On Saturday, 30 August 2014 at 03:54:41 UTC, Orvid King wrote:
On 8/29/2014 2:52 PM, "Marc =?UTF-8?B?U2Now7x0eiI=?=
<[email protected]>" wrote:
On Friday, 29 August 2014 at 19:01:51 UTC, Andrei Alexandrescu
wrote:
On 8/29/14, 3:53 AM, "Marc Schütz" <[email protected]>" wrote:
Jacob Carlborg just recently brought this up in another
thread. Isn't it
kind of consensus that calling a destructor from the GC is
not a good
idea because of the restrictions that apply in this context?
Andrei even
wanted to deprecate destructors for classes because of this.
Maybe a
better direction would be to separate the concepts of
destruction and
finalization, and introduce two kinds of "destructors" for
them.
I think we need to stay with what we have. Adding a distinct
kind of
destructor might be interesting. -- Andrei
Our idea was that an additional destructor (let's call it a
finalizer)
would be helpful because it is backward compatible. The
compiler could
make some validity checks on it, at the least make it nothrow,
maybe
@nogc (but I believe we can relax this restriction), pure (?).
Disallowing access to references (because they could pointer
to already
destroyed objects) is unfortunately not feasible, because we
can't
distinguish GC pointers from other ones. To avoid the need for
code
duplication, finalizers could always be called implicitly by
destructors
(assuming everything that is allowed in finalizers is also
permitted in
destructors).
Calling destructors from the GC could later be phased out. It
is
technically not a breaking change, because there never was a
guarantee
that they'd be called anyway.
I would say that all of those restrictions, except for nothrow,
are dependent on the current GC implementation. It is possible
to write the GC in such a way that you can do GC allocations in
a destructor, as well as access any GC references you want. The
only thing with the GC references is that there's no way to
guarantee that the referenced objects won't have already had
their destructor called when the current destructor is being
called.
Hmmm... could the GC zero those references that it already
destroyed, before calling the finalizer? Don't know how this
would affect performance, but it would only be necessary if a
finalizer exists (could even be restricted to those references
that are accessible from non-trivial finalizers, i.e. if a struct
has GCed pointers and an embedded struct member with a finalizer,
but no finalizer of its own, the compiler would probably generate
one that only calls the member's finalizer, but this would have
no access to its parent's pointers).
You're right that many of the restrictions are only necessary
because of the current GC implementation. Even the fact that
garbage collection can happen in any thread could theoretically
be changed. Even more complicated: I can imagine that with the
upcoming allocator work there could be several different GC
implementations, even used in parallel in the same program, each
with different capabilities and restrictions. It's clear that
this requires coordination.
