More radical ideas about gc and reference counting

[Rainer Schuetze via Digitalmars-d]

On 14.05.2014 12:56, "Marc Schütz" <schuetzm at gmx.net>" wrote:
> On Tuesday, 13 May 2014 at 19:50:52 UTC, Rainer Schuetze wrote:
>> class C { C next; }
>>
>> shared(C) list;
>>
>> C newC()
>> {
>>     return new C; // allocated on the local heap?
>
> Yes, because it doesn't say "new shared(C)".
>
>> }
>>
>> void prependToList(C c)
>> {
>>     synchronized(list)
>>     {
>>         C lst = cast(C) list;
>>         c.next = lst;  // anything special happening here?
>>         list = cast(shared(C)) c;  // and here?
>
> The casts are a problem. You're basically lying to the compiler, so this
> cannot work. Instead, it should be something like:
>
>      auto newC() {
>          return new isolated(C);
>      }
>
>      void prependToList(isolated(C) c) {
>          synchronized(list) {
>              // transfer to shared heap
>              // afterwards, c is no longer usable ("consumed")
>              shared(C) tmp = c.makeShared();
>              tmp.next = list;
>              list = tmp;
>          }
>      }


Sorry for the late reply.

I currently don't know of a feasible way to replace casting away 
"shared" if you want to do more than just "tmp.next = list;", for 
example call "bool check(C c);" on tmp or list? I can't implement this 
with a shared object, because any operations on "shared" tend to not 
exist (ok, maybe I could for this very simple example, so consider C 
contains a container to operate on). Also, the object is already locked, 
I should not have to do it again in a function if it only accepts a 
shared C.

How can I prepend something to the list that is not isolated? I guess I 
cannot, as it might create references to same object both shared and 
unshared.

My gut feeling tells me that this ownership handling through the type 
system might work, but it is very restrictive and does not really 
contribute to the first goal of D listed on the front page: "convenience".


>
> This uses the "isolated" concept suggested by deadalnix here:
> http://forum.dlang.org/thread/yiwcgyfzfbkzcavuqdwz@forum.dlang.org?page=1
>
>>     }
>> }
>>
>> void callMeFromMultipleThreads()
>> {
>>     prependToList(newC());
>> }
>