In apple doc singleton:
static MyGizmoClass *sharedGizmoManager = nil;
+ (MyGizmoClass*)sharedManager {
if (sharedGizmoManager == nil) {
sharedGizmoManager = [[super allocWithZone:NULL] init];
}
return sharedGizmoManager;
}
+ (id)allocWithZone:(NSZone *)zone {
return [[self sharedManager] retain];
}
- (id)copyWithZone:(NSZone *)zone {
return self;
}
- (id)retain {
return self;
}
- (NSUInteger)retainCount {
return NSUIntegerMax; //denotes an object that cannot be released
}
- (void)release {
//do nothing
}
- (id)autorelease {
return self;
}
I have a question:
+ (id)allocWithZone:(NSZone *)zone {
return [[self sharedManager] retain];
}
retain should omit, but why remain retain?
Because return [self sharedManager] or [[self sharedManager] retain] is equal.
The sharedGizmoManager is owned by its own class, but any client code that tries to use allocWithZone: to create an instance will expect to own the returned reference (that's the rule for alloc/allocWithZone:). The retain does exactly that -- gives (partial) ownership to the calling code over the existing instance.
The client code, since it has an owning reference, is required to send release at some point, when it's done using the object. If there was no "extra" retain, this would actually release the ownership claim that the object has on itself, and the object could be (improperly) deallocated.
In a sentence, the retain is there to make the singleton, which is in fact managing its own lifetime, compatible with the rules that client code will be following.
In Objective-C, how to write a singleton with ARC? In ARC, it is not allowed to overwrite the release, autorelease, retain, retainCount methods, how to avoid a object to be released? I know without ARC, a classic singleton would like below:
#interface SingletonObject
+ (SingletonObject*)sharedObject;
#end
SingletonObject *sharedObj;
#implementation SingletonObject
+ (id)allocWithZone:(NSZone *)zone
{
if (sharedObj == nil) {
//So the code [[SingletonObject alloc] init] is equal with [SingletonObject sharedObject]
sharedObj = [super allocWithZone:zone];
}
return sharedObj;
}
+ (void)initialize
{
if (self == [SingletonObject class]) {
sharedObj = [[SingletonObject alloc] init];
}
}
+ (SingletonObject*)sharedObject
{
return sharedObj;
}
- (id)retain
{
return self;
}
- (unsigned)retainCount
{
return UINT_MAX; //denotes an object that cannot be released
}
- (oneway void)release
{
//do nothing
}
- (id)autorelease
{
return self;
}
- (id)init {
self = [super init];
if (self) {
//...
}
return self;
}
#end
Is it safe to just remove the retain, retainCount, release, autorelease methods? Thanks!
You only need one method to implement a class that supports the singleton pattern:
+ (instancetype)sharedInstance
{
static id _sharedInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_sharedInstance = [[self alloc] init];
});
return _sharedInstance;
}
Copy/paste that into any class and it'll have a shared instance. Any code beyond that is just added complexity that really isn't necessary. I'll sometimes add:
- (void)dealloc
{
*(char*)0x42 = 'b';
// no super, ARC all the way
}
That'll cause a very specific crash if my shared instance is ever deallocated due to a bug. (Yes, hex 0x42 is not 42, but it leaves a nice 0x000000042 in a register in the crash log, making it immediately identifiable what happened.)
Yes.
-sharedObject should be +sharedObject, though (class method). Just make sure you always use that.
I created a Singleton class named DataManager. I've set up some caching. However, when attempting to call the cache instance methods from within another instance method of DataManager, I'm getting EXC_BAD_ACCESS error on the line:
NSMutableArray *stops = [[DataManager sharedInstance] getCacheForKey:#"stops"];
DataManager.h
#interface DataManager : NSObject {
FMDatabase *db;
NSMutableDictionary *cache;
}
+ (DataManager *)sharedInstance;
// ... more methods
- (id)getCacheForKey:(NSString *)key;
- (void)setCacheForKey:(NSString *)key withValue:(id)value;
- (void)clearCache;
DataManager.m
- (NSArray *)getStops:(NSInteger)archived {
NSMutableArray *stops = [[DataManager sharedInstance] getCacheForKey:#"stops"];
if (stops != nil) {
NSLog(#"Stops: %#", stops);
return stops;
}
stops = [NSMutableArray array];
// set stops...
