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
initializeto 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 theinitializemessage 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
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;
}
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;
}
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
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.
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;
}
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;
}
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;
}
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;
}