Singleton in Objective-C

Mac recommendations to create singlton:

static MyGizmoClass *sharedGizmoManager = nil;

+ (MyGizmoClass*)sharedManager
{
@synchronized(self) {
if (sharedGizmoManager == nil) {
[[self alloc] init]; // assignment not done here
}
}
return sharedGizmoManager;
}

+ (id)allocWithZone:(NSZone *)zone
{
@synchronized(self) {
if (sharedGizmoManager == nil) {
sharedGizmoManager = [super allocWithZone:zone];
return sharedGizmoManager; // 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;
}

Macro by Matt Gallagher

#define SYNTHESIZE_SINGLETON_FOR_CLASS(classname) \
\
static classname *shared##classname = nil; \
\
+ (classname *)shared##classname \
{ \
@synchronized(self) \
{ \
if (shared##classname == nil) \
{ \
shared##classname = [[self alloc] init]; \
} \
} \
\
return shared##classname; \
} \
\
+ (id)allocWithZone:(NSZone *)zone \
{ \
@synchronized(self) \
{ \
if (shared##classname == nil) \
{ \
shared##classname = [super allocWithZone:zone]; \
return shared##classname; \
} \
} \
\
return nil; \
} \
\
- (id)copyWithZone:(NSZone *)zone \
{ \
return self; \
} \
\
- (id)retain \
{ \
return self; \
} \
\
- (NSUInteger)retainCount \
{ \
return NSUIntegerMax; \
} \
\
- (void)release \
{ \
} \
\
- (id)autorelease \
{ \
return self; \
}

If you #import this header at the top of a class implementation, then all you need to do is write:

SYNTHESIZE_SINGLETON_FOR_CLASS(MyClassName);

inside the @implementation MyClassName declaration and your class will become a singleton. You will also need to add the line:

+ (MyClassName *)sharedMyClassName;

to the header file for MyClassName so the singleton accessor method can be found from other source files if they #import the header.

Once your class is a singleton, you can access the instance of it using the line:

[MyClassName sharedMyClassName];

Note: A singleton does not need to be explicitly allocated or initialized (the alloc and init methods will be called automatically on first access) but you can still implement the default init method if you want to perform initialization.