Thread safe instantiation of a singleton

Question

Which one synchronization method to use to ensure a singleton remains a singleton?

+(Foo*)sharedInstance
{
   @synchronized(self)
   {
      if (nil == _shar         


        

Answer 1

If anyone cares, here is another macro for the same thing :)

IMHO, it provides greater flexibility compared to the other variations.

#define SHARED_INSTANCE(...) ({\
    static dispatch_once_t pred;\
    static id sharedObject;\
    dispatch_once(&pred, ^{\
        sharedObject = (__VA_ARGS__);\
    });\
    sharedObject;\
})

Usage, one-line initialization:

+ (instancetype) sharedInstanceOneLine {
    return SHARED_INSTANCE( [[self alloc] init] );
}

Usage, multi-line initialization (notice curly braces around the block of code):

+ (instancetype) sharedInstanceMultiLine {
    return SHARED_INSTANCE({
        NSLog(@"creating shared instance");
        CGFloat someValue = 84 / 2.0f;
        [[self alloc] initWithSomeValue:someValue]; // no return statement
    });
}

Usage in the right part of an assignment:

- (void) someMethod {
    MethodPrivateHelper *helper = SHARED_INSTANCE( [[MethodPrivateHelper alloc] init] );
    // do smth with the helper
}
// someMethod should not call itself to avoid deadlock, see bbum's answer

This modification utilizes two language features: the GCC compound expressions extension, which is also supported by Clang, and the C99 variadic macros support.

After pre-processing, the output will look like (you can test it yourself by invoking Product > Perform Action > Preprocess "YourClassName.m" in Xcode 5):

+ (instancetype) sharedInstanceOneLine {
    return ({
        static dispatch_once_t pred;
        static id sharedObject;
        dispatch_once(&pred, ^{
            sharedObject = ( [[self alloc] init] );
        });
        sharedObject; // this object will be returned from the block
    });
}

+ (instancetype) sharedInstanceMultiLine {
    return ({
        static dispatch_once_t pred;
        static id sharedObject;
        dispatch_once(&pred, ^{
            sharedObject = ({
                NSLog(@"creating shared instance");
                CGFloat someValue = 84 / 2.0f;
                [[self alloc] initWithSomeValue:someValue];
            });
        });
        sharedObject;
    });
}

- (void) someMethod {
    MethodPrivateHelper *helper = ({
        static dispatch_once_t pred;
        static id sharedObject;
        dispatch_once(&pred, ^{
            sharedObject = ( [[MethodPrivateHelper alloc] init] );
        });
        sharedObject;
    });
}