On Sunday, 15 September 2013 at 12:17:34 UTC, Dmitry Olshansky
wrote:
15-Sep-2013 12:39, monarch_dodra пишет:
On Saturday, 14 September 2013 at 06:18:02 UTC, Jonathan M
Davis wrote:
I have a few issues with ref counted.
First, if you *ever* place one in an array or an AA, then you
will leak.
It is NOT designed for simply incorporating reference, but for
deterministic finalization.
Another problem with built-in AAs...
Second, it has an elaborate postblit and opAssign. This is not
a big
issue in itself, but their sole existence does cause dmd to
generate
code that is sub-optimal. It also means it can't take the
"optimal"
route in a lot of algorithms (array has to emplace each element
individually, for example).
Seriously we could do a better job then that.. For instance
blit the whole data, then call postblits on that range.
Exception safety would be trickier to achieve. Even better one
day I now expect compiler to be able to know that e.g.
RefCounted is registered as ARC-capable type then the compiler
should elide call to ref-counting.
Well, if a struct has no elaborate opAssign, then "=" *is*
blit+postblit. And it doesn't get much more efficient than that.
If not, you have to pull out the memcpy, and things just go
downhill from there.
Finally, a RefCounted will implicitly cast to its payload. I
think this
is *horrible*. Before you know it, the Payload will have
"jettisoned"
its wrapper, and you'll be operating on your value-type
payload "raw".
For example:
void foo(T t);
RefCounted!T myRecCounted;
foo(myRefCounted); //Passes. Oops!
Well it does a copy. Which is IMO perfectly fine.
If you want to prevent that (at least in theory) you should
simply mark postblit of T as disabled.
It depends how you look at it. If the original intent was to have
a type with reference semantics, because your value type was so
damn big (eg, PRNG), then that would *not* be OK (and the
original point I was making that RefCounted is a bad contender
for "just" reference semantics).
If you are *trully* using it for deterministic finalization, then
yeah, it's OK.
