On 9 Aug 2011, at 12:05 PM, David Duncan wrote:
> On Aug 8, 2011, at 11:43 PM, Roland King wrote:
>
>> After a bit of googling I came across some posts which explained
>> Block_Copy() would treat a variable adorned with __attribute__((NSObject))
>> similarly to NSObjects and retain them. So changing the method signature to
>> this
>>
>> -(void)convert:(__attribute__((NSObject))CGImageRef)image
>> withBlock:(ImageResizedBlock)callbackBlock
>>
>> appears to do the right thing in my testing, the CGImageRef is CFRetain()ed
>> when the block is enqueued and I assume CFRelease()d later. I prefer this
>> syntax, as I find it self-documenting;
>>
>> I can't find anywhere apart from blog posts and the block specification
>> itself which says this works. Does anyone know if it's officially supported,
>> for gcc and clang and something that apple is likely to support going
>> forward into ARC
>
> The ARC specification defines a "retainable pointer" as including those
> pointers that have the __attribute__((NSObject)) decoration, so this should
> behave as expected under ARC.
> <http://clang.llvm.org/docs/AutomaticReferenceCounting.html#objects>
I am curious, though, in that CGImageRef is not toll-free bridged to a
Foundation class. I had the impression that the API contract was that only
bridged CF classes were guaranteed to implement reference-counting methods. The
clang document I read seemed to say that the purpose of
__attribute__((NSObject)) was to reassure the compiler that the argument type
was an alias for an NSObject subclass.
At the page you cite, I see:
> A retainable object pointer is either a null pointer or a pointer to a valid
> object. [Further qualifications.] ... Otherwise ARC does not enforce the
> Objective-C type system as long as the implementing methods follow the
> signature of the static type. It is undefined behavior if ARC is exposed to
> an invalid pointer.
>
> For ARC's purposes, a valid object is one with well-behaved retaining
> operations. Specifically, the object must be laid out such that the
> Objective-C message send machinery can successfully send it the following
> messages:
>
> • retain, taking no arguments and returning a pointer to the object.
> • release, taking no arguments and returning void.
> • autorelease, taking no arguments and returning a pointer to the
> object.
I believe the OP when he says it works in his setup, but I wonder if the API
promises it will always work.
Have I missed something?
— F
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