Question

Well, i am still confused about objective c properties and instance variables.
I create a LocationManager-object in my viewDidLoad. On the one hand the LocationMan is just an instance variable on the other hand it is declared as a property. Have a look at the examples:

First example:

Header:

CLLocationManager* _locationMan;

Implementation:

CLLocationManager* theManager = [[[CLLocationManager alloc] init] autorelease];
_locationMan = theManager;
_locationMan.delegate = self;

Second Example

Header:

CLLocationManager* _locationMan;
@property (retain, nonatomic) CLLocationManager* locationMan;

Implementation:

self.locationMan = [[[CLLocationManager alloc] init] autorelease];
self.locationMan.delegate = self;

Whats the difference between those examples except of that the second one is working and the first one not? What’s going on there with the memory management?

Answer 1

The problem you have in your first example:

CLLocationManager* theManager = [[[CLLocationManager alloc] init] autorelease];

is caused by the use of autorelease. autorelease can be seen as meaning: at some point in the near future, release automatically this object. Given the way autorelease is implemented, by means of a release pool, this usually happens the next time the control flow return to the main loop and the release pool is emptied.

So, in your first case, you are creating the object and storing it in your ivar; but soon it will be released, and since you don’t have explicitly retained it anywhere else, it will end up being deallocated. When you access it after that, you get the error. If you had not used autorelease, everything would have worked correctly:

CLLocationManager* theManager = [[CLLocationManager alloc] init]; //-- this is correct
_locationMan = theManager;  //-- because you assign directly to the ivar

In your second example, creation is just the same, meaning, the object will also be flagged for autorelease. But, in this case, you assign it to a property which has got the retain modifier. This means that the object will be automatically retained when assigning