[[DataManager sharedInstance] setCacheForKey:#"stops" withValue:stops];
return stops;
}
It seems to occur when called from another view controller. That is the first view controller no error, second view controller, error.
This is my first attempt at a Singleton, so I'm sure I am making a simple mistake. But I am failing to see it myself.
Note: I've tried [self getCache...] with the same result.
UPDATE
Here is my Singleton implementation. Adapted from http://www.galloway.me.uk/tutorials/singleton-classes/
+ (DataManager *)sharedInstance {
#synchronized(self) {
if (!instance) {
instance = [[super allocWithZone:NULL] init];
}
}
return instance;
}
+ (id)allocWithZone:(NSZone *)zone {
return [[self sharedInstance] retain];
}
- (id)copyWithZone:(NSZone *)zone {
return self;
}
- (id)retain {
return self;
}
- (unsigned)retainCount {
return UINT_MAX;
}
- (oneway void)release {
// never release
}
- (id)autorelease {
return self;
}
- (id)init {
if (self = [super init]) {
if (db == nil){
BourbonAppDelegate *appDelegate = (BourbonAppDelegate *)[[UIApplication sharedApplication] delegate];
[appDelegate createEditableCopyOfDatabaseIfNeeded];
db = [[FMDatabase alloc] initWithPath:[appDelegate getDBPath]];
}
if (![db open]) {
NSAssert(0, #"Failed to open database.");
[db release];
return nil;
}
[db setTraceExecution:YES];
[db setLogsErrors:TRUE];
cache = [NSMutableDictionary dictionary];
NSLog(#"cache: %#", cache);
}
return self;
}
Your cache object is autoreleased, so it's no longer in memory when you try to access it.
Use [NSMutableDictionary alloc] init] instead of [NSMutableDictionary dictionary] to get an instance that is retained.
Not a direct answer to your question, which was already answered.
However I'd just like to point out that your singleton implementation is sub-optimal. #synchronized is very expensive, and you can avoid using it every time you access the singleton:
if (!instance) {
#synchronized(self) {
if (!instance) {
instance = [[super allocWithZone:NULL] init];
}
}
}
An even better way to initialize a singleton would be:
+ (DataManager *)sharedInstance {
static DataManager *instance;
static dispatch_once_t donce;
dispatch_once(&donce, ^{
instance = [[self alloc] init];
});
return instance;
}
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 9 years ago.
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
My singleton accessor method is usually some variant of:
static MyClass *gInstance = NULL;
+ (MyClass *)instance
{
#synchronized(self)
{
if (gInstance == NULL)
gInstance = [[self alloc] init];
}
return(gInstance);
}
What could I be doing to improve this?
Another option is to use the +(void)initialize method. From the documentation:
The runtime sends initialize to each class in a program exactly one time just before the class, or any class that inherits from it, is sent its first message from within the program. (Thus the method may never be invoked if the class is not used.) The runtime sends the initialize message to classes in a thread-safe manner. Superclasses receive this message before their subclasses.
So you could do something akin to this:
static MySingleton *sharedSingleton;
+ (void)initialize
{
static BOOL initialized = NO;
if(!initialized)
{
initialized = YES;
sharedSingleton = [[MySingleton alloc] init];
}
}
#interface MySingleton : NSObject
{
}
+ (MySingleton *)sharedSingleton;
#end
#implementation MySingleton
+ (MySingleton *)sharedSingleton
{
static MySingleton *sharedSingleton;
#synchronized(self)
{
if (!sharedSingleton)
sharedSingleton = [[MySingleton alloc] init];
return sharedSingleton;
}
}
#end
[Source]
Per my other answer below, I think you should be doing:
+ (id)sharedFoo
{
static dispatch_once_t once;
static MyFoo *sharedFoo;
dispatch_once(&once, ^ { sharedFoo = [[self alloc] init]; });
return sharedFoo;
}
Since Kendall posted a threadsafe singleton that attempts to avoid locking costs, I thought I would toss one up as well:
#import <libkern/OSAtomic.h>
static void * volatile sharedInstance = nil;
+ (className *) sharedInstance {
while (!sharedInstance) {
className *temp = [[self alloc] init];
if(!OSAtomicCompareAndSwapPtrBarrier(0x0, temp, &sharedInstance)) {
[temp release];
}
}
return sharedInstance;
}
Okay, let me explain how this works:
Fast case: In normal execution sharedInstance has already been set, so the while loop is never executed and the function returns after simply testing for the variable's existence;
Slow case: If sharedInstance doesn't exist, then an instance is allocated and copied into it using a Compare And Swap ('CAS');
Contended case: If two threads both attempt to call sharedInstance at the same time AND sharedInstance doesn't exist at the same time then they will both initialize new instances of the singleton and attempt to CAS it into position. Whichever one wins the CAS returns immediately, whichever one loses releases the instance it just allocated and returns the (now set) sharedInstance. The single OSAtomicCompareAndSwapPtrBarrier acts as both a write barrier for the setting thread and a read barrier from the testing thread.
