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Objective-C, Property memory leak in iOS App

Posted on 2010-11-15
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Last Modified: 2012-05-10
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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Question by:NV9
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Accepted Solution

by:
mad_mac earned 2000 total points
ID: 34144258
From my memory and my understanding.

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

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Will give you a reference count of 2 on the object, the alloc and init will return an object with the reference count set to 1, then assuming you have a retain on your property definition this will increase the count to 2.

The arrayWithContentsOfFile works as this returns an auto release reference, so when the current thread ends it will release the object.

The final example works because you are releasing the initial object allocation.

You should be able to validate this by looking at the retainCount property of the object.
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Author Closing Comment

by:NV9
ID: 34151959
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!
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