Casts that makes no sense (!)

[Steven Schveighoffer]

On Mon, 08 Mar 2010 09:38:08 -0500, Ary Borenszweig <ary at esperanto.org.ar>  
wrote:

> The following compiles:
>
> import std.stdio;
>
> interface I {
> }
>
> class A : I {
> }
>
> class B {
> }
>
> int main() {
>    I i = new A();
>    A a = cast(A) i;
>    B b = cast(B) i; // shouldn't compile
>    B c = cast(B) a; // shouldn't compile
>
>    writeln(a);
>    writeln(b);
>    writeln(c);
>
>    return 0;
> }
>
> But two lines there doesn't make sense:
>
> B b = cast(B) i;
>
> An instance of I can never be a B, so why the cast is allowed?

class C: B, I {}

I i = new C;
B b = cast(B)i; // should work

> B c = cast(B) a;
>
> An instance of A can never be an A, so why the cast is allowed?

Aside from the typo, I agree with you there, this should never be  
possible, because a derived class can not inherit both B and A.

> I think these casts should result in an error. This can prevent some  
> bugs.
>
> Java and C# work like that. You can't cast an object of instance of type  
> A to type B if both types are classes and B isn't a supertype or subtype  
> of A.

This rule makes sense.  On the other hand, I wonder how the new ability to  
cast to any type works in terms of classes.  Can you define cast!(A)() in  
B for instance?  I suppose you could make a caveat that either the class  
must inherit A or define an opCast for A.

-Steve