Slides from Automatic Reference Counting by Giuseppe Arici @ Pragma Night

1. Automatic
Reference Counting
Giuseppe Arici
Pragma Night @ Talent Garden

2. Memory Management
• Manual Reference Counting
• Higher level abstraction than malloc / free
• Straightforward approach, but must adhere to conventions and rules
• Garbage Collection
• Only available for OS X 10.5+, but depracated from 10.8+
• Not available on iOS due to performance concerns
• Automatic Reference Counting (ARC)
• Makes memory management the job of the compiler (and runtime)
• Available for: partially iOS 4+ or OS X 10.6+ / fully iOS 5+ or OS X 10.7+
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3. Manual Reference Counting
Reference Counting
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4. Manual Reference Counting
(Only) Objective-C objects are reference counted:
• Objects start with retain count of 1 when created
• Increased with retain
• Decreased with release, autorelease
• When count equals 0, runtime invokes dealloc
1 2 1 0
alloc retain release release
dealloc
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5. Objects you create
For objects you create with [[SomeClass alloc] init] or
[myInstance copy] (without autoreleasing):
• Retain should not need to be called
• Release when you are done using it in the {code block}
- (void)someMethod {
NSObject *localObject = [[NSObject alloc] init];
_instanceArray = [[NSArray alloc] initWithObjects:localObject, nil];
[localObject release];
}
- (void)dealloc {
[_instanceVariable release];
[super dealloc];
}
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6. Objects you don’t create
For objects you don’t create (e.g. get from methods):
• Retain only when saving to instance (or static) variable
• Release only if you retained it by saving it (as in above case)
- (void)someMethod {
id localObject = [anArray objectAtIndex:0];
_instanceVariable = [localObject retain];
}
- (void)dealloc {
[_instanceVariable release];
[super dealloc];
}
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7. Autorelease
What if you create an object and you are returning it from a
method, how would you be able to release it?
- (NSArray *)objects {
NSArray *localArray = [[NSArray alloc] init]; ✇
}
return localArray;
Leak !
☠
- (NSArray *)objects {
NSArray *localArray = [[NSArray alloc] init];
return [localArray release];
} Crash !
- (NSArray *)objects {
NSArray *localArray = [[NSArray alloc] init]; ☺
return [localArray autorelease];
} Enjoy !
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8. Autorelease
• Instead of explicitly releasing something, you mark it for a
later release
• An object called autorelease pool manages a set of
objects to release when the pool is released
• Add an object to the release pool by calling autorelease
@autoreleasepool {
// code goes here
}
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
// code goes here
[pool release];
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9. Autorelease
• Autorelease is NOT a Garbage Collector !
It is deterministic ⌚
• Objects returned from methods are understood to be
autoreleased if name is not in implicit retained set
(alloc/new/copy/mutableCopy/init)
• If you spawn your own thread, you’ll have to create your
own NSAutoreleasePool
• Stack based: autorelease pools can be nested
Friday Q&A 2011-09-02: Let's Build NSAutoreleasePool
http://www.mikeash.com/pyblog/friday-qa-2011-09-02-lets-build-nsautoreleasepool.html
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10. The Memory Management Rule
Everything that increases the retain count with
alloc, [mutable]copy[WithZone:], new or retain
is in charge of the corresponding [auto]release.
From C++ to Objective-C
http://pierre.chachatelier.fr/programmation/objective-c.php
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11. Automatic Reference Counting
“Automatic Reference Counting (ARC) in Objective-C
makes memory management the job of the compiler. By
enabling ARC with the new Apple LLVM compiler, you
will never need to type retain or release again,
dramatically simplifying the development process, while
reducing crashes and memory leaks. The compiler has a
complete understanding of your objects, and releases
each object the instant it is no longer used, so apps run
as fast as ever, with predictable, smooth performance.”
(Apple, “iOS 5 for developers” – http://developer.apple.com/technologies/ios5)
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12. Automatic Reference Counting
• The Rule is still valid, but it is managed by the compiler
• No more retain, release, autorelease nor dealloc
• New lifetime qualifiers for objects, which includes zeroing
weak references (only available on iOS 5+ & OS X 10.7+)
• Apple provides a migration tool which is build into Xcode
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13. ARC Settings: Project | File
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14. Requirements
• iOS 5+ & OS X 10.7+ Full Support :)
• iOS 4 & OS X 10.6 (64-bit) No Runtime :(
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15. How it works !?
