Objective-C Singleton Demo

main.m

#import <Foundation/Foundation.h>
#import "Singleton.h"

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        NSLog(@"Objective-C Singleton Demo");
        [[Singleton sharedSingleton] sayHello];
        [[Singleton sharedSingleton] sayHello];
    }
    return 0;
}

Singleton.h

#import <Foundation/Foundation.h>

@interface Singleton : NSObject
+(Singleton*)sharedSingleton;
-(void)sayHello;
@end

Singleton.m

#import "Singleton.h"

@implementation Singleton
static Singleton* _sharedSingleton = nil;
+(Singleton *) sharedSingleton{
    @synchronized([Singleton class])
	{
		if (!_sharedSingleton)
			[[self alloc] init];
        
		return _sharedSingleton;
	}
    return nil;
}

+(id)alloc
{
	@synchronized([Singleton class])
	{
		NSAssert(_sharedSingleton == nil, @"Attempted to allocate a second instance of a singleton.");
		_sharedSingleton = [super alloc];
		return _sharedSingleton;
	}
	return nil;
}

-(id)init {
	self = [super init];
	if (self != nil) {
		// initialize stuff here
	}
    
	return self;
}

-(void)sayHello {
	NSLog(@"Hello World!");
}

@end