Re: Casts that makes no sense (!)

[Steven Schveighoffer]

On Mon, 08 Mar 2010 09:38:08 -0500, Ary Borenszweig <a...@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