static MyClass *sharedInst = nil;
+ (id)sharedInstance
{
#synchronize( self ) {
if ( sharedInst == nil ) {
/* sharedInst set up in init */
[[self alloc] init];
}
}
return sharedInst;
}
- (id)init
{
if ( sharedInst != nil ) {
[NSException raise:NSInternalInconsistencyException
format:#"[%# %#] cannot be called; use +[%# %#] instead"],
NSStringFromClass([self class]), NSStringFromSelector(_cmd),
NSStringFromClass([self class]),
NSStringFromSelector(#selector(sharedInstance)"];
} else if ( self = [super init] ) {
sharedInst = self;
/* Whatever class specific here */
}
return sharedInst;
}
/* These probably do nothing in
a GC app. Keeps singleton
as an actual singleton in a
non CG app
*/
- (NSUInteger)retainCount
{
return NSUIntegerMax;
}
- (oneway void)release
{
}
- (id)retain
{
return sharedInst;
}
- (id)autorelease
{
return sharedInst;
}
Edit: This implementation obsoleted with ARC. Please have a look at How do I implement an Objective-C singleton that is compatible with ARC? for correct implementation.
All the implementations of initialize I've read in other answers share a common error.
+ (void) initialize {
_instance = [[MySingletonClass alloc] init] // <----- Wrong!
}
+ (void) initialize {
if (self == [MySingletonClass class]){ // <----- Correct!
_instance = [[MySingletonClass alloc] init]
}
}
The Apple documentation recommend you check the class type in your initialize block. Because subclasses call the initialize by default. There exists a non-obvious case where subclasses may be created indirectly through KVO. For if you add the following line in another class:
[[MySingletonClass getInstance] addObserver:self forKeyPath:#"foo" options:0 context:nil]
Objective-C will implicitly create a subclass of MySingletonClass resulting in a second triggering of +initialize.
You may think that you should implicitly check for duplicate initialization in your init block as such:
- (id) init { <----- Wrong!
if (_instance != nil) {
// Some hack
}
else {
// Do stuff
}
return self;
}
But you will shoot yourself in the foot; or worse give another developer the opportunity to shoot themselves in the foot.
- (id) init { <----- Correct!
NSAssert(_instance == nil, #"Duplication initialization of singleton");
self = [super init];
if (self){
// Do stuff
}
return self;
}
TL;DR, here's my implementation
#implementation MySingletonClass
static MySingletonClass * _instance;
+ (void) initialize {
if (self == [MySingletonClass class]){
_instance = [[MySingletonClass alloc] init];
}
}
- (id) init {
ZAssert (_instance == nil, #"Duplication initialization of singleton");
self = [super init];
if (self) {
// Initialization
}
return self;
}
+ (id) getInstance {
return _instance;
}
#end
(Replace ZAssert with our own assertion macro; or just NSAssert.)
A thorough explanation of the Singleton macro code is on the blog Cocoa With Love
http://cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html.
I have an interesting variation on sharedInstance that is thread safe, but does not lock after the initialization. I am not yet sure enough of it to modify the top answer as requested, but I present it for further discussion:
// Volatile to make sure we are not foiled by CPU caches
static volatile ALBackendRequestManager *sharedInstance;
// There's no need to call this directly, as method swizzling in sharedInstance
// means this will get called after the singleton is initialized.
