Struct with default ctor (Was: [dmd-beta] dmd 2.064 beta take 2)

[Idan Arye]

On Sunday, 19 May 2013 at 13:13:07 UTC, Andrei Alexandrescu wrote:
> On 5/19/13 9:11 AM, deadalnix wrote:
>> On Sunday, 19 May 2013 at 13:08:53 UTC, Andrei Alexandrescu 
>> wrote:
>>> Sounds like a race problem unrelated to null. With non-null 
>>> objects
>>> the race would have manifested itself in a different way, 
>>> perhaps even
>>> more pernicious.
>>>
>>
>> It is both a race condition and a null problem.
>
> No, it's just a race condition.
>
>> And having non nullable
>> type would have been a compile time error instead of days of 
>> debugging.
>
> No, the race condition would have stayed.
>
>
> Andrei

I believe this claim requires an explanation:

It's a good practice to initialize references(and all other types 
of variables) as soon as possible - and if possible, right away 
in the declaration. If that reference started as null, it's safe 
to assume it was not possible to initialized it at declaration, 
so it was intentionally initialized with null(if there was no 
initialization Java would scream at you).

Now, let's assume that reference was non-nullable. It is safe to 
assume that this change would not remove the obstacle that 
prevented that reference from being initialized right away in the 
declaration - so you still need to initialize it to something 
else - let's call that something `Nil`. Nil is an object that 
tells you that the reference has not yet been initialized.

So, in the original bug the reference "could be seen
as null where it was assumed everywhere to be set.". But now we 
don't have null - so that piece of code that thought the 
reference is null would now think that it is... what? The 
initialization value? No! Because we didn't switch from 
initializing the reference with null to initializing it with the 
later initialization value - we couldn't do it. Instead, we had 
to use Nil. So now, the reference 'could be seen as Nil where it 
was assumed everywhere to be set'...

Now, if you are lucky and your Nil is the better-kind-of-null 
that is used in dynamic object oriented languages, you'll get a 
nil exception - which is just as good as null exception. But if 
you are not so lucky, and you had to declare Nil as a blank 
object of the type of that reference, you are going to have 
logical bug, which is far worse than exceptions...