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Objective-C, Property memory leak in iOS App

I'm using instruments to check for leaks and I'm running into a leak that I've solved but can't seem to understand the theory behind it. I'm looking for someone to explain it to me. I've read http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html but it's not helping me understand.

Here's the scoop:

I had this line in a viewWillAppear: method.

 
self.catPiecesArray = [[NSArray alloc] initWithContentsOfFile:filePath]; // creates memory leak.

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I thought I was releasing it here:

 
- (void)viewDidUnload {
    self.catPiecesArray = nil;
}

- (void)dealloc {
    [self.catPiecesArray release];
}

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But this leaks!

So I tried this which does not leak:
 
self.catPiecesArray = [NSArray arrayWithContentsOfFile:filePath];

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This made me curious as to whether this would also work— which it does:

 
NSArray *testArr = [[NSArray alloc] initWithContentsOfFile:filePath];
self.catPiecesArray = testArr;
[testArr release];

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Like I said I don't understand why the later 2 examples work but my original code does not. Can anyone enlighten me, please. Thanks!
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mad_mac

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Thanks— that was helpful. I NSLogged out the ratainCounts at various points along the flow and that really helped clear the picture. Thanks for your help!