Safe way to create singleton with init method in Objective-C

Question

I would like to take the GCD approach of using shared instances to the next step so I created the following code:

@implementation MyClass

static id sharedInstance;

#pragma mark Initialization

+ (instancetype)sharedInstance {
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

- (instancetype)init {
    if (sharedInstance) {
        return sharedInstance;
    }
    @synchronized(self) {
        self = [super init];
        if (self) {
            sharedInstance = self;
        }
        return self;
    }
}

@end

I assume the sharedInstance method seems to be ok but I am unsure about the init method. The reason for creating this is that I don't want people using my SDK, to use the init method, and if they do ... make it bullet proof.

Answer 1

I would like to suggest new ways of solving your problem.

You can use NS_UNAVAILABLE in the header file just like this:

//Header file
@interface MyClass : NSObject
+ (instancetype)sharedInstance
- (instancetype)init NS_UNAVAILABLE;
//...
@end

In this case init function will not be available from outside, will not be suggested for autocompletion, and you'll be able to normally use the init method inside implementation file.

As you are making a singleton class I would suggest you to make new method unavailable too by adding this line to the header file:

+ (instancetype)new NS_UNAVAILABLE;

There is also an old way of making methods unavailable (which can be used in header too):

- (instancetype) init __attribute__((unavailable("Use 'sharedInstance' instead of 'init' as this class is singleton.")));

This can be used if you want to prompt some message about unavailability.