pass-by-ref semantics for structs (was Deque impl.)

[Rainer Schuetze]

On 01.02.2013 09:06, Dmitry Olshansky wrote:
> 01-Feb-2013 00:32, Rainer Schuetze пишет:
>>
>>
>> - how do you reference count immutable containers? You'll have to cast
>> the payload to mutable and assume it is not in read-only memory.
>
> Containers of immutables can have ref-count easily.
> And I'd say fully immutable containers are rare case and can rely on GC.

Do you want different implementations of the containers depending on the 
mutable/immutable/shared modifier? Would that be possible?

>
>>
>> - how do you do reference counting when accessing shared containers in
>> multiple threads?
>
> Atomic Inc/Dec.
>

Not good enough as shown in the discussion with Steven.

>> Consider clearing the last reference to a shared
>> reference counted object while another thread reads this reference. With
>> usual atomic operations on the reference count only:
>>
>>    1. the reading thread reads the payload pointer
>>    2. the writing thread reads the payload pointer, decrements the
>> reference count and frees the array
>>    3. the reading thread increments the reference count, but accesses
>> the deallocated array.
>
> Can't happen as reading thread got to have a reference and so does the
> writing thread, then refcount >= 2.

Can happen with a single shared reference accessed from two threads.