Ruling out arbitrary cost copy construction?

[Andrei Alexandrescu]

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.

Walter and I discussed the matter again today and we're on the brink of 
deciding that cheap copy construction is to be assumed. This simplifies 
the language and the library a great deal, and makes it perfectly good 
for 95% of the cases. For a minority of types, code would need to go 
through extra hoops (e.g. COW, refcounting) to be compliant.

I'm looking for more feedback from the larger D community. This is a 
very important decision that marks one of the largest departures from 
the C++ style. Taking the wrong turn here could alienate many 
programmers coming from C++.

So, everybody - this is really the time to speak up or forever be silent.


Andrei