+ (MySingleton *)simpleSharedInstance
{
return (MySingleton *)sharedInstance;
}
+ (MySingleton*)sharedInstance
{
#synchronized(self)
{
if (sharedInstance == nil)
{
sharedInstance = [[MySingleton alloc] init];
// Replace expensive thread-safe method
// with the simpler one that just returns the allocated instance.
SEL origSel = #selector(sharedInstance);
SEL newSel = #selector(simpleSharedInstance);
Method origMethod = class_getClassMethod(self, origSel);
Method newMethod = class_getClassMethod(self, newSel);
method_exchangeImplementations(origMethod, newMethod);
}
}
return (MySingleton *)sharedInstance;
}
Short answer: Fabulous.
Long answer: Something like....
static SomeSingleton *instance = NULL;
#implementation SomeSingleton
+ (id) instance {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
if (instance == NULL){
instance = [[super allocWithZone:NULL] init];
}
});
return instance;
}
+ (id) allocWithZone:(NSZone *)paramZone {
return [[self instance] retain];
}
- (id) copyWithZone:(NSZone *)paramZone {
return self;
}
- (id) autorelease {
return self;
}
- (NSUInteger) retainCount {
return NSUIntegerMax;
}
- (id) retain {
return self;
}
#end
Be sure to read the dispatch/once.h header to understand what's going on. In this case the header comments are more applicable than the docs or man page.
I've rolled singleton into a class, so other classes can inherit singleton properties.
Singleton.h :
static id sharedInstance = nil;
#define DEFINE_SHARED_INSTANCE + (id) sharedInstance { return [self sharedInstance:&sharedInstance]; } \
+ (id) allocWithZone:(NSZone *)zone { return [self allocWithZone:zone forInstance:&sharedInstance]; }
#interface Singleton : NSObject {
}
+ (id) sharedInstance;
+ (id) sharedInstance:(id*)inst;
+ (id) allocWithZone:(NSZone *)zone forInstance:(id*)inst;
#end
Singleton.m :
#import "Singleton.h"
#implementation Singleton
+ (id) sharedInstance {
return [self sharedInstance:&sharedInstance];
}
+ (id) sharedInstance:(id*)inst {
#synchronized(self)
{
if (*inst == nil)
*inst = [[self alloc] init];
}
return *inst;
}
+ (id) allocWithZone:(NSZone *)zone forInstance:(id*)inst {
#synchronized(self) {
if (*inst == nil) {
*inst = [super allocWithZone:zone];
return *inst; // assignment and return on first allocation
}
}
return nil; // on subsequent allocation attempts return nil
}
- (id)copyWithZone:(NSZone *)zone {
return self;
}
- (id)retain {
return self;
}
- (unsigned)retainCount {
return UINT_MAX; // denotes an object that cannot be released
}
- (void)release {
//do nothing
}
- (id)autorelease {
return self;
}
#end
And here is an example of some class, that you want to become singleton.
#import "Singleton.h"
#interface SomeClass : Singleton {
}
#end
#implementation SomeClass
DEFINE_SHARED_INSTANCE;
#end
The only limitation about Singleton class, is that it is NSObject subclass. But most time I use singletons in my code they are in fact NSObject subclasses, so this class really ease my life and make code cleaner.
This works in a non-garbage collected environment also.
#interface MySingleton : NSObject {
}
+(MySingleton *)sharedManager;
#end
#implementation MySingleton
static MySingleton *sharedMySingleton = nil;
+(MySingleton*)sharedManager {
#synchronized(self) {
if (sharedMySingleton == nil) {
[[self alloc] init]; // assignment not done here
}
}
return sharedMySingleton;
}
+(id)allocWithZone:(NSZone *)zone {
#synchronized(self) {
if (sharedMySingleton == nil) {
sharedMySingleton = [super allocWithZone:zone];
return sharedMySingleton; // assignment and return on first allocation
}
}
return nil; //on subsequent allocation attempts return nil
}
-(void)dealloc {
[super dealloc];
}
-(id)copyWithZone:(NSZone *)zone {
return self;
}
-(id)retain {
return self;
}
-(unsigned)retainCount {
return UINT_MAX; //denotes an object that cannot be release
}
-(void)release {
//do nothing
}
-(id)autorelease {
return self;
}
-(id)init {
self = [super init];
sharedMySingleton = self;
//initialize here
return self;
}
#end
Shouln't this be threadsafe and avoid the expensive locking after the first call?
