }
>
> referenceTest.d(11): Error: function referenceTest.myFunc 
> (Parent* obj) is not callable using argument types (Child*)
> referenceTest.d(11): Error: cannot implicitly convert 
> expression (& myChild) of type Child* to Parent*

You'd get the same behavior problem in C++. Where you can't pass 
a "Child**" when asking for a "Parent**". Long story short, if 
you could, you'd be able to place a parent instance inside a 
child instance, and mess everything up:

void myFunc(Parent* obj)
{
	static Parent par;
	if(!par) par = new Parent();
	obj = ∥
}
void main() {
	Child myChild = new Child();
	myFunc(&myChild);
	//Here, myChild is a reference to a Parent => Type system broken
}