On 31/10/2010 03:56, Andrei Alexandrescu wrote:
On 10/30/2010 09:40 PM, Michel Fortin wrote:
On 2010-10-30 20:49:38 -0400, Andrei Alexandrescu
<[email protected]> 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
I would also go for "2. Constant-cost copy construction", but I can't
really make a case for it. I can only state that it seems to me that the
benefits in library simplification (both in implementation and API)
would be far more valuable than the drawbacks ("Makes value types
difficult to define").
It should be considered that those 5% types (large value types) will not
strictly need to have a refcounting+COW support to be usable in
containers and algorithms: just store pointers-to-the-value-type in the
container, instead of using the value type directly. (Basicly, use them
as reference types). So it's not the end of the world for some value
type if it doesn't implement refcounting+COW.
--
Bruno Medeiros - Software Engineer
