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@interface MySingleton : NSObject

// ...

+ (instancetype) sharedInstance;

+ (instancetype) alloc  __attribute__((unavailable("alloc not available, call sharedInstance instead")));
- (instancetype) init   __attribute__((unavailable("init not available, call sharedInstance instead")));
+ (instancetype) new    __attribute__((unavailable("new not available, call sharedInstance instead")));

// ...

@end

@implementation MySingleton

// ...

+ (instancetype) sharedInstance {
    static id sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[super alloc] initInstance];
    });
    return sharedInstance;
}
 
- (instancetype) initInstance {
    self = [super init];
    // Do any other initialisation stuff here
    // ...
    return self;
}
 
// ...
 
@end