+ (MySingleton*)sharedInstance
{
if (sharedInstance == nil) {
#synchronized(self) {
if (sharedInstance == nil) {
sharedInstance = [[MySingleton alloc] init];
}
}
}
return (MySingleton *)sharedInstance;
}
Here's a macro that I put together:
http://github.com/cjhanson/Objective-C-Optimized-Singleton
It is based on the work here by Matt Gallagher
But changing the implementation to use method swizzling as described here by Dave MacLachlan of Google.
I welcome comments / contributions.
How about
static MyClass *gInstance = NULL;
+ (MyClass *)instance
{
if (gInstance == NULL) {
#synchronized(self)
{
if (gInstance == NULL)
gInstance = [[self alloc] init];
}
}
return(gInstance);
}
So you avoid the synchronization cost after initialization?
For an in-depth discussion of the singleton pattern in Objective-C, look here:
Using the Singleton Pattern in Objective-C
KLSingleton is:
Subclassible (to the n-th degree)
ARC compatible
Safe with alloc and init
Loaded lazily
Thread-safe
Lock-free (uses +initialize, not #synchronize)
Macro-free
Swizzle-free
Simple
KLSingleton
You don't want to synchronize on self... Since the self object doesn't exist yet! You end up locking on a temporary id value. You want to ensure that no one else can run class methods ( sharedInstance, alloc, allocWithZone:, etc ), so you need to synchronize on the class object instead:
#implementation MYSingleton
static MYSingleton * sharedInstance = nil;
+( id )sharedInstance {
#synchronized( [ MYSingleton class ] ) {
if( sharedInstance == nil )
sharedInstance = [ [ MYSingleton alloc ] init ];
}
return sharedInstance;
}
+( id )allocWithZone:( NSZone * )zone {
#synchronized( [ MYSingleton class ] ) {
if( sharedInstance == nil )
sharedInstance = [ super allocWithZone:zone ];
}
return sharedInstance;
}
-( id )init {
#synchronized( [ MYSingleton class ] ) {
self = [ super init ];
if( self != nil ) {
// Insert initialization code here
}
return self;
}
}
#end
Just wanted to leave this here so I don't lose it. The advantage to this one is that it's usable in InterfaceBuilder, which is a HUGE advantage. This is taken from another question that I asked:
static Server *instance;
+ (Server *)instance { return instance; }
+ (id)hiddenAlloc
{
return [super alloc];
}
+ (id)alloc
{
return [[self instance] retain];
}
+ (void)initialize
{
static BOOL initialized = NO;
if(!initialized)
{
initialized = YES;
instance = [[Server hiddenAlloc] init];
}
}
- (id) init
{
if (instance)
return self;
self = [super init];
if (self != nil) {
// whatever
}
return self;
}
static mySingleton *obj=nil;
#implementation mySingleton
-(id) init {
if(obj != nil){
[self release];
return obj;
} else if(self = [super init]) {
obj = self;
}
return obj;
}
+(mySingleton*) getSharedInstance {
#synchronized(self){
if(obj == nil) {
obj = [[mySingleton alloc] init];
}
}
return obj;
}
- (id)retain {
return self;
}
- (id)copy {
return self;
}
- (unsigned)retainCount {
return UINT_MAX; // denotes an object that cannot be released
}
- (void)release {
if(obj != self){
[super release];
}
//do nothing
}
- (id)autorelease {
return self;
}
-(void) dealloc {
[super dealloc];
}
#end
I know there are a lot of comments on this "question", but I don't see many people suggesting using a macro to define the singleton. It's such a common pattern and a macro greatly simplifies the singleton.
Here are the macros I wrote based on several Objc implementations I've seen.
Singeton.h
/**
#abstract Helps define the interface of a singleton.
#param TYPE The type of this singleton.
#param NAME The name of the singleton accessor. Must match the name used in the implementation.
#discussion
Typcially the NAME is something like 'sharedThing' where 'Thing' is the prefix-removed type name of the class.
*/
#define SingletonInterface(TYPE, NAME) \
+ (TYPE *)NAME;
/**
#abstract Helps define the implementation of a singleton.
#param TYPE The type of this singleton.
