putting more smarts into a == b

[Steven Schveighoffer]

On Sun, 27 Sep 2009 10:32:29 -0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> 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?

 From the assembly, it appears that the compiler is comparing the reference  
values directly.  Which is not what you want, you want the opEquals from  
Object.

-Steve