8

I'm afraid that asking this question may result in some downvotes, but after making some not satisfying research I decided to take a risk and ask more experienced people...

There are many questions here referring to some specific problems connected with the XCode Analayzer Tool. It seems to be very helpful solution. But I would like to ask you - as a beginner in iOS world - what kind of memory management stuff cannot be noticed by this tool.

In other words, are there any common memory management aspects, about which the iOS beginners should think "Oh, be careful with that, because in this case XCode Analyzer may not warn you about your mistake"...

For instance, I've found here Why cannot XCode static analyzer detect un-released retained properties? that:

(...)the analyzer can't reliably detect retain/release issues across method/library boundaries(...)

It sounds like a good hint to consider, but maybe you know about some other common issues...

Community
  • 1
  • 1
radekEm
  • 4,617
  • 6
  • 32
  • 45
  • 1
    Just because a tool can detect a leak does not mean you should not think about memory management less carefully. It's a tool, not replacement of careful thought. – Lie Ryan Mar 08 '13 at 15:15

1 Answers1

7

The analyzer is very good at finding the routine leaks that plague new programmers writing non-ARC code (failures to call release, returning objects of the wrong retain count, etc.).

In my experience, there are a couple of types of memory issues it does not find:

  • It cannot generally identify strong reference cycles (a.k.a. retain cycles). For example, you add a repeating NSTimer to a view controller, unaware that the timer maintains a strong reference to the view controller, and if you don't invalidate the timer (or do it in the wrong place, such as the dealloc method), neither the view controller nor the timer will get released.

  • It cannot find circular logic errors. For example, if you have some circular references where view controller A presents view controller B, which in turn presents a new copy of A (rather than dismissing/popping to get back to A).

  • It cannot find many non-reference counting memory issues. While it's getting better in dealing with Core Foundation functions, if you have code that is doing manual memory allocations (such as via malloc and free), the static analyzer may be of limited use. The same is true whenever you're using non-reference counting code (e.g. you use SQLite sqlite3_prepare_v2 and fail to call sqlite3_finalize).

I'm sure that's not a complete list of what it doesn't find, but those are the common issues I see asked about on Stack Overflow for which the static analyzer will be of limited help. But the analyzer is still a wonderful tool (it finds issues other than memory issues, too) and for those individuals not using ARC, it's invaluable.

Having said that, while the static analyzer is an under-appreciated first line of defense, you really should use Instruments to find leaks. See Locating Memory Issues in Your App in the Instruments User Guide. That's the best way to identify leaks.

Rob
  • 415,655
  • 72
  • 787
  • 1,044
  • Could you illuminate a bit more why it would be bad to `invalidate` your timer in dealloc? – Mark Bernstein Mar 08 '13 at 16:34
  • 3
    @MarkBernstein The `dealloc` is only called when there are no more strong references to the view controller. So, because the repeating timer is maintaining a strong reference to the view controller, it will never be called. Putting the `invalidate` in the `dealloc` is analogous to saying "when there are no more strong references to this view controller, go ahead and remove the timer's strong reference to this view controller." You generally put the `invalidate` in `viewWillDisappear` or something like that, depending upon the details of your app. – Rob Mar 08 '13 at 16:40