Alright, then how do we solve refcounting of constant objects (see
Michel Fortin's question)? My solution involves casting away const and
keeping immutability information at runtime. Is that acceptable?
Andrei
On 10/31/10 5:32 PM, Shin Fujishiro wrote:
I think cheap copy construction is must. Since any accesses to sealed
range elements involve copy construction.
I read older discussions and found that they mostly focused on swap
and moveX. But the problem lies not only in the swap operation. It's
everywhere and move doesn't help.
Note that *every* access to an element of sealed range involves a copy
construction. When sorting a sealed range of BigInts (Array!BigInt),
even an innocent comparison creates two BigInt copies:
if (!less(r[i], r[p])) // Creates two temporary copies.
I think these copies can't be elided since original objects exist in a
container at the same time. Still the compared elements could be moved
out and then assigned back, but that's terribly awkward.
We mostly don't want to move out elements - just want to read them.
Sealed ranges are inherently copy intensive, so if you will put forward
the concept, copy cost of elements must be assumed to be cheap IMO.
Shin
Andrei Alexandrescu<[email protected]> 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<[email protected]>
Newsgroups: digitalmars.D
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
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