My Reference Safety System (DIP???)

[via Digitalmars-d]

On Monday, 2 March 2015 at 20:04:49 UTC, deadalnix wrote:
> On Monday, 2 March 2015 at 13:30:39 UTC, Zach the Mystic wrote:
>> On Monday, 2 March 2015 at 08:59:11 UTC, deadalnix wrote:
>>> On Monday, 2 March 2015 at 00:37:05 UTC, Zach the Mystic 
>>> wrote:
>>>>> I'm sure many inc/dec can still be removed.
>>>>
>>>> Do you agree or disagree with what I said? I can't tell.
>>>
>>> Yes, but I think this is overly conservative.
>>
>> I'm arguing a rather liberal position: that only in a very 
>> exceptional case do you need to protect a variable for the 
>> duration of a function. For the most part, it's not necessary. 
>> What am I conserving?
>
> I let the night go over that one. Here is what I think is the 
> best road forward :
>  - triggering postblit and/or ref count bump/decrease is 
> prohibited on borrowed.
>  - Acquiring and releasing ownership does.
>
> Now that we have this, let's get back to the exemple :
> class C {
>     C c;
>
>     // Make ti refconted somehow, doesn't matter. Andrei's 
> proposal for instance.
> }
>
> void boom() {
>     C c = new C();
>     c.c = new C();
>
>     foo(c, c.c);
> }
>
> void foo(ref C c1, ref C c2) {
>     // Here is where things get different. c1 is borrowed, so 
> you can't
>     // do c1.c = null before acquiring c1.c beforehand. That 
> means the
>     // compiler needs to get a local copy of c1.c, bump the 
> refcount
>     // to get ownership before executing c1.c = null and 
> decrease
>     // the refcount. The ownership expire when the function 
> returns
>     // so c2 is free when foo returns.
>     c1.c = null;
>     // c2 is dead.
> }
>
> The definition is a bit wonky ATM and most likely needs to be 
> refined, but I think this is the way forward with that issue. 
> It allow elision of a lot of ref increase/decrease by the 
> compiler, which is very important to get refcounting works fast.

Interesting approach. I will have to think about that. But I 
think it does not really work. Your example hides the fact that 
there are actually two types involved (or can be): an RC wrapper, 
and the actual class. foo() would need to take at least `c1` as 
the wrapper type `RC!C`, not `C` itself, otherwise it couldn't 
copy it. But that defeats the purpose of borrowing, that it 
neutralizes the actual memory management strategy; foo() should 
know whether `c1` is reference counted or not.