iPhone Memory Management

Objective-C 2.0Memory management for iPhone applications

Vadim Ziminsenior software developer at IT&T Consulting

1Saturday, 5 December 2009

Introduction

• iPhone and Objective-C

• Language features

• Objective-C 2.0: what’s new?


Why my app crashes?

• Bad memory access

• Internal exceptions

• Run out of memory

• Hang and killed by OS

• Killed by user


Main syntax differencesCall a method:

object->Method();[object message];

Passing method arguments:person->SetNameAndAge(name, age);[person setName:name andAge:age];

Declaration:void Person::SetNameAndAge(string name, int age);-(void)setName:(NSString*)name andAge:(int)age;

Access to current object’s method:this->DoSomething();[self doSomething];


Basic rules and principles

• Reference counting

• Creating and destroying objects

• Object ownership

• Autorelease pool

• Code conventions


Reference counting

• New object created with reference count=1

• Send retain/release message to increase/decrease reference count

• Object deallocated automatically when reference count becomes 0


Creating object:Person* person = [[Person alloc] init]; // ref_count =1 now

Using object:[person setName:@”Alex”];[person setAge:15];NSLog([person personInfo]); // print to stdout

Destroying (releasing) object:[person release];// person pointer is invalid now[person addPet:[Pet anyPet]]; // crash!!!person = nil; // на всякий случай)


Object ownership• Any object may have one or more owner

• Object’s creator becomes its 1st owner

• Other owners must retain object

• Owner is responsible for releasing object

• Owner must never destroy object directly

• Object w/o owners deallocated automatically

• Weak references used to avoid inf. loops


@interface Person : NSObject {NSString* name;int age;NSMutableArray* pets;

}...@implementation Person

-(id)initWithName:(NSString*)aName andAge:(int)yearsOld {self = [super init];name = [aName retain]; // got ownershipage = yearsOld;pets = [[NSMutableArray alloc] init]; // created and ownedreturn self;

}

-(void)dealloc {[name dealloc]; //wrong! will crash later[name release];[pets release]; [super dealloc]; // very important!

}


Autoreleased objects

• What is Autorelease pool?

• Use -autorelease method when you don’t want to own the object

• autorelease mean “Release later”

• Every thread should have it’s own pool


NSAutorelease pool = [[NSAutoreleasePool alloc] init];...

{Person* person = [[Person alloc] init]; // retainCount ==1...[person autorelease]; // retainCount==1

} // out of person pointer scope...[pool release]; // memory allocated for Person released


@interface Person : NSObject {NSString* name;int age;NSMutableArray* pets;

}...@implementation Person...-(NSString*)personInfo {

NSString* info = [[NSString alloc] initWithFormat:@”%@, %d years old, owns %d pets”, name, age, [pets count]];return [info autorelease]; // we are not responsible for this object now!

}

-(NSArray*)pets { // safe accessorreturn [[pets copy] autorelease]; // return read-only copy

}

-(void)setName:(NSString*)newName { // safe setter[name autorelease]; // why not just “release”?name = [newName retain];

}


...@implementation Person...-(void)startPlayWithPets {

[NSThread detachNewThreadSelector:@selector(playWithPets) toTarget:self withObject:nil];

}

-(void)playWithPets {// without pool every autoreleased object will leak memoryNSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];NSArray* petsToPlay = [self pets] ; // our safe accessor returned autoreleased object, it is implicitly added to our thread’s autorelease poolBOOL allPetsAreHappy = NO;while(! allPetsAreHappy) {

... // some code, that may create autoreleased objects}[pool release]; // memory is freed, petsToPlay released as well

}


Conventions• Syntax like [[Class alloc] init] and [obj copy] creates

objects with ref_count==1, you are responsible for releasing (autoreleasing) it

• You are not responsible for releasing objects created by “convenience constructors” like

-[aString stringByAppendingString:otherString] or +[NSNumber numberWithFloat:1.0f]

• You are not responsible for objects obtained by (property) getters and should not delegate such responsibility to others when implementing getters


@interface Pet : NSObject { ... }...@implementation Pet// this method retuns object with retainCount =1 by convention-(id)initWithName:(NSString*)petName {

self = [super init];name = [petName retain];return self;

}// convenience constructor, autoreleased object+(id)anyPet {

NSSet* petClasses = [NSSet setWithObjects:[Dog class], [Cat class],nil];Class aClass = [petClasses anyObject];NSSet* petNames = [NSSet setWithObjects:@”Max”, @”Sharik”,@”Murka”,nil];NSSrting* aName = [petNames anyObject];return [[[aClass alloc] initWithName:aName] autorelease];

}


Objective-C @property

• Writability (readonly, readwrite)

• Setter semantic (assign, copy, retain)

• Atomicity (atomic, nonatomic)

Declaration:@propery (writability, setter, atomicity) type name;@synthesize name=ivarName;

Usage:object.propertyName = newValue;value = object.propertyName;


@property (nonatomic, retain) NSString* name;

// equivalent to pair of methods-(NSString*)name {

return name;}

-(void)setName:(NSString*)aName {if (name != aName) {

[name release];name = [aName retain];

}}


@property (atomic, copy) NSString* name;// equivalent to pair of methods-(NSString*)name {

NSString* retVal=nil;@synchronized(self) {

retVal = [[name retain] autorelease];}return retVal;

}

-(void)setName:(NSString*)aName {@synchronized(self) {

if (name != aName) {[name release];name = [aName retain];

}}

}18Saturday, 5 December 2009

Common mistakes

• no retain/release balance

• [self release]

• missed [super dealloc]

• reassign of pointer without old value release

• redefined -retain -release methods


Optimizations

• Use C structures instead of classes

• Keep immutable objects

• Release now instead of autorelease if possible

• Nested Autorelease pools

• Use UIKit recommendations


Debugging tools

• Static code analyzer (Mac OS X 10.6)

• Instruments (ObjectAlloc, Leaks)


Code analyze:


Instruments:


The end

by Vadim Ziminphone: +38(093)5826658e-mail: [email protected]


mailto:[email protected]

mailto:[email protected]

Technology

iPhone Memory Management