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First of all, as I understand it, init in Objective-C, functionally is similar to a constructor in Java, as it is used to initialize instance variables and prepare a class to do some work. Is this correct?

I understand that NSObject implements init and as such it does not need to be declared in any .h files.

But how about custom implementation of init for a given class, for example: 

(id) initWithName:(NSString *) name

Should declaration like this be listed as part of .h, or it is not necessary? Is it done by convention or is there any other reasoning?

taskinoor
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James Raitsev
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2 Answers2

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init is by no means similar to constructor in Java/C++. The constructor always executes when the object is created. But the execution of init is up to you. If you don't send init message after alloc then it will not execute. 

// init does not execute here
MyObject *obj = [MyObject alloc];

And this will work without any problems if you derive from NSObject, as init of NSObject does nothing.

You do not need to add init in the header file, because it is inherited from NSObject but you need to add custom init methods (that are not inherited) to the header file. Note that init methods are just normal methods with a naming convention, but technically there is no difference from other methods.

If you do not specify your custom init methods in the header file, but send that message to an object, the compiler will generate a warning. There will be no compile error. So if you decide to ignore the warning then you can omit that from header too. But you will get a runtime crash if the method is not actually implemented. So it's better to add all methods that are not inherited in header file.

Cypress Frankenfeld
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taskinoor
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    Thank you. If i initialize my class with a custom init method, does it mean that I should always [super init] to make sure NSObjet's init is called at least once? – James Raitsev Jul 19 '11 at 15:43
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    Yes you should do this always even if NSObject's init does no initialization. This is a good practice and there is no problem when you call this. And for most of the other classes calling super class initialization is required. So the summary is: call base class's initialization method always. – taskinoor Jul 19 '11 at 15:49
  • I'm not sure why I got a downvote here. A comment explaining my fault would have helped me. – taskinoor Feb 27 '13 at 05:35
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Yes you have to declare it if you want to be able to call this personalized initialisation method (initWithName). And the first think you have to do in that method is to call [super init];.

taskinoor
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Rabskatran
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    It'd be better to say that the initializer should call its superclass's designated initializer. The designated initializer isn't necessarily `-init`, and you're free to do other work before calling the designated initializer. – Caleb Jul 19 '11 at 15:30
  • You don't **have** to declare it. It will still be callable without the declaration, but you'll get compiler warnings when trying to use it from other classes. – Richard Jul 19 '11 at 15:30