Making singleton class secure

15 Feb 2011

There comes a time for a project, when you can’t resist using a Singleton class, for things that can’t be afforded to duplicate. Say, for example I’m creating a World class and I want only a single world to exist in my entire project. So, next time anyone is referring to World, he’s referring to this:

static World *_world = nil;

@implementation World

+(World *)sharedWorld
{
	if(!_world)
		_world = [[self alloc] init];
	return _world;
}

-(id)init
{
	self = [super init];
	if(self)
	{
		//create world
		[self setup_world];
	}
	return self;
}

And then whenever someone wants to refer to world:

[[World sharedWorld] scheduleSnowfall];

Look cool? wait a sec. Suppose, at some random time somebody does something like:

World *newOne = [[World alloc] init];
[newOne scheduleSnowfall];
[newOne release];

This will create a new instance of the World and scheduleSnowfall in there. (Everybody living in this world will still be having sunshine).

So, the best fix is throwing an exception, thats like creating a new World totally illegal. How you do that?

+(World *)sharedWorld
{
	if(!_world)
		_world = [[self alloc] __secret_init];
	return _world;
}

-(id)init
{
	NSException *warning = [NSException exceptionWithName:@"World" reason:@"Illegally created: are you God?" userInfo:nil];
	@throw warning;
}

-(id)__secret_init
{
	self = [super init];
	if(self)
	{
		//create world
		[self setup_world];
	}
	return self;
}

So, now for anybody trying to be the next GOD, will see this exception ruining the dreams:

*** Terminating app due to uncaught exception ‘World’, reason: ‘Illegally created: are you God?’