Should I alloc an NSMUtableArray instance variable?

Question

This is probably a silly question but I have been thinking it over for a while with no obvious answer.

I have a small app that I have been working on and am having a problem with my NSMutableArray instance variable. It was always (null) until I added an [[NSMutableArray alloc]init] to the viewDidLoad.

I am pretty new to the Objective C world so I am trying to get an understanding of this issue. I was under the impression that with I instantiated my class the instance variables were all alloc'd. When I add items to the array without the alloc/init it remains (null) with a retain count of 0.

The array in question was declared as a (retain) property.

I can add code if this isn't clear.

Answer 1

Any instance variables that are objects are actually just pointers that are initialized to nil (0). That is why the item isn't retained and added to the array, since messages to nil objects return nil/0.

You need to alloc and init the object in your class's init, and then release it in the dealloc.

Answer 2

The retain/assign/copy qualifiers on the property declaration are about how the memory for the property value is managed in the getters and setters that the compiler synthesizes for you. (The documentation discusses them in detail, and gives example code for each kind.)

That's completely orthogonal to whether your instance variables are initialized for you or not. Declaring an ivar is just reserving storage for the value; for Objective-C classes, that's a pointer to an instance. The runtime will initialize those ivars to zero for you, but you're still responsible for creating the objects you want to store there. (The same is true in similar languages like Java or C#: declaring an Array instance variable just gives you space for a reference, it doesn't create the array for you.)