The document discusses memory management techniques in Objective-C, including manual reference counting (MRR) and automatic reference counting (ARC). It covers how MRR works by incrementing and decrementing retain counts, and issues like memory leaks and dangling pointers that can occur if the counts are not balanced. ARC is introduced as a way to automate MRR by having the compiler insert the appropriate memory management calls. The document also discusses retain cycles, where strong references between objects prevent deallocation, and ways to prevent leaks like breaking retains explicitly.

1. Study group
Memory management
2014/12/16
JoeHsieh

2. Outline
• Item 29 : Understand Reference Counting
• Item 30 : Use ARC to Make Reference Counting
Easier.

3. Manual Retain Release
(MRR)

4. Manual Retain Release(1/2)
• Tracks how many owners the object has.When it
reaches zero, the OS allowed to destroy it.

5. Manual Retain Release(2/2)
• increment
• alloc : Creates an object and claims ownership of it.
• retain : Claims ownership of an existing object
• copy : Copy an object and claims ownership of it
• decrement
• release : Relinquishes ownership of an object and destroy it
immediately
• autorelease : Relinquishes ownership of an object but defer its
destruction

6. In case of retain/release is
not balanced
• Forgets to release, retain cycle, etc… : memory leak
• Releases too many times, etc… : dangling
pointer(zombie)

7. Example
Memory Management in Property Accessors

8. CarStore

9. Crash?

10. Dangling pointer and
memory leak.
setter will not retain inventory
superstore forget to release

11. Fixes it, but it’s still memory
leak?

12. Finally…

13. Automatic Reference Counting
(ARC)
In order to solve the verbose problem of MMR, savior is
coming!
@property (strong, nonatomic) NSMutableArray *inventory;
For property accessor

14. ARC
• Complier will add retain, release and autorelease
for you automatically.You cannot add these by
yourself.
• Memory leak and dangling pointer exist still.
• ARC is only for Objective-C.That means memory
management of CoreFundation and C is still
MRR.(malloc, free, CFRetain, CFRelease)

15. Retain Cycle

16. How to prevent leak ?
• Pointer to any parent must not be retained.

17. Retain Cycle Example
• strong delegate
• retain variables in block

18. How to solve leak ?
• Breaks retain relationship explicitly.

19. Retain Cycle Example
• Timer
• NSURLConnectionDelegate

20. Retain Cycle Demo
PrinterVC
Printer

21. References
• http://rypress.com/tutorials/objective-c/memory-
management
• http://www.cocoawithlove.com/2009/07/rules-to-
avoid-retain-cycles.html
• https://www.mikeash.com/pyblog/friday-
qa-2010-04-30-dealing-with-retain-cycles.html