How should I initialize an array that's a member of a UITableViewController?

Question

The data source for my table view is a plain NSMutableArray that will be populated as the app runs, but will be empty when the Table View first loads. The class interface looks like this...

@interface ViewController_iPhone : UITableViewController  {

    NSMutableArray *serverList; 
}

@property (retain, readonly) NSMutableArray *serverList;

@end

My questions are...

1. Currently, I initialize it in the viewDidLoad method like so...

   serverList = [[NSMutableArray alloc] initWithCapacity:1];

   I do it this way because the array needs to be valid in order for my numberOfRowsInSection method to avoid crashing when reading the count of the array (which will be zero) when the view first loads. My current approach of using initWithCapacity just feels a little clunky since I just need an empty, but valid array object that will return a count value of zero when the view loads. How should I be initializing my serverList array?

2. While playing around, I noticed that when I try and initialize the serverList array this way...

   serverList = [NSMutableArray arrayWithCapacity:1];

   it crashes on that line. Why?

Thanks in advance for all your help!

Answer 1

Note that capacity does not mean any objects are in the array. Using -initWithCapacity: simply sets aside a chunk of space for the array. Nothing is in the array even with a non-zero capacity.

The following initializers:

self.serverList = [[[NSMutableArray alloc] init] autorelease];
self.serverList = [[[NSMutableArray alloc] initWithCapacity:capacity] autorelease];
self.serverList = [NSMutableArray array];
self.serverList = [NSMutableArray arrayWithCapacity:capacity];

should all work, however.

Make sure you specify self so that the property is accessed. Using serverList by itself will not work.

Make sure you autorelease any alloc-init of a property which you are retaining, otherwise you will have a memory leak.

You do not need an array with a specified capacity. This is just a performance convenience for setting aside contiguous memory, and useful if you have a rough idea how much space your array will need up-front. An array that is created through alloc-init will also be empty, with a count of zero.

Answer 2

Here's a key concept to learn about member variables and properties: Member variables are not the same as properties.

That is, when accessing a member variable in your class:
serverList = [NSArray array]; is not the same as self.serverList = [NSArray array];

serverList by itself means you're accessing it directly.
self.serverList means you're using the getter/setter methods to access it.

Normally this isn't that big of a deal when dealing with basic variable types. However, when your property uses retain or copy, that means your setter method will automatically retain it when you use it, but it won't do such when you access it directly.

That means: serverList = [NSArray array]; will not retain the array. self.serverList = [NSArray array]; will retain the array.

It should be noted that [NSMutableArray arrayWithCapacity:1]; (and 99% of other methods that aren't alloc) will return an object that is autoreleased. If you want to keep it for later use, as you need to in this case, then you must retain it in some form or fashion.

Answer 3

I somehow missed the simplest approach and found that when I simply create the array like so...

serverList = [[NSMutableArray alloc] init];

and release it in the dealloc method, everything works great!