iOS "Hiding" init method

MickeyT · Feb 18, 2013

I have written a class where one of its instance variables needs to point to the object which created it. I have written an initWithName:andContainer method where one of the second argument requires an instance of the creating object. A weak pointer is then created to this "containing" object.

Normally, I would also override the init method, call the above method and pass some dummy parameters. However, in this case I don't want init to be "available" because I can't pass a dummy parameter for the containing object. These objects must never be created without an "owning" container.

Is there a way to stop init being available, or do I just set it up so that the init method returns nil which would, I assume, cause an exception to be thrown and thus force the use of the initWithName:andContainer method?

Thank you

ArtOfWarfare · Feb 18, 2013

MickeyT said:
do I just set it up so that the init method returns nil which would, I assume, cause an exception to be thrown

Returning nil would not cause an exception to be thrown unless you somehow set it to throw yourself. In Obj-C, nil evaluates to zero and any calls to it will simply return zero.

There's an answer on StackOverflow over here suggesting that you should throw an NSInternalInconsistencyException:

http://stackoverflow.com/questions/...o-make-the-init-method-private-in-objective-c

Edit: Actually, I think I like the answer below it more... doesNotRecognizeSelector: sounds like exactly what you want:

http://stackoverflow.com/a/5772821/901641

MickeyT · Feb 18, 2013

Returning nil would not cause an exception to be thrown unless you somehow set it to throw yourself. In Obj-C, nil evaluates to zero and any calls to it will simply return zero.

Sorry - what I meant was that when an attempt is subsequently made to use the object, I thought an exception would be thrown because the [super init] method would never have been called and so it wouldn't have been set up properly.

But I realised just now that you're absolutely right - I coincidentally didn't initialise an NSString object just now, and when I use it there's no exception, it just doesn't work.

Thanks for the reply - I'll have a look over those links and see if I'm happy before resolving the thread.

Many thanks.

PhoneyDeveloper · Feb 18, 2013

I wouldn't call [self release] and then [super whatever] since you'll probably be dealloced from the [self release] call. At any rate, it's neurotic to be trying to do memory management to avoid a memory leak when on the next line you're going to deliberately crash your app, as suggested in one of those links.

Any of the suggested solutions is fine.

ArtOfWarfare · Feb 18, 2013

PhoneyDeveloper said:
I wouldn't call [self release] and then [super whatever] since you'll probably be dealloced from the [self release] call. At any rate, it's neurotic to be trying to do memory management to avoid a memory leak when on the next line you're going to deliberately crash your app, as suggested in one of those links.

Not necessarily... you could catch the exception somewhere, could you not?

PBG4 Dude · Feb 18, 2013

If I understand you correctly, you're saying that Object A instantiates Object B and requires a pointer to Object A?

If so, you should set up Object A to be a delegate of Object B.

MickeyT · Feb 19, 2013

If so, you should set up Object A to be a delegate of Object B.

Forgive me - what is the difference between what I am doing and setting up as a delegate?

Whenever I have set something as the delegate of something else, I was under the impression that "delegate" was just the name that was given to the instance variable I was assigning the object to (and I was using the setter method setDelegate to do that). In this case, that's what I do, but my instance variable isn't called "delegate", it is called something else.

And I am doing this via an init method, because I absolutely want this to always happen.

dejo · Feb 19, 2013

MickeyT said:
Forgive me - what is the difference between what I am doing and setting up as a delegate?

Delegation usually has a protocol associated with it. And delegate methods that may need to be implemented. All you're doing, I believe, is capturing a simple relationship between two objects, a parent-child one, as it were.

MickeyT · Feb 19, 2013

All you're doing, I believe, it capturing a simple relationship between two objects, a parent-child one, as it were.

That's right. Whether what I'm doing is a good way or a bad way to achieve what I'm setting out to do is possibly irrelevant at this stage; I want to do it so that if it turns into a nightmare then I will learn from that.

Essentially, I might end up with a hierarchy of perhaps three or four levels. When I'm down at level four, I want to be able to access instance variables at level one if the situation warrants it. Chances are the view controller dealing with the level four object won't have any knowledge of the level one object, and so I was thinking I would need a way of moving up and down the chain without having to bring the level one object with me everywhere.

That might not make sense - the app is going to be navigation controller based and so I was making my model layer to sort of follow the same drill down convention.

Duncan C · Feb 19, 2013

MickeyT said:
I have written a class where one of its instance variables needs to point to the object which created it. I have written an initWithName:andContainer method where one of the second argument requires an instance of the creating object. A weak pointer is then created to this "containing" object.

Normally, I would also override the init method, call the above method and pass some dummy parameters. However, in this case I don't want init to be "available" because I can't pass a dummy parameter for the containing object. These objects must never be created without an "owning" container.

Is there a way to stop init being available, or do I just set it up so that the init method returns nil which would, I assume, cause an exception to be thrown and thus force the use of the initWithName:andContainer method?

Thank you

One of the stack overflow links posted by another poster suggested adding a compiler directive that would throw a compile-time error:

-(id)init __attribute__((unavailable("init not available")));

That seems like the best option to me. You get warned at compile time so you know not to use the init method.

MickeyT · Feb 20, 2013

Thank you everyone - very helpful.

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