iOS Memory Management Basics

iOS &
Memory Management
Basics
Cameron Barrie - @whalec

iOS &
Memory Management
Memory Management Programming Guide:
https://developer.apple.com/iphone/library/
documentation/Cocoa/Conceptual/MemoryMgmt/
MemoryMgmt.html

iOS Memory Management

alloc/copy/retain/release/autorelease

Program received signal: "EXC_BAD_ACCESS".

ObjectiveC objects contain a ‘count’ of how many
other objects are currently holding onto them.

You increment that count with the retain message.
You decrement that count with the release message.

When that count hits 0.
The object is deallocated from memory

Memory Management Rules:

1. You only release objects you own:

[[UPGameViewController alloc] initWithNibName:@"UPGameView" bundle:nil]


NSArray *topLevelObjects = [[NSBundle mainBundle]
loadNibNamed:@"UPLeaderboardViewCell"
owner:nil options:nil];
NSMutableArray *mutableTopLevelObjects = [topLevelObjects mutableCopy];



2. Or objects you take ownership of:




[[UIFont fontWithName:@"HoboStd" size:25] retain]




[[UIFont fontWithName:@"HoboStd" size:25] retain]

Retaining an object simply increments an objects retainCount by 1.

Releasing an object
Release an object by sending it the release message

Releasing an object


[mutableTopLevelObjects release];

Releasing an object



Releasing an object doesn’t remove it from memory.
It simply decrements the objects retainCount by 1.
When an objects retainCount is equal to 0.
The object is deallocated by the runtime automatically.

Releasing an object

[topLevelObjects retainCount]; // => 1
[topLevelObjects retain];

[topLevelObjects release];

Releasing an object doesn’t remove it from memory.
It simply decrements the objects retainCount by 1.
When an objects retainCount is equal to 0.
The object is deallocated by the runtime automatically.

So what about the other objects?

They’re what’s known as autorelease objects.


// This object will be automatically released at some point
// in the future when the autorelease pool is drained
NSString *aString = [NSString stringWithFormat:@"a string with an int %i", 1];



You can mark your own objects as autorelease objects



You can mark your own objects as autorelease objects
[topLevelObjects autorelease];
[topLevelObjects retainCount]; // => 1 since it will be released later

NSAutoreleasePool
“Cocoa always expects there to be an autorelease pool available”
- Memory Management Programming Guide: Autorelease Pools

// main.m
// The main entry point into the app sets up
// an autorelease pool for us automatically
int main(int argc, char *argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
int retVal = UIApplicationMain(argc, argv, nil, nil);
[pool release];
return retVal;
}

Common mistakes

Common mistakes
// You need to mark zero for autorelease or release it
- (void)reset {
NSNumber *zero = [[NSNumber alloc] initWithInteger:0];
[self setCount:zero];
}

Common mistakes
// You need to mark zero for autorelease or release it
- (void)reset {
NSNumber *zero = [[NSNumber alloc] initWithInteger:0];
}

// zero is an autorelease object already.
// you haven't used alloc/copy anywhere
// you are over releasing the zero object
- (void)reset {
NSNumber *zero = [NSNumber numberWithInteger:0];
[zero release];
}

Common confusion
// Adding an object to a collection retains the object
// It transfers ownership to the parent collection
// Since alloc/copy was never called you don’t need
// to release convenienceNumber
NSMutableArray *array = [[NSMutableArray alloc] init];
NSUInteger i = 0;
for (i; i < 10; i++) {
NSNumber *convenienceNumber = [NSNumber numberWithInteger:i];
[array addObject:convenienceNumber];
}

Common confusion
// Adding an object to a collection retains the object
// It transfers ownership to the parent collection
// You need to release allocedNumber here since you alloced it.
NSMutableArray *array = [[NSMutableArray alloc] init];
NSUInteger i = 0;
for (i; i < 10; i++) {
NSNumber *allocedNumber = [[NSNumber alloc] initWithInteger: i];
[array addObject:allocedNumber];
[allocedNumber release];
}

Returning Objects from Methods

// Correct - You don't own the string object so you should return it as such.
- (NSString *)fullName {
NSString *string = [NSString stringWithFormat:@"%@ %@",
self.firstName, self.lastName];
return string;
}

return string;
}

// Incorrect - You're returning an object that has already been released.
NSString *string = [[[NSString alloc] initWithFormat:@"%@ %@",
self.firstName, self.lastName]
release];
return string;
}

return string;
}

// Incorrect - You're returning an object that has already been released.
release];
return string;
}

// Correct - You've created an autorelease object.
autorelease];
return string;
}

Accessor Method - @property/@synthesize

// MyClass.h
@property(nonatomic, readwrite, retain) NSString *myString

// MyClass.h
@property(nonatomic, readwrite, retain) NSString *myString

// MyClass.m
@synthesize myString = _myString;
-(id)init
{
if ((self = [super init]))
{
self.myString = @"A string to be retained";
}
return self;
}

-(void)dealloc
{
[_myString release], _myString = nil;

[super dealloc];
}


// MyClass.h
// Will retain the variable sent to it.
@property(nonatomic, readwrite, retain) NSString *myString;

// Will not retain the variable sent to it.
@property(nonatomic, readwrite, assign) NSString *myWeakString;

// MyClass.m
// Will retain the variable sent to it.

// The @synthesize produces a setter/getter combo(readwrite)
-(NSString *)myString
{
return _myString;
}

-(void)setMyString:(NSString *)myString
{
if (myString != _myString)
{
[_myString release];
_myString = [myString retain];
}
}


// MyClass.m
// Will not retain the variable sent to it.
@synthesize myWeakString = _myWeakString;

// The @synthesize produces a setter/getter combo(readwrite)
-(NSString *)myWeakString
{
return _myWeakString;
}

-(void)setMyWeakString:(NSString *)myWeakString
{
_myWeakString = myWeakString;
}

Accessor Methods - @property/@synthesize
// MyClass.m
// By synthesizing in this way you are protecting your iVar so it can
// only be accessed through the setter.
// It also means you can pass the argument myString to methods.

-(id)initWithMyString:(NSString *)myString
{
{
self.myString = myString;
}
return self;
}

+(id)myClassWithMyString:(NSString *)myString
{
return [[self initWithMyString:myString] autorelease];
}

Accessor Methods - @property/@synthesize
// MyClass.m
// By synthesizing in this way you are protecting your iVar so it can
// only be accessed through the setter.
// It also means you can pass the argument myString to methods.
@synthesize myString;

-(id)initWithMyString:(NSString *)cantBeCalledMyStringNow
{
{
// Woops I assigned straight to the iVar. So it’s not retained now
myString = cantBeCalledMyStringNow;
}
return self;
}

+(id)myClassWithMyString:(NSString *)cantBeCalledMyStringNow
{
return [[self initWithMyString: cantBeCalledMyStringNow] autorelease];
}

iOS Memory Management Basics

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (19)

Similaire à iOS Memory Management Basics

Similaire à iOS Memory Management Basics (20)

Dernier

Dernier (20)

iOS Memory Management Basics

Notes de l'éditeur