Pragma Night @ Talent Garden

Automatic Reference Counting

Giuseppe Arici

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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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Manual Reference Counting

Reference Counting

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Manual Reference Counting

retain

2

alloc

1

release

1

release

0

(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

dealloc

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Objects you createFor 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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Objects you don’t createFor 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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AutoreleaseWhat 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 autorelease];}

- (NSArray *)objects { NSArray *localArray = [[NSArray alloc] init]; return [localArray release];}

- (NSArray *)objects { NSArray *localArray = [[NSArray alloc] init]; return localArray;}

✇Leak !

☠Crash !

☺Enjoy !

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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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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 NSAutoreleasePoolhttp://www.mikeash.com/pyblog/friday-qa-2011-09-02-lets-build-nsautoreleasepool.html

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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-Chttp://pierre.chachatelier.fr/programmation/objective-c.php

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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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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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ARC Settings: Project | File

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Requirements

• iOS 5+ & OS X 10.7+ Full Support :)

• iOS 4 & OS X 10.6 (64-bit) No Runtime :(

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How it works !?

ARC consists of 2 main components

Frontend Compiler Optimizer

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• 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

ARC Frontend Compiler

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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 !

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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ARC Frontend Compiler 1/3

- (void)someMethod { Foo *foo = [[Foo alloc] init]; [foo bar]; [foo release];}

- (void)someMethod { Foo *foo = [[Foo alloc] init]; [foo bar];

}

- (void)someMethod { Foo *foo = [[Foo alloc] init]; [foo bar]; obj_release(foo);}

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ARC Frontend Compiler 2/3- (void)someMethod { Foo *localFoo = [self foo]; [localFoo bar];

self.foo = [Builder newFoo]; [localFoo bar];

}

- (void)someMethod { Foo *localFoo = objc_retain([self foo]); [localFoo bar];

self.foo = [Builder newFoo]; [localFoo bar]; objc_release(localFoo);}

☠Crash !

☺Enjoy !

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ARC Frontend Compiler 3/3

- (Foo *) foo{ return [[_foo retain] autorelease];}

- (Foo *) foo{ return _foo;}

- (Foo *) foo{ return objc_retainAutoreleaseReturnValue(_foo);}

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• 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

ARC Optimizer

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ARC Optimizer 1/2

retainautoreleaseretainrelease

retain

releasestack

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ARC Optimizer 1/2

retain

releasestack

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ARC Optimizer 2/2- (Foo *) foo{ return objc_retainAutoreleaseReturnValue(_foo);}

- (void)someMethod { Foo *f = objc_retainAutoreleasedReturnValue([self foo]); [f bar];

self.foo = [Builder newFoo]; [f bar]; objc_release(f);}

called method

caller method

Autorelease Pool

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Variable Ownership Qualifiers

• __strong (default)

• __weak

• __unsafe_unretained

• __autoreleasing

Only For Object Type Variables !( id | AnyClass : NSObject )

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Retain Cycles 1/5

Child

View

Parent

Controller

__strong

+1_view

__strong

as View Delegate

+1_delegate

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Retain Cycles 2/5

Child

View

Parent

Controller

__strong

+1_view

__unsafe_unretained

as View Delegate

=_delegate

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Retain Cycles 3/5

Child

View

Parent

Controller

__strong

+1_view

__weak

as View Delegate

=_delegate

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release

Retain Cycles 4/5

Child

View

Parent

Controller

=☠_view

__weak

as View Delegate

_delegate

dealloc

☺nil

New with ARC iniOS 5+ & OS X 10.7+

_view

_delegate

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Retain Cycles 5/5

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__strong

/* not-ARC */{ id obj1 = [[NSObject alloc] init]; [obj1 release];}

/* ARC */{ id __strong obj1 = [[NSObject alloc] init]; id obj2 = [[NSObject alloc] init];}

• Strong reference for the object

• Default for ‘id’ and Object Type Variables !

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__weak

/* not-ARC */{ id obj1 = [[NSObject alloc] init]; id obj2 = obj1; [obj1 release];}

/* ARC */{ id __strong obj1 = [[NSObject alloc] init]; id __weak obj2 = obj1;}

• No ownership of the object

• When discarded, the variable is assigned to nil

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__unsafe_unretained/* ARC */{ id __unsafe_unretained obj = [[NSObject alloc] init];}

• UNSAFE !

• You must take care of the variables manually

• __weak only for iOS 5+ / OSX 10.7+

Assigning retained object to unsafe_unretained variable; object will be released after assignment [-Warc-unsafe-retained-assign]

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__autoreleasing

- (BOOL) performWithError:(NSError **)error;

- (BOOL) perfomrWithError:(NSError * __autoreleasing *)error;

@autoreleasepool{ id __strong obj = [NSMutableArray array];}

• Pointer to ‘id’ or to ‘Object Type Variables Pointer’ is qualified with __autoreleasing as default

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Variables Initialization

/* not-ARC */id __strong obj1 = nil;id __weak obj2 = nil;id __unsafe_unretained obj3;id __autoreleasing obj4 = nil;

/* ARC */id __strong obj1;id __weak obj2;id __unsafe_unretained obj3;id __autoreleasing obj4;

• Any variables that are qualified with __strong, __weak and __autoreleasing are initialized with nil

☠Crash !

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Property

* Note: new copied object is assigned

*

Object Property Modifier Variable Ownership Qualifier

assign __unsafe_unretained

unsafe_unretained __unsafe_unretained

weak __weak

retain __strong

strong __strong

copy __strong

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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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Coding With ARC Rules

What is forbidden ?

What can be done but with care ?

What is mandatory ?

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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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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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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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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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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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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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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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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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Rule 09/12

• Follow the naming rule for methods relatedto 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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False Positive

-(NSString*) copyRightString NS_RETURNS_NOT_RETAINED;

-(NSString*) copyRightString;

• 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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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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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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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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( _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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( __bridge_retained cast )

/* not-ARC */id object = [[NSObject alloc] init];void *pointer = object;[(id)pointer retain];

/* ARC */id object = [[NSObject alloc] init];void *pointer = (__bridge_retained void *)object;

• From ARC to Core Foundation

• __bridge_retained cast will retain the object just after the assignment is done

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( __bridge_transfer cast )

/* not-ARC */id object = (id)pointer;[object retain];[(id)pointer release];

/* ARC */void *pointer = &bytes;id object = (__bridge_transfer id)pointer;

• From Core Foundation to ARC

• __bridge_transfer cast will release the object just after the assignment is done

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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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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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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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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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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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ARC Books

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NSLog(@”Thank you!”);

giuseppe.arici@pragmamark.org