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

[Andrei Alexandrescu]

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.