putting more smarts into a == b

[Andrei Alexandrescu]

Frank Benoit wrote:
> Andrei Alexandrescu schrieb:
>> Frank Benoit wrote:
>>> Andrei Alexandrescu schrieb:
>>>> Consider two objects a and b with a of class type. Currently, the
>>>> expression a == b is blindly rewritten as a.opEquals(b). I argue it
>>>> should be rewritten into a call to an (imaginary/inlined) function
>>>> equalObjects(a, b), with the following definition:
>>>>
>>>> bool equalObjects(T, U)(T a, U b) if (is(T == class))
>>>> {
>>>>     static if (is(U == class))
>>>>     {
>>>>         if (b is null) return a is null;
>>>>         if (a is null) return b is null;
>>>>     }
>>>>     else
>>>>     {
>>>>         enforce(a !is null);
>>>>     }
>>>>     return a.opEquals(b);
>>>> }
>>>>
>>>> This hoists the identity test outside the opEquals call and also deals
>>>> with null references. What do you think?
>>>>
>>>>
>>>> Andrei
>>> What about interfaces?
>> Good question! What do they do now? I ran this:
>>
>> interface A {}
>> class Widget : A {}
>>
>> void main() {
>>     auto a = cast(A) new Widget;
>>     A b = null;
>>     writeln(a == b);
>>     writeln(b == a);
>> }
>>
>> To my surprise, the program printed false twice. If I replace A with
>> Widget inside main, the program prints false then crashes with the
>> mythical segfault :o).
>>
>> So how are interfaces compared?
>>
>>
>> Andrei
> 
> Hm, i would have expected it not to compile, because A does not have
> opEquals.
> 
> In DWT, I cast always first to Object.
> Java> if( intf1.equals(intf2) ){
> D1.0> if( ((cast(Object)intf1).opEquals( cast(Object)intf2 )){

I think in D the cast is inserted automatically. Walter?

Andrei