On Feb 21, 2009, at 12:39 PM, Stuart Malin wrote:
I've seen the idiom [[property retain] autorelease] used in getter
accessors, and just re-read the Memory Management Programming Guide
(MMPG) to understand this. Now I do, and see how the perhaps once
canonical approach of having a setter release-then-retain (if the
new value is not equal to the present value) can lead to references
persisting to a released object (e.g., if the setter is invoked
after the getter has issued references). In fact, the MMPG goes so
far as to say this about this approach:
"...because of the potential dangers of invalidating objects
prematurely, use of this technique should be used sparingly and well
documented."
Hmm. Google finds the Accessor Methods article of the Memory
Management Guide when I search for that phrase, but the article
doesn't actually contain the phrase. I guess it's been edited out.
In any case, I'm not aware of any such issue with any of the
techniques listed on the page now.
So, I am curious as to why the default (retain) behavior of
@synthesize does just this?
What makes you think it does? There is no concrete promise about how
synthesized accessors are implemented, except that they conform to the
declared property attributes (retain vs. copy vs. assign, etc.).
There is one place where some pseudo-code is used to _illustrate_ what
_might_ be going on inside a synthesized setter, but the documentation
clearly says that the actual implementation may differ.
And, is there a way to tell @synthesize to use one of the other
approaches, or do I need to make such a property @dynamic and handle
the setter/getter myself?
You should trust that Apple is synthesizing just about the most
appropriate setter imaginable, given the declared property
attributes. Unless you need to do something else within the setter --
that is, the setter has additional side effects -- just use @synthesize.
Cheers,
Ken
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