iPhone Memory Management

Objective-C 2.0
Memory management for iPhone applications

Vadim Zimin
senior software developer at IT&T Consulting

Saturday, 5 December 2009 1

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 differences
Call 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 ownership
age = yearsOld;
pets = [[NSMutableArray alloc] init]; // created and owned
return 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;
}
...
...
-(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 accessor
return [[pets copy] autorelease]; // return read-only copy
}

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


...
...
-(void)startPlayWithPets {
[NSThread detachNewThreadSelector:@selector(playWithPets)
toTarget:self withObject:nil];
}

-(void)playWithPets {
// without pool every autoreleased object will leak memory
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
NSArray* petsToPlay = [self pets] ; // our safe accessor returned
autoreleased object, it is implicitly added to our thread’s autorelease pool
BOOL 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
Declaration:
@propery (writability, setter, atomicity) type name;
@synthesize name=ivarName;
Usage:
object.propertyName = newValue;
value = object.propertyName;

• Writability (readonly, readwrite)
• Setter semantic (assign, copy, retain)

• Atomicity (atomic, nonatomic)

@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];
}
}
}

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 Zimin
phone: +38(093)5826658
e-mail: vad@wareous.com

iPhone Memory Management

More Related Content

What's hot (20)

Viewers also liked (8)

Similar to iPhone Memory Management (20)

Recently uploaded (20)

iPhone Memory Management