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When we assign memory to a class variable, should we release it or set it to nil in dealloc method? What is the best practice?

vikingosegundo
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Abhinav
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1 Answers1

11

Best practice:

[foo release]; // ensures that memory is released
foo = nil; // ensures that there is no dangling pointer to released memory

Other notes:

When you assign to a property declared to retain,

// in your .h
@property (retain) MyObject *foo;

// in your .m
self.foo = bar; // bar is retained; whatever foo previously pointed at is released

it will release what it was previously pointing at and retain the new object being assigned.

So, you can use:

self.foo = nil;

and it will release whatever foo was pointing at. However, if your property was not declared to have retain storage semantics, this will not implicitly release whatever foo was pointing at. Also, as Ryan pointed out, a property can be overridden to have side effects. For this reason, it is best to follow the pattern of always using:

[foo release];

To ensure that you don't have a dangling pointer to released memory, you can follow this up with:

foo = nil;

If you are not using properties with retain semantics, you need to release whatever was stored in the variable:

[foo release];

EDIT: Also see the following answer to another question that explains this:

iPhone - dealloc - Release vs. nil

Community
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Greg
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    It is safer to always use -release instead of the property setter, as setters can be overridden to have side-effects. You don't want side-effects happening in your dealloc method. – Ryan Nov 18 '10 at 23:53
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    Also, this blog post clarified a great deal for me http://www.red-sweater.com/blog/1423/dont-coddle-your-code – griotspeak Nov 19 '10 at 02:20
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    I've often found that explicitly assigning `nil` after the release can mask bugs. It's a bug if anyone is accessing the ivar afterwards, and assigning `nil` both allows this to succeed without any apparent error and prevents the usage of any of the memory debugging tools. – Andrew Jun 10 '11 at 07:42
  • Great answer - At first I did this: [self.myVar release]; self.myVar = nil; This caused a crash in the dealloc section of myVar, it was being released twice> However is I bypass the property, and simply do [myVar release] myVar = nil; All is fine. Why did the first method not work? – Marc Mar 05 '12 at 10:52