在Objective-C中分配属性

Question

I have the following questions about allocating properties in Objective-C,

if I have the following property:

@property (nonatomic, retain) NSArray *arr;
@synthize arr=_arr;

should I allocate this array like:

arr=[[NSArray alloc]init];

and if I should to, where is the best place to to allocate it?

I know I should release it, but retaining a property will increase its retain count by 1 and allocating it will increase it by another 1, am I right.

Answer 1

You can allocate the array in two ways:

1. Set the ivar directly with a retained value, like this:

    _arr = [[NSArray alloc] init]; 

2. Set the property with an autoreleased value like this:

    self.arr = [NSArray array]; 

You can do this in your class's init method, although you've not said what superclass this is using so I'm not sure how the init method should look. If it's an NSObject, it will look like this:

- (id)init
{
    if ((self = [super init]))
    {
         self.arr = [NSArray array];
    }
    return self;
}

But if it's a UIViewController, you'll need to use initWithNibName:bundle, like this:

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)bundleOrNil
{
    if ((self = [super itWithNibName:nibNameOrNil bundle:bundleOrNil]))
    {
         self.arr = [NSArray array];
    }
    return self;
}

Then you need to release it in your dealloc method, like this:

- (void)dealloc
{
    [_arr release];
    [super dealloc];
}

However, you should bear in mind that NSArrays can't be changed once they're created, so you probably either want to initialise it with some objects, like this:

self.arr = [NSArray arrayWithObjects:object1, object2, nil];

Or if you want to add objects to it later, you should define it as an NSMutableArray, like this:

@property (nonatomic, retain) NSMutableArray *arr;

And init it with:

self.arr = [NSMutableArray array];

That will let you add objects to it later.

By the way, if you get errors in your dealloc method, it probably means that your project is using ARC, which is a new technology in iOS 5 that means you don't need to write release and retain statements any more.

If you aren't using ARC I suggest you do so because it will save you having to worry about this retain/release stuff - any iOS developer who is just starting out now should really use ARC for every project, but because it was only introduced in iOS 5, a lot of the Objective-C books don't cover it.