Question

Can Someone explain me few things regarding the Singleton implementation in Apple’s documentation here.

Link: – http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html

Go to Creating a Singleton Instance in the link.

I tried but couldn’t understand few things:-

• What does [super allocWithZone:NULL] in + (MyGizmoClass*)sharedManager does.?
• Why does allocWithZone call sharedManger method and return a retain call on its return type when retain itself returns self ?
• If the Singleton has some instance variables in it, where should they be initialized ?

If anyone could briefly explain working of allocWithZone and sharedManager methods here , a lot of these questions would automatically be answered.

Answer 1

That implementation is generally considered over_kill. There is a lot of protections against a programmer trying to mis-use the singleton which is generally not considered necessary.

Here is an example of a more simple implementation from Yoga:

+ (id)sharedFoo
{
    static dispatch_once_t once;
    static MyFoo *sharedFoo;
    dispatch_once(&once, ^ { sharedFoo = [[self alloc] init]; });
    return sharedFoo;
}