Re: Understanding CFMakeCollectable (was: EXC_BAD_ACCESS when -fobjc-gc)
Re: Understanding CFMakeCollectable (was: EXC_BAD_ACCESS when -fobjc-gc)
- Subject: Re: Understanding CFMakeCollectable (was: EXC_BAD_ACCESS when -fobjc-gc)
- From: Michael Ash <email@hidden>
- Date: Wed, 7 Oct 2009 18:01:56 -0400
On Wed, Oct 7, 2009 at 5:48 PM, Gabriel Zachmann <email@hidden> wrote:
> Thanks for your response.
>
>> No. CFRetain & CFRelease continue to work the same regardless of GC. That
>> is, the reference count field still exists, but Obj-C objects in GC start
>> life with a 0 retain count and -retain/-release/-retainCount/-autorelease
>> are no-op'd. CF objects still start life with a retain count of 1, and thus
>> you need to release them in order for them to participate in GC.
>
> So in other words, the purpose of CFMakeCollectable() is to decrease the
> ref-count to 0 in the GC world, and only there, is that correct?
The purpose is to decrement the ref-count by 1, not necessarily to 0,
in the GC world, and leave it alone in non-GC. To put it more simply,
CFMakeCollectable is just, if(GC) CFRelease(obj).
>> Since you do not want to release them in a ref counted environment,
>> CFMakeCollectable (and NSMakeCollectable) need to do nothing in ref-counted
>> (or your objects would vanish) and CFRelease (not -release) in a GC
>> environment.
>
> So, when I have old code like this:
>
> CFTypeRef obj = CFCreateType( ... );
> // do something with obj
> CFRelease( obj );
>
> I always need to transform it into this:
>
> CFTypeRef obj = CFCreateType( ... );
> // do something with obj
> if ([NSGarbageCollector defaultCollector] == NULL )
> CFRelease( obj );
> CFMakeCollectable( obj );
Since CFMakeCollectable does CFRelease under GC and nothing under
non-GC, your code, paraphrased, is like this:
"If we're not under garbage collection, then CFRelease the object. If
we're under garbage collection, then CFRelease the object."
This is obviously a bit redundant. Much simpler to just use CFRelease
with no conditionals. This is valid under both GC and non-GC.
> Or does CFMakeCollectable() always have to be used like this?
>
> CFTypeRef obj = CFMakeCollectable( CFCreateType( ... ) );
The major use of CFMakeCollectable, and its friend NSMakeCollectable
(which does the same thing and just has a different declared return
type), is to give reasonable "autorelease" semantics to
toll-free-bridged objects in dual-mode code. For example, where you
would previously have written something like:
NSString *str = [(id)CFStringCreateFoo() autorelease];
For dual-mode code you would write:
NSString *str = [NSMakeCollectable(CFStringCreateFoo()) autorelease];
And for GC-only code you could write:
NSString *str = NSMakeCollectable(CFStringCreateFoo());
Or simply:
NSString *str = (id)CFStringCreateFoo();
CFRelease(str);
For pure CF code that never touches Objective-C, no changes are required.
I imagine you've read it already, but if not, there's a good
discussion on CoreFoundation usage under garbage collection here:
http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/GarbageCollection/Articles/gcCoreFoundation.html
Mike
_______________________________________________
Cocoa-dev mailing list (email@hidden)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden