Question

I have a question about how instance variables work and when to use the @property. Here is an example interface file I am working with

@interface PackageModel : NSObject {
    NSString *tracking;
    NSString *carrier;
    NSString *status;
    NSMutableDictionary *events;

    // Connection ivars
    NSMutableData *receivedData;

    // Parsing ivars
    int tagLevel;
    NSMutableArray *tagTree;
    NSString *parentTag;
    NSString *currentTag;
}

@property (nonatomic, retain) NSMutableData *receivedData;

- (id)initWithTrackingString:(NSString *)string;
- (void)getPackageDataWithEvents;
- (void)printMe;

@end

How can I access these in the file’s code. Can I access tracking, carrier, and status in this class’s methods just by using something like

tracking = [[NSString alloc] initWithString:@"Hello World"];

Also, what variables need to be put in the dealloc? Only the variables I have in the @property/@synthesize? Or do I need to release all the instance variables in the dealloc method.

I am just looking for some clarification on how instance variables work in Objective-C. Thanks.

Answer 1

@property declarations are nothing more than compiler-generated getter and setter methods. That’s it. Just the methods. Obviously, you have to be getting and setting something, so we create the ivar that goes along with the getters and setters. In your example above, your compiler is generating:

- (NSMutableData *) receivedData;
- (void) setReceivedData:(NSMutableData *)newReceivedData;

The getter returns the value in the receivedData instance variable, and the setter changes the value in the receivedData instance variable.

(Side note: with the 64-bit runtime, you can skip declaring the instance variable, but I still like to put it in just to be explicit)

As for what you should do in your dealloc method, you need to release the instance variables that have been retained or copyed. So in your example, you’ll need to do [receivedData release]; in your dealloc method, because when you set the receivedData ivar, you retained the new value (that’s what the retain means on the @property line). If you don’t specify either retain or copy in the @property declaration, then it defaults to assign, and you don’t have to do anything. Beyond this, you’ll need to release any other instance variables that you retained yourself (ie, not through an @property (retain) setter).

For your last question, yes you can just do:

tracking = [[NSString alloc] initWithFormat:@"Hello, %@!", @"world"];

Inside your own class, you have direct access to all your own instance variables (and any protected and public ivars of your super classes).