Collection callbacks and toll free bridging

[Adam R. Maxwell]

On Wednesday, April 02, 2008, at 08:06AM, "Christiaan Hofman" <cmhofman at gmail.com> wrote:
>
>On 2 Apr 2008, at 4:45 PM, Adam R. Maxwell wrote:
>
>>
>> On Apr 2, 2008, at 7:25 AM, Christiaan Hofman wrote:
>>> Hi all,
>>>
>>> I have some questions about how toll free bridging handles  
>>> callbacks for collections. I've searched the docs but came out  
>>> empty. Basically I have the following 2 questions:
>>>
>>> 1. Are Cocoa accessors doing anything besides some checking and  
>>> wrapping CF functions?
>>> 2. What callbacks are used by collections initialized using Cocoa?
>>>
>>> Let me make this a bit more explicit for the example of  
>>> CFMutableDictionaryRef/NSMutableDictionary. NSMutableDictionary is  
>>> documented to copy the keys. So a more concrete version of my  
>>> question is: are the keys copied by the Cocoa methods, or by the CF  
>>> key callbacks?
>>>
>>> Context: I need a dictionary for keys that do not conform to  
>>> NSCopying. So can I use just use CFDictionaryCreateMutable to  
>>> create the dictionary and then use Cocoa methods for the rest?
>>
>> No, NSMutableDictionary is weird about this.  You must not use - 
>> setObject:forKey: because the copying is done in that method.  Mike  
>> Ash pointed this out to me on cocoa-dev a couple of years ago:
>>
>> http://www.cocoabuilder.com/archive/message/cocoa/2006/5/13/163439
>>
>> My bug report on this was duped to his, and the conclusion is that  
>> if you use setObject:forKey:, it will create a copy of your key,  
>> then invoke your custom callbacks /on the copy/:
>>
>> http://www.cocoabuilder.com/archive/message/cocoa/2006/7/10/167259
>>
>> Once you've used CFDictionaryAddValue, you can safely use  
>> objectForKey: and other Cocoa methods, though.  I still think the  
>> setObject:forKey: behavior is wrong, but at least I can avoid it.
>>
>> -- 
>> adam
>
>Thanks Adam. Now my question was general, and this is just a specific  
>example.

Ah, okay; sounded as if you were interested in NSMutableDictionary in particular.

> Does anyone know the general story for any collection? Or is  
>the general story that I cannot trust toll free bridging for anything?

Only Apple knows, and I think all you'll get out of them is weasel words like the response  to Mike's bug:).  Within that reply, though, it sounds as if custom callbacks are guaranteed to be invoked and other behavior will be documented.  NSSet is documented as retaining its keys instead of copying, so it works as expected.  Likewise NSArray.  OTOH, even with custom callbacks, I wouldn't try passing e.g. an int to -[NSMutableArray addObject:]; it feels safer to use CFArray functions for that.

If you get a better response, please let me know!  I'm pretty disillusioned with toll-free bridging since it's not quite as transparent as the docs indicate.

-- 
adam