ARC consists of 2 main components
Frontend Compiler Optimizer
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16. ARC Frontend Compiler
• For every local variable inserts retain and
[auto]release appropriately in the block scope { }
• For every owned instance variable inserts release
appropriately in the dealloc method
• Call the [super dealloc] in dealloc method
• Generates errors when a variable ownership is
not set correctly
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17. Method Family / Ownership
Method Name Family Object Ownership Action
alloc/new/copy/mutableCopy/init Create and have ownership of it
retain Take ownership of it
release Relinquish it
dealloc Dispose of it
Everything Else No ownership !
alloc/new/copy/mutableCopy/init “ The Implicit Retained Set ”
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18. ARC Frontend Compiler 1/3
- (void)someMethod {
Foo *foo = [[Foo alloc] init];
[foo bar];
}
- (void)someMethod {
Foo *foo = [[Foo alloc] init];
[foo bar];
[foo release];
}
- (void)someMethod {
Foo *foo = [[Foo alloc] init];
[foo bar];
obj_release(foo);
}
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19. ARC Frontend Compiler 2/3
- (void)someMethod {
Foo *localFoo = [self foo];
[localFoo bar];
self.foo = [Builder newFoo];
[localFoo bar];
☠
}
Crash !
- (void)someMethod {
Foo *localFoo = objc_retain([self foo]);
[localFoo bar];
self.foo = [Builder newFoo];
[localFoo bar]; ☺
}
objc_release(localFoo);
Enjoy !
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20. ARC Frontend Compiler 3/3
- (Foo *) foo
{
return _foo;
}
- (Foo *) foo
{
return [[_foo retain] autorelease];
}
- (Foo *) foo
{
return objc_retainAutoreleaseReturnValue(_foo);
}
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21. ARC Optimizer
• Optimize the retain and release statements by
removing them if they are inserted multiple times
by the ARC Frontend Compiler
• Ensures that performance is not affected by calling
retain and release multiple times
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22. ARC Optimizer 1/2
retain
autorelease
retain
release
stack
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23. ARC Optimizer 1/2
retain
release
stack
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24. ARC Optimizer 2/2
- (Foo *) foo called method
{
return objc_retainAutoreleaseReturnValue(_foo);
}
Autorelease Pool
- (void)someMethod {
Foo *f = objc_retainAutoreleasedReturnValue([self foo]);
[f bar];
self.foo = [Builder newFoo];
[f bar];
objc_release(f);
} caller method
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25. Variable Ownership Qualifiers
• __strong (default)
• __weak
• __unsafe_unretained
• __autoreleasing
Only For Object Type Variables !
( id | AnyClass : NSObject )
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26. Retain Cycles 1/5
Parent Child
__strong
_view
+1 +1
_delegate
__strong
Controller View
as View Delegate
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27. Retain Cycles 2/5
Parent Child
__strong
_view
= +1
_delegate
__unsafe_unretained
Controller View
as View Delegate
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28. Retain Cycles 3/5
Parent Child
__strong
_view
= +1
_delegate
__weak
Controller View
as View Delegate
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29. Retain Cycles 4/5
dealloc
Parent Child
release
☠
_view
New with ARC in
iOS 5+ & OS X 10.7+
=
_delegate
__weak
Controller View ☺nil
as View Delegate
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30. Retain Cycles 5/5
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31. __strong
/* ARC */
{
id __strong obj1 = [[NSObject alloc] init];
id obj2 = [[NSObject alloc] init];
}
/* not-ARC */
{
id obj1 = [[NSObject alloc] init];
[obj1 release];
}
• Strong reference for the object
• Default for ‘id’ and Object Type Variables !
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32. __weak
/* ARC */
{
id __strong obj1 = [[NSObject alloc] init];
id __weak obj2 = obj1;
}
/* not-ARC */
{
id obj1 = [[NSObject alloc] init];
id obj2 = obj1;
[obj1 release];
}
• No ownership of the object
• When discarded, the variable is assigned to nil
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33. __unsafe_unretained
/* ARC */
{
id __unsafe_unretained obj = [[NSObject alloc] init];
}
Assigning retained object to unsafe_unretained variable; object will be
released after assignment [-Warc-unsafe-retained-assign]
• UNSAFE !
