Objective C double locking singleton example

PCSingletonExample.h

//
//  PCSingletonExample.h
//  Lab11
//
//  Created by Chris Zelenak on 12/6/13.
//  Copyright (c) 2013 Bootstrapping iOS. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@interface CoreDataLayer : NSObject

+(CoreDataLayer *) sharedInstance;

@property NSManagedObjectContext * context;

@end

PCSingletonExample.m

//
//  PCSingletonExample.m
//  Lab11
//
//  Created by Chris Zelenak on 12/6/13.
//  Copyright (c) 2013 Bootstrapping iOS. All rights reserved.
//

#import "PCSingletonExample.h"

@implementation CoreDataLayer

+(CoreDataLayer *) sharedInstance {
    static CoreDataLayer * _inst;
    if(_inst == nil){
        @synchronized([self class]){
            if(_inst == nil){
                _inst = [[CoreDataLayer alloc] init];
            }
        }
    }
    return _inst;
}

-(id) init{
    self = [super init];
    if(self){
        self.context = nil; //  some core data setup
    }
    return self;
}


@end