What should my Objective-C singleton look like? [closed]

Answer 1

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?

Answer 2

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;
}

Answer 3

@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]

Answer 4

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.

Answer 5

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).

Answer 6

KLSingleton is:

1. Subclassible (to the n-th degree)
2. ARC compatible
3. Safe with alloc and init
4. Loaded lazily
5. Thread-safe
6. Lock-free (uses +initialize, not @synchronize)
7. Macro-free
8. Swizzle-free
9. Simple

KLSingleton