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

[Rainer Schuetze]

On 31.01.2013 23:37, Steven Schveighoffer wrote:
> On Thu, 31 Jan 2013 17:31:38 -0500, Rainer Schuetze <r.sagitario at gmx.de>
> wrote:
>
>> Any reference that is accessible from multiple threads, e.g. a global
>> shared variable.
>>
>> shared(Container) allObjects;
>>
>> Without any thread having a reference to it, the payload's reference
>> count is 1. The reading thread above increments the counter when
>> making a local copy, but the writing thread decrements the counter
>> when assigning Container.init. If both run concurrently, with the
>> sequence 1 to 3 above, trouble is inevitable.
>
> Right, but isn't that an issue with having ANY shared variable that
> isn't protected by a lock?

True, as soon as you start mutating the variable. In case of simple 
reference/pointer without reference count, the change is atomic though 
(depending on the CPU or the implementation of "shared") and the reader 
thread just gets the old or the new pointer, but never an invalid pointer.

My main point actually was that a lot of people seem to think that 
reference counting with atomic incrementing/decrementing the counter 
(like it is used in COM) is thread safe in general. I think they are wrong.

>
> I was thinking you had passed the pointer in via a message or something.
>

I agree, message passing should not have this issue. It needs that 
single reference being accessed from different threads.