Challenge: write a reference counted slice that works as much as possible like a built-in slice

[Andrei Alexandrescu]

On 11/12/21 10:15 PM, Timon Gehr wrote:
> On 12.11.21 23:09, Andrei Alexandrescu wrote:
>> On 2021-11-12 16:14, Timon Gehr wrote:
>>> On 12.11.21 18:44, H. S. Teoh wrote:
>>>> Yes.  So actually, this*could*  be made to work if the RC payload is
>>>> only allowed to be allocated from an RC allocator.  The allocator would
>>>> allocate n+8 bytes, for example, and return a pointer to offset 8 of 
>>>> the
>>>> allocated block, which is cast to whatever type/qualifier(s) needed.
>>>> Offset 0 would be the reference count.  The copy ctor would access the
>>>> reference count as *(ptr-8), which is technically outside the
>>>> const/immutable/whatever payload.  When the ref count reaches 0, the
>>>> allocator knows to deallocate the block starting from (ptr-8), which is
>>>> the start of the actual allocation.
>>>
>>> You still need language support. Reaching mutable data through an 
>>> immutable pointer violates transitivity assumptions.
>>
>> Indeed, but not indirectly. You can soundly access a pointer to 
>> mutable data if you use the immutable pointer as a key in a global 
>> hashtable.
> 
> Actually, you can't.
> 
> On 08.11.21 22:42, Andrei Alexandrescu wrote:
>> - work in pure code just like T[] does

Indeed if you add the purity requirement you can't, thanks.