Casts that makes no sense (!)

[Ary Borenszweig]

Steven Schveighoffer wrote:
> 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

Ah, good one! I just tested it in Java and it also compiles, so I was wrong.

> 
>> 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.

Where's the typo?

> 
>> 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.

Yes, you'd always have to also check if an opCast is present.

> 
> -Steve