Array Slices and Interior Pointers

[Alex Rønne Petersen]

On 11-12-2012 21:24, Dmitry Olshansky wrote:
> 12/11/2012 11:23 PM, Alex Rønne Petersen пишет:
>> On 11-12-2012 20:09, Dmitry Olshansky wrote:
>>> 12/11/2012 4:04 AM, Alex Rønne Petersen пишет:
>>>> http://xtzgzorex.wordpress.com/2012/12/11/array-slices-and-interior-pointers/
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Destroy.
>>>>
>>>
>>> Aside from the fact that I can use slices without GC just fine? :)
>>> The base pointers would then be either counted, released manually or
>>> implicitly as part of stack unwinding.
>>
>> Yes, in theory. But that's not how most idiomatic D code written today
>> works.
>>
>
> I'd mention that the most of idiomatic D code is agnostic with respect
> to the origin of slice. The major reason to use slices is to avoid
> allocations and thus the allocation scheme is not important up to the
> point of explicit copy.
>
> And at that point e.g. Phobos plays it safe and does everything that has
> to copy or incrementally build via GC. And it gets bashed for it every
> once in a while. To put simply it's because there is no concept of
> allocators in idiomatic D code _yet_.
>
> And separating slices and allocation mechanism behind them is the key of
> usability of slices as they stand. If we add stuff that makes them 50%
> more bulky and helps only a certain scheme of GC memory allocation we
> are screwed.

Then our current slice design is broken.

int[] arr;
arr.length = 1024; // guess where this memory comes from?

>
> Also what would direct operations with ptr field translate to in your
> scheme e.g.:
> arr.ptr = arr.ptr+x;
>
> where ptr+x is hidden by some function and not obvious to the compiler?

Exactly what you wrote. Remember, the ptr field doesn't change meaning.

>
> Same question with slicing a raw pointer - what will the base contain?
> (The pointer _might_ have been interior.)

Of course you have to take care not to slice an interior pointer and let 
the base pointer go out of scope.

>
>>>
>>> I personally think that managed VMs are going to have to emulate slices
>>> and pointers as an array object + one or pair of offsets. In fact it
>>> could be implemented as an abstract object with implementation depending
>>> on where you did get that pointer from.
>>>
>
>


-- 
Alex Rønne Petersen
alex at lycus.org
http://lycus.org