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POCalling -retainCount Considered Harmful
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copied!<p><strong>Or, Why I Didn't Use <code>retainCount</code> On My Summer Vacation</strong></p> <p>This post is intended to solicit detailed write-ups about the whys and wherefores of that infamous method, <code>retainCount</code>, in order to consolidate the relevant information floating around SO.*</p> <ol> <li><p>The basics: What are the official reasons to not use <code>retainCount</code>? Is there ever any situation <em>at all</em> when it might be useful? What should be done instead?** Feel free to editorialize. </p></li> <li><p>Historical/explanatory: Why does Apple provide this method in the <a href="http://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intf/NSObject"><code>NSObject</code> protocol</a> if it's not intended to be used? Does Apple's code rely on <code>retainCount</code> for some purpose? If so, why isn't it hidden away somewhere?</p></li> <li><p>For deeper understanding: What are the reasons that an object may have a different retain count than would be assumed from user code? Can you give any examples*** of standard procedures that framework code might use which cause such a difference? Are there any known cases where the retain count is <em>always</em> different than what a new user might expect?</p></li> <li><p>Anything else you think is worth metioning about <code>retainCount</code>?</p></li> </ol> <hr> <p><sub>* Coders who are new to Objective-C and Cocoa often grapple with, or at least misunderstand, the reference-counting scheme. Tutorial explanations may mention retain counts, which (according to these explanations) go up by one when you call <code>retain</code>, <code>alloc</code>, <code>copy</code>, etc., and down by one when you call <code>release</code> (and at some point in the future when you call <code>autorelease</code>).</sub></p> <p><sub>A budding Cocoa hacker, Kris, could thus quite easily get the idea that checking an object's retain count would be useful in resolving some memory issues, and, lo and behold, there's a method available on every object called <code>retainCount</code>! Kris calls <code>retainCount</code> on a couple of objects, and this one is too high, and that one's too low, and what the heck is going on?! So Kris makes a post on SO, "What's wrong with my memory management?" and then a swarm of <bold>, <large> letters descend saying "Don't do that! You can't rely on the results.", which is well and good, but our intrepid coder may want a deeper explanation.</sub></p> <p><sub>I'm hoping that this will turn into an FAQ, a page of good informational essays/lectures from any of our experts who are inclined to write one, that new Cocoa-heads can be pointed to when they wonder about <code>retainCount</code>. </sub></p> <p><sub>** I don't want to make this too broad, but specific tips from experience or the docs on verifying/debugging retain and release pairings may be appropriate here.</sub></p> <p><sub> ***In dummy code; obviously the general public don't have access to Apple's actual code.</sub></p>

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