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copied!<p>There are two totally different answers depending on whether or not you're using ARC. Since the ARC environment is pretty straight forward, I'll just answer from a pre-ARC perspective. The main thing to understand is that all of the talk around property retain semantics is just a distraction from the main point: <strong>If you take ownership, you must relinquish it</strong>. Relinquishing ownership is done via <code>-release</code>. So, what you need to understand is "What counts as taking ownership?"</p> <p>By convention, any time you send a message to class that contains any of [<code>new</code>, <code>alloc</code>, <code>retain</code>, <code>copy</code>], you now own that object. The compiler generates setter methods for Objective C Properties depending on the ownership policies you specify. For example...</p> <pre><code>@property (..., retain) NSMutableArray *myArray; @synthesize myArray = _myArray; </code></pre> <p>This generates a method that looks like this<sup>1</sup>:</p> <pre><code>- (void)setMyArray:(NSMutableArray *)myArray { // This is oversimplified, just to illustrate the important point. // Again, this is NOT the way a setter would actually be synthesized. [_myArray release]; _myArray = [myArray retain]; } </code></pre> <p>So now, when you say something like <code>self.myArray = someArray</code>, you know you've sent a retain message to <code>someArray</code>, and you're responsible for releasing it later. The ideal way to do that is to say <code>self.myArray = nil</code>, because it releases the old value before retaining <code>nil</code>. Note that it's also perfectly safe to send that message, even if you never set anything to the <code>myArray</code> property, because sending messages to <code>nil</code> is okay. That's why it's common to <em>always</em> set owned properties to <code>nil</code> when you're done with them, regardless of how they're being used. </p> <p><hr></p> <p><sup>1</sup> For a study on how accessors should actually work, see <a href="http://cocoawithlove.com/2010/06/assign-retain-copy-pitfalls-in-obj-c.html" rel="nofollow">this article</a>.</p>

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