Eliminate class allocators and deallocators?

[Steven Schveighoffer]

On Thu, 08 Oct 2009 08:31:00 -0400, Denis Koroskin <2korden at gmail.com>  
wrote:

> On Thu, 08 Oct 2009 15:48:56 +0400, Steven Schveighoffer  
> <schveiguy at yahoo.com> wrote:
>
>> On Thu, 08 Oct 2009 07:26:37 -0400, Denis Koroskin <2korden at gmail.com>  
>> wrote:
>>
>>> On Thu, 08 Oct 2009 14:48:19 +0400, Steven Schveighoffer  
>>> <schveiguy at yahoo.com> wrote:
>>>
>>>> On Wed, 07 Oct 2009 17:54:35 -0400, Denis Koroskin  
>>>> <2korden at gmail.com> wrote:
>>>>
>>>>> On Wed, 07 Oct 2009 23:00:06 +0400, Sean Kelly  
>>>>> <sean at invisibleduck.org> wrote:
>>>>>
>>>>>> auto x = cast(MyClass) malloc(MyClass.classinfo.init.length);
>>>>>
>>>>> I would expect a dynamic cast to occur at this line. Which will  
>>>>> either result in an access violation (since you are trying to cast a  
>>>>> garbage to an object) or result in a null being returned.
>>>>
>>>> malloc returns void *, so no dynamic cast.
>>>>
>>>> -Steve
>>>
>>> I know malloc returns void*. I didn't know you can hijack type system  
>>> that easily.
>>>
>>> But then, if no dynamic cast takes place why cast(Object)cast(void*)0  
>>> cannot be evaluated at compile time?
>>
>> Your message made me test it :)
>>
>> import std.stdio;
>>
>> void *foo()
>> {
>>      return cast(void*)0;
>> }
>>
>> void main()
>> {
>>      auto o = cast(Object)foo();
>>      writefln("here!");
>>      o.opEquals(o);
>> }
>>
>> outputs:
>>
>> here!
>> Segmentation fault
>>
>> So, no dynamic cast (dynamic cast would have looked at the classinfo of  
>> null, segfaulting before the output).
>>
>
> No, IIRC, casting null to Object is perfectly valid and returns null.

Oh yeah :)  I forgot.  Should have returned 1 instead of 0.

I did also verify via obj2asm that dynamic cast was not being called :)

>
> But you are right, casting void* to Object does a reinterpret cast  
> instead
> of dynamic cast. I'm not sure if that's a good design decision, though.

It's the only sane decision.  void * is really the developer's way of  
saying "I'll handle the typing from here, thanks" to the compiler.  So the  
compiler can't expect to interpret void * as anything special.  There's no  
deterministic way to detect an object anyways, so the compiler can't make  
any assumptions without the typesystem.

>
>> So I would say, the fact that compile time evaluation doesn't work is a  
>> bug maybe?
>>
>> -Steve
>
> Probably. Not only it doesn't work at compile time, it doesn't work at  
> all!
>
> void main()
> {
>      auto o = cast(Object)cast(void*)0; // Error: cannot cast void* to  
> object.Object
> }

Hm.. strange that my example compiles and yours does not.  I'd think it to  
be the same thing.  That definitely should be flagged as a bug.

I used dmd 2.033.

-Steve