Understanding instance variable and property in Objective-C 2.0

Question

My development environment:

Xcode 4.6.2

non-Automatic Reference Counting

For example, suppose we have a view controller called CertainViewController, where a NSArray * typed property called listData has been declared, with attributes retain and nonatomic. listData is to be loaded to a table view. I would do it like this:

// CertainViewController.h
@interface CertainViewController : UIViewController
{
}
@property (retain, nonatomic) NSArray *listData;

// CertainViewController.m
@implementation CertainViewController

@synthesize listData;

- (void) viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.
    
    // someValue is ready
    self.listData = someValue;
    // Release someValue
}

- (void) dealloc
{
    self.listData = nil;
}

While some other developers would explicitly specify the instance variable _listData for property listData and do it like this:

// CertainViewController.h
@interface CertainViewController : UIViewController <UITableViewDelegate, UITableViewDataSource>
{
    NSArray *_listData;
}

@property (retain, nonatomic) NSArray *listData;

// CertainViewController.m
@implementation CertainViewController

@synthesize listData = _listData;

- (void) viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.
    
    // someValue is ready
    self.listData = someValue;
    // Release someValue
}

- (void) dealloc
{
    [_listData release];
}

Are the above two implementations totally equivalent? Or any tiny differences? The explicit appointing instance variable as _listData just mean to be use [_listData release];?

Any hint is welcome and thank you for help :D

In dealloc method you should (it is recommended) after release assign _listData = nil; — stosha, May 24 '13 at 04:03

score 2 · Answer 1 · answered May 24 '13 at 02:16

2

They're essentially the same, but if you want to know the exact difference, check out the documentation: http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html

answered May 24 '13 at 02:16

Matt Becker

2,338
1
28
36

score 2 · Answer 2 · answered May 24 '13 at 02:25

2

With the latest clang compiler you don't need to declare the instance variable, the compiler does it for you automatically.

Actually you don't even need to declare @synthesize anymore either. As soon as you declare your @property, the compiler will generate the getter, setter and the instance variable like if you had written:

@synthesize listData = _listData;

answered May 24 '13 at 02:25

Sylvain Guillopé

1,358
9
21

unless your code needs to run in 32 bit Mac land then you still need your backing iVar. – Grady Player May 24 '13 at 02:28
You might occasionally still want to declare an instance variable in the header - to allow access in a sub-class or category, for example. – Jasper Blues May 24 '13 at 02:50
1

@JasperBlues Yes indeed. I figured I'd provide the simplest answer that covers most cases related to declaring a `@property` – Sylvain Guillopé May 24 '13 at 03:29
Yup, sounds good. . . I think those edge cases are interesting - and can come in handy for certain things. – Jasper Blues May 24 '13 at 03:32

Understanding instance variable and property in Objective-C 2.0

2 Answers2