• You must take care of the variables manually
• __weak only for iOS 5+ / OSX 10.7+
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34. __autoreleasing
@autoreleasepool
{
id __strong obj = [NSMutableArray array];
}
- (BOOL) performWithError:(NSError **)error;
- (BOOL) perfomrWithError:(NSError * __autoreleasing *)error;
• Pointer to ‘id’ or to ‘Object Type Variables Pointer’ is
qualified with __autoreleasing as default
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35. Variables Initialization
/* ARC */
id __strong obj1;
id __weak obj2;
id __unsafe_unretained obj3;
id __autoreleasing obj4;
/* not-ARC */
id __strong obj1 = nil;
id
id
__weak obj2 = nil;
__unsafe_unretained obj3;
☠
id __autoreleasing obj4 = nil; Crash !
• Any variables that are qualified with __strong,
__weak and __autoreleasing are initialized with nil
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36. Property
Object Property Modifier Variable Ownership Qualifier
assign __unsafe_unretained
unsafe_unretained __unsafe_unretained
weak __weak
retain __strong
strong __strong
copy __strong *
* Note: new copied object is assigned
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37. Property
• Same ownership rules as instance variables
• Manually declared instance variables has to have
the same ownership qualifier as the property
• Copy: copies the object by using copyWithZone
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38. Coding With ARC Rules
What is forbidden ?
What can be done but with care ?
What is mandatory ?
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39. Rule 01/12
• Forget about using retain, release,
retainCount and autorelease
{
id obj1 = [[NSObject alloc] init];
/* ... */
[obj1 release];
}
error: ARC forbids explicit message send of 'release'
[obj release];
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40. Rule 02/12
• Forget about using NSAllocateObject,
NSDeallocateObject, NSZone & Co.
/* NSObject.h */
/***********!
Object Allocation / Deallocation! *******/
!
FOUNDATION_EXPORT id
NSAllocateObject(Class, NSUInteger, NSZone *)
NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
error: 'NSAllocateObject' is unavailable:
not available in automatic reference counting mode
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41. Rule 03/12
• Forget about using NSAutoreleasePool
Use @autoreleasepool { ... } Instead
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
/* ... */
[pool drain]; // [pool release];
error: 'NSAutoreleasePool' is unavailable:
not available in automatic reference counting mode
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42. Rule 04/12
• Forget about calling super dealloc
explicitly
- (void) dealloc
{
free(buffer);
[super dealloc];
}
error: ARC forbids explicit message send of 'dealloc'
[super dealloc]; not available in automatic reference counting mode
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43. Rule 05/12
• Object Type Variables Cannot Be
Members of struct or union in C
struct Data
{
NSMutableArray *array;
};
error: ARC forbids Objective-C objs in structs or unions
NSMutableArray *array;
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44. Rule 06/12
• Exceptions use must be exceptional
• For performance purpose, ARC code leaks on exceptions
• In Obj-C ARC is not exception-safe for normal releases by default
• -fobjc-arc-exceptions / -fno-objc-arc-exceptions: enable / disable
• In Obj-C++ -fobjc-arc-exceptions is enabled by default
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45. Rule 07/12
• IBOutlet must be declared as weak
• Outlets should be defined as declared properties
• Outlets should generally be weak
• Top-level objects (or, in iOS, a storyboard scene) should be strong
Resource Programming Guide
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46. Rule 08/12
• Be careful with performSelector
#pragma clang diagnostic push
#pragma clang diagnostic ignored
"-Warc-performSelector-leaks"
[self performSelector:@selector(someMethod)];
#pragma clang diagnostic pop
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47. Rule 09/12
• Follow the naming rule for methods related
to object creation: implicit retained set
• If begins { alloc/new/copy/mutableCopy }, the caller has ownership
• If begins { init }, the method has to: *
• be an instance method
• return an object of type ‘id’ or an object of type of its [super|sub]class
• return an object not registered in autoreleasepool: the caller has ownership
- (id)init...;
* A special case: “ + (void) initialize ”
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48. False Positive
-(NSString*) copyRightString;
-(NSString*) copyRightString NS_RETURNS_NOT_RETAINED;
• NS_RETURNS_RETAINED
• NS_RETURNS_NOT_RETAINED
This macro is ONLY to be used in exceptional circumstances, not to
annotate functions which conform to the Cocoa naming rules.
