&
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

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/