Re: __weak pointer collection firing prematurely???
Re: __weak pointer collection firing prematurely???
- Subject: Re: __weak pointer collection firing prematurely???
- From: John McCall <email@hidden>
- Date: Thu, 16 Aug 2012 15:10:57 -0700
On Aug 15, 2012, at 6:00 PM, Britt Durbrow wrote:
> On Aug 15, 2012, at 4:46 PM, John McCall <email@hidden> wrote:
>> On Aug 13, 2012, at 11:38 PM, Britt Durbrow wrote:
>>> OK, I think I've isolated the issue... basically, __weak, @public, and -fno-objc-arc don't like to play nice together. If you set a @public __weak instance variable (may also apply to stack variables, I haven't tested that yet) in a file compiled with -fno-objc-arc, and then access it in a file compiled with ARC turned on, the variable gets zeroed out.
>>>
>>> I have reduced it to a simple test project, and I'm going to file a RADAR.
>>
>> Please do; maybe we can find a way to warn about the assignment to a __weak object in non-ARC. That said, your code is buggy; we do *not* guarantee anything about the representation of a weak object, and it is not legal to access one in non-ARC code except by explicitly calling one of the specified runtime functions (and we don't encourage you to do that).
>>
>> John.
>
> Done. rdar://12101247
>
> The ARC documentation does not seem to indicate that the appropriate runtime function is not emitted by the compiler when -fno-objc-arc is in effect... and given that it's automatic *retain counting* that I thought I was turning off; and not other parts of the runtime system, it's kinda counterintuitive (well, it is to me at least :-) to have the compiler silently generate erroneous code - especially when the result is a value changing on a *read* operation!
Unless specifically indicated, everything in the ARC documentation is only enabled/valid under -fobjc-arc. ARC's __weak semantics are new to ARC. We did repurpose an existing "keyword" from GC, and it has roughly similar behavior, but that bites us here: __weak was always ignored when GC was disabled, so changing that is source-breaking.
> I would expect either: a) the compiler would always emit the correct runtime call for a __weak assignment no matter if ARC is on or off; or b) the compiler would throw an error when an attempt to access a __weak variable is made without ARC turned on (I don't think a warning is sufficient; given that it really does mess things up).
We'll see whether we can emit a warning. An error is unlikely.
John.
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