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49. Rule 10/12
• Switch cases must be wrapped in scope
with { ... }
switch (value)
{
case 0:
{
NSObject *obj= [[NSObject alloc] init];
NSLog(@"obj for value 0: %@", obj);
}
break;
}
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50. Rule 11/12
• The old style singleton which redefines
retain and release is not ARC compatible
• Singletons should be redefined with
dispatch_once
static Foo *sharedInstance;
+ (id)sharedInstance {
static dispatch_once_t predicate;
dispatch_once(&predicate, ^{
sharedInstance = [[Foo alloc] init];
});
return sharedInstance;
}
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51. Rule 12/12
• 'id' and 'void *' Have to Be Cast Explicitly
id object = [[NSObject alloc] init];
void *pointer = object;
Error: implicit conversion of a non-Objective-C pointer type 'void *'
to 'id' is disallowed with ARC
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52. ( _bridge cast )
id object1 = [[NSObject alloc] init];
void *pointer = (__bridge void *)object1;
id object2 = (__bridge id)pointer;
• More dangerous than an __unsafe_unretained
qualified variable !
• You have to manage ownership of the object
yourself carefully or it crashes
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53. ( __bridge_retained cast )
/* ARC */
id object = [[NSObject alloc] init];
void *pointer = (__bridge_retained void *)object;
/* not-ARC */
id object = [[NSObject alloc] init];
void *pointer = object;
[(id)pointer retain];
• From ARC to Core Foundation
• __bridge_retained cast will retain the object just
after the assignment is done
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54. ( __bridge_transfer cast )
/* ARC */
void *pointer = &bytes;
id object = (__bridge_transfer id)pointer;
/* not-ARC */
id object = (id)pointer;
[object retain];
[(id)pointer release];
• From Core Foundation to ARC
• __bridge_transfer cast will release the object just
after the assignment is done
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55. Toll-Free Bridging
/* NSObject.h */
// After using a CFBridgingRetain on an NSObject, the caller
// must take responsibility for calling CFRelease at an
// appropriate time
NS_INLINE CF_RETURNS_RETAINED CFTypeRef
CFBridgingRetain(id X)
{
return (__bridge_retained CFTypeRef)X;
}
NS_INLINE id
CFBridgingRelease(CFTypeRef CF_CONSUMED X)
{
return (__bridge_transfer id)X;
}
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56. ARC MACRO
// define some LLVM3 macros if the code is
// compiled with a different compiler (ie LLVMGCC42)
#ifndef __has_ feature
#define __has_feature(x) 0
#endif
#ifndef __has_extension
// Compatibility with pre-3.0 compilers
#define __has_extension __has_feature
#endif
#if __has_feature(objc_arc) && __clang_major__ >= 3
#define ARC_ENABLED 1
#endif
Check @ compile-time if ARC is enabled
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57. tl;dr What Is ARC ?
• Automatic memory management of Obj-C objects
• Compiler obeys and enforces existing conventions
• Full interoperability with manual retain and release
• New runtime features
• Weak pointers
• Advanced performance optimizations
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58. tl;dr What ARC Is NOT ?
• No new runtime memory model
• No automation for malloc/free, CF, etc.
• No garbage collector
• No heap scans
• No whole app pauses
• No non-deterministic releases
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59. ARC References
• Clang: Automatic Reference Counting documentation
• Apple: Transitioning to ARC Release Notes
• Apple: Xcode New Features User Guide: Automatic Reference Counting
• WWDC 2011 – Session 323 – Introducing Automatic Reference Counting
• WWDC 2012 – Session 406/416 – Adopting Automatic Reference Counting
• Mike Ash: Friday Q&A about Automatic Reference Counting
• Mike Ash: Zeroing weak references without ARC
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60. ARC Books
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61. NSLog(@”Thank you!”);
giuseppe.arici@pragmamark.org
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