On Aug 8, 2012, at 16:21 , Graham Cox <graham....@bigpond.com> wrote:
> This opens up a whole big can of worms about implementing copy.
>
> If a superclass implements copies using NSCopyObject, then any pointer ivars
> we add in a subclass need to be additionally -retained. But if the superclass
> implements copy by alloc/initing a new object and setting properties, then
> this extra step would lead to an over-retain and so leak.
>
> But how can we know what approach the superclass has taken? It's not
> documented and we don't have the source. Aren't we supposed to be immune from
> implementation details like this? Or is it just a case of after a while the
> experienced Cocoaista "just knows" what to do in each case? This is only just
> rearing its head for me after almost 10 years...
It seems to me that you need to think of 'copyWithZone:' as a kind of 'init…'
method, and it should therefore *not* use properties to change the instance
variables that belong to the subclass, but change them directly. This is
actually documented:
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Protocols/NSCopying_Protocol/Reference/Reference.html
> "Your options for implementing this protocol are as follows:
> […]
> • Implement NSCopying by invoking the superclass’s copyWithZone: when
> NSCopying behavior is inherited. If the superclass implementation might use
> the NSCopyObject function, make explicit assignments to pointer instance
> variables for retained objects."
In other words, after calling 'super copyWithZone:', you should immediately fix
(zero or overwrite) or retain your instance variables, if they're strong.
That doesn't work with ARC, of course. But this is also addressed in
documentation:
http://developer.apple.com/library/ios/#releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html
> "What do I have to do when subclassing NSCell or another class that uses
> NSCopyObject?
>
> Nothing special. ARC takes care of cases where you had to previously add
> extra retains explicitly. With ARC, all copy methods should just copy over
> the instance variables."
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