#param NAME The name of the singleton accessor. Must match the name used in the interface.
#discussion
Typcially the NAME is something like 'sharedThing' where 'Thing' is the prefix-removed type name of the class.
*/
#define SingletonImplementation(TYPE, NAME) \
static TYPE *__ ## NAME; \
\
\
+ (void)initialize \
{ \
static BOOL initialized = NO; \
if(!initialized) \
{ \
initialized = YES; \
__ ## NAME = [[TYPE alloc] init]; \
} \
} \
\
\
+ (TYPE *)NAME \
{ \
return __ ## NAME; \
}
Example of use:
MyManager.h
#interface MyManager
SingletonInterface(MyManager, sharedManager);
// ...
#end
MyManager.m
#implementation MyManager
- (id)init
{
self = [super init];
if (self) {
// Initialization code here.
}
return self;
}
SingletonImplementation(MyManager, sharedManager);
// ...
#end
Why a interface macro when it's nearly empty? Code consistency between the header and code files; maintainability in case you want to add more automatic methods or change it around.
I'm using the initialize method to create the singleton as is used in the most popular answer here (at time of writing).
With Objective C class methods, we can just avoid using the singleton pattern the usual way, from:
[[Librarian sharedInstance] openLibrary]
to:
[Librarian openLibrary]
by wrapping the class inside another class that just has Class Methods, that way there is no chance of accidentally creating duplicate instances, as we're not creating any instance!
I wrote a more detailed blog here :)
To extend the example from #robbie-hanson ...
static MySingleton* sharedSingleton = nil;
+ (void)initialize {
static BOOL initialized = NO;
if (!initialized) {
initialized = YES;
sharedSingleton = [[self alloc] init];
}
}
- (id)init {
self = [super init];
if (self) {
// Member initialization here.
}
return self;
}
My way is simple like this:
static id instanceOfXXX = nil;
+ (id) sharedXXX
{
static volatile BOOL initialized = NO;
if (!initialized)
{
#synchronized([XXX class])
{
if (!initialized)
{
instanceOfXXX = [[XXX alloc] init];
initialized = YES;
}
}
}
return instanceOfXXX;
}
If the singleton is initialized already, the LOCK block will not be entered. The second check if(!initialized) is to make sure it is not initialized yet when the current thread acquires the LOCK.
I've not read through all the solutions, so forgive if this code is redundant.
This is the most thread safe implementation in my opinion.
+(SingletonObject *) sharedManager
{
static SingletonObject * sharedResourcesObj = nil;
#synchronized(self)
{
if (!sharedResourcesObj)
{
sharedResourcesObj = [[SingletonObject alloc] init];
}
}
return sharedResourcesObj;
}
I usually use code roughly similar to that in Ben Hoffstein's answer (which I also got out of Wikipedia). I use it for the reasons stated by Chris Hanson in his comment.
However, sometimes I have a need to place a singleton into a NIB, and in that case I use the following:
#implementation Singleton
static Singleton *singleton = nil;
- (id)init {
static BOOL initialized = NO;
if (!initialized) {
self = [super init];
singleton = self;
initialized = YES;
}
return self;
}
+ (id)allocWithZone:(NSZone*)zone {
#synchronized (self) {
if (!singleton)
singleton = [super allocWithZone:zone];
}
return singleton;
}
+ (Singleton*)sharedSingleton {
if (!singleton)
[[Singleton alloc] init];
return singleton;
}
#end
I leave the implementation of -retain (etc.) to the reader, although the above code is all you need in a garbage collected environment.
The accepted answer, although it compiles, is incorrect.
+ (MySingleton*)sharedInstance
{
#synchronized(self) <-------- self does not exist at class scope
{
if (sharedInstance == nil)
sharedInstance = [[MySingleton alloc] init];
}
return sharedInstance;
}
Per Apple documentation:
... You can take a similar approach to synchronize the class methods of the associated class, using the Class object instead of self.
Even if using self works, it shouldn't and this looks like a copy and paste mistake to me.
The correct implementation for a class factory method would be:
+ (MySingleton*)getInstance
{
#synchronized([MySingleton class])
{
if (sharedInstance == nil)
sharedInstance = [[MySingleton alloc] init];
}
return sharedInstance;
}