Leaks & Zombies

& Leaks Zombies A quick intro to iPhone memory management Teemu Kurppa Co-Founder, Huikea.com

Basics NSString* s = [[NSString alloc] init]; ➊ [s release];

Basics NSString* s = [[NSString alloc] init]; ➊ [s retain]; ➋ [s release]; ➊ [s release];

Autorelease NSString* s = [[[NSString alloc] init] autorelease]; ➊

Autorelease NSAutoreleasePool* pool = [[NSAutoreleasePool] alloc] init]; ... NSString* s = [[[NSString alloc] init] autorelease]; ➊ ... [pool release];

Factory methods NSString* s = [NSString string]; ➊ [pool release]; why? because: @implementation NSString + (NSString*)string { return [[[NSString alloc] init] autorelease]; } @end

An incorrect setter - (void)setFoo:(Bar*)aFoo { [_foo release]; _foo = [aFoo retain]; // Incorrect, why? }

A correct setter - (void)setFoo:(Bar*)aFoo { if (aFoo != _foo) { [_foo release]; _foo = [aFoo retain]; } }

A correct setter - (void)setFoo:(Bar*)aFoo { [_foo autorelease]; _foo = [aFoo retain]; }

4 Rules of Thumb 1 Use autorelease for temporaries 2 Retain & release members 3 Use properties to get correct setters 4 Break rule 2 to avoid retain loops

Rule 1: Use autorelease for temporaries - (void) foo { NSString* s = [NSString string]; ➊ or NSString* s = [[[NSString alloc] init] autorelease]; ➊ }

Rule 2: Retain & release members @interface Bar { NSString* _name; } @end - (id) init { if (self = [super init]) { _name = [[NSString alloc] init]; } ➊ return self; } - (void) dealloc { [_name release]; [super dealloc]; }

Rule 3: Use properties to get correct setters @interface Bar { NSString* _name; } @property(retain) NSString* name; @end @implementation @synthesize name = _name; - (void) dealloc { [_name release]; // still important! [super dealloc]; }

Rule 4: Break rule 2 to avoid retain loops @interface Bar { id _delegate; } @property(assign) id delegate; // Note: not retain! @end @interface Foo { Bar* _bar; } @end in Foo: _bar = [[Bar alloc] init]; _bar.delegate = self;

What about bugs?

A leak - (void)init { if (self = [super init]) { _foo = [[NSString alloc] init]; ➊ } - (void)dealloc { [super dealloc]; }

Premature delete - (void)init { if (self = [super init]) { _foo = [NSString string]; } - (void)foo { NSLog(@”Foo is %@”, _foo); }

Double-deletion - (void) f { Bar* bar = [NSString string]; ➊ _foo = bar; } ... [pool release]; // - (void)dealloc { [_foo release]; !!! -1 [super dealloc]; }

Zombies to help NSZombieEnabled

by lunchtimemama http://www.ﬂickr.com/photos/lunchtimemama/97685452/sizes/o/ by Rachel Cobcroft http://www.ﬂickr.com/photos/felix42/453311029/sizes/o/

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Leaks & Zombies

by Teemu Kurppa

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