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

[Steven Schveighoffer]

On Thu, 31 Jan 2013 15:55:11 -0500, Rainer Schuetze <r.sagitario at gmx.de>  
wrote:

>
>
> On 31.01.2013 21:48, Steven Schveighoffer wrote:
>> On Thu, 31 Jan 2013 15:32:23 -0500, Rainer Schuetze <r.sagitario at gmx.de>
>> wrote:
>>
>>> - 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.
>>
>> The reference count part is not immutable.
>>
>> e.g.:
>>
>> struct refcountstruct
>> {
>>     int count;
>>     immutable(containerimpl) impl;
>> }
>
> What about immutable(refcountstruct)? Maybe only implicitely as field of  
> an immutable class.

You wouldn't be able to do that.  The reference count needs to be  
tail-immutable, meaning the reference count is not immutable, but the data  
it protects is.

Anticipating your answer, no, there isn't a good way to do this at the  
moment, it's something D definitely lacks.

>
>>
>>>
>>> - how do you do reference counting when accessing shared containers in
>>> multiple threads? 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.
>>
>> I would be concerned if a thread can get ahold of a reference counted
>> pointer without having the counter incremented already on its behalf.
>>
>
> You have to read the pointer to increment the counter to get hold of the  
> reference.

Right, but where do you get the original pointer from?  Wouldn't that  
reference have incremented the count?

-Steve