[phobos] Fwd: Re: Ruling out arbitrary cost copy construction?

[Sean Kelly]

On Oct 30, 2010, at 8:57 PM, Andrei Alexandrescu wrote:

> I am highly interested in the opinion of Phobos contributors in the matter of copy construction (just posted the message below).
> 
> Andrei
> 
> -------- Original Message --------
> Subject: Re: Ruling out arbitrary cost copy construction?
> Date: Sat, 30 Oct 2010 22:56:24 -0500
> From: Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org>
> Newsgroups: digitalmars.D
> 
> On 10/30/2010 09:40 PM, Michel Fortin wrote:
>> On 2010-10-30 20:49:38 -0400, Andrei Alexandrescu
>> <SeeWebsiteForEmail at erdani.org> said:
>> 
>>> On 10/30/10 2:24 CDT, Don wrote:
>>>> At the moment, I think it's impossible.
>>>> Has anyone succesfully implemented refcounting in D? As long as bug 3516
>>>> (Destructor not called on temporaries) remains open, it doesn't seem to
>>>> be possible.
>>>> Is that the only blocker, or are there others?
>>> 
>>> I managed to define and use RefCounted in Phobos. File also uses
>>> hand-made reference counting. I think RefCounted is a pretty good
>>> abstraction (unless it hits the bug you mentioned.)
>> 
>> I like the idea of RefCounted as a way to automatically make things
>> reference counted.
> 
> Unfortunately it's only a semi-automated mechanism.
> 
>> But like File and many similar ref-counted structs, it has this race
>> condition (bug 4624) when stored inside the GC heap. Currently, most of
>> Phobos's ref-counted structs are race-free only when they reside on the
>> stack or if your program has only one thread (because the GC doesn't
>> spawn threads if I'm correct).
>> 
>> It's a little sad that the language doesn't prevent races in destructors
>> (bug 4621).
> 
> I hope we're able to solve these implementation issues that can be seen
> as independent from the decision at hand.

Bug 4621 is tricky.  The appropriate fix is probably to have per-thread heaps, at least for data with finalizers.  It's a good direction to head anyway, so "stop the world" collections only occur when a shared allocation needs memory.