Destroying structs (literally)

[Orvid King via Digitalmars-d]

On 8/30/2014 4:22 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm at gmx.net>" 
wrote:
> On Saturday, 30 August 2014 at 03:54:41 UTC, Orvid King wrote:
>> On 8/29/2014 2:52 PM, "Marc =?UTF-8?B?U2Now7x0eiI=?=
>> <schuetzm at gmx.net>" wrote:
>>> On Friday, 29 August 2014 at 19:01:51 UTC, Andrei Alexandrescu wrote:
>>>> On 8/29/14, 3:53 AM, "Marc Schütz" <schuetzm at gmx.net>" wrote:
>>>>> Jacob Carlborg just recently brought this up in another thread.
>>>>> Isn't it
>>>>> kind of consensus that calling a destructor from the GC is not a good
>>>>> idea because of the restrictions that apply in this context? Andrei
>>>>> even
>>>>> wanted to deprecate destructors for classes because of this. Maybe a
>>>>> better direction would be to separate the concepts of destruction and
>>>>> finalization, and introduce two kinds of "destructors" for them.
>>>>
>>>> I think we need to stay with what we have. Adding a distinct kind of
>>>> destructor might be interesting. -- Andrei
>>>
>>> Our idea was that an additional destructor (let's call it a finalizer)
>>> would be helpful because it is backward compatible. The compiler could
>>> make some validity checks on it, at the least make it nothrow, maybe
>>> @nogc (but I believe we can relax this restriction), pure (?).
>>> Disallowing access to references (because they could pointer to already
>>> destroyed objects) is unfortunately not feasible, because we can't
>>> distinguish GC pointers from other ones. To avoid the need for code
>>> duplication, finalizers could always be called implicitly by destructors
>>> (assuming everything that is allowed in finalizers is also permitted in
>>> destructors).
>>>
>>> Calling destructors from the GC could later be phased out. It is
>>> technically not a breaking change, because there never was a guarantee
>>> that they'd be called anyway.
>>
>> I would say that all of those restrictions, except for nothrow, are
>> dependent on the current GC implementation. It is possible to write
>> the GC in such a way that you can do GC allocations in a destructor,
>> as well as access any GC references you want. The only thing with the
>> GC references is that there's no way to guarantee that the referenced
>> objects won't have already had their destructor called when the
>> current destructor is being called.
>
> Hmmm... could the GC zero those references that it already destroyed,
> before calling the finalizer? Don't know how this would affect
> performance, but it would only be necessary if a finalizer exists (could
> even be restricted to those references that are accessible from
> non-trivial finalizers, i.e. if a struct has GCed pointers and an
> embedded struct member with a finalizer, but no finalizer of its own,
> the compiler would probably generate one that only calls the member's
> finalizer, but this would have no access to its parent's pointers).
>
> You're right that many of the restrictions are only necessary because of
> the current GC implementation. Even the fact that garbage collection can
> happen in any thread could theoretically be changed. Even more
> complicated: I can imagine that with the upcoming allocator work there
> could be several different GC implementations, even used in parallel in
> the same program, each with different capabilities and restrictions.
> It's clear that this requires coordination.

The references issue can be gotten around by marking an allocation that 
needs finalization as if it were alive. It does mean however that 
finalizable allocations will live through more than one collection. I 
believe this is how .Net currently handles them, as I don't remember 
anything in the spec about restrictions on what's referenced in 
destructors, nor have I had issues referencing otherwise dead 
allocations in them.