This is a preferred way to set up a singleton.

SingletonClass.h

#import <Foundation/Foundation.h>

@interface SingletonClass : NSObject
{
    
}

// Properties of the singleton

+(SingletonClass *)sharedSingleton;

@end

SingletonClass.m

#import "SingletonClass.h"

@implementation SingletonClass

static SingletonClass *theSingleton = nil;

+(SingletonClass *)sharedSingleton {
    if (theSingleton != nil) {
        return theSingleton;
    }
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        theSingleton = [[SingletonClass alloc] init];
    });
    
    return theSingleton;
}

// Called the first time the Singleton is used
- (id)init
{
    self = [super init];
    
    if (self) {
        // Post initialization code such as setting other properties
    }
    
    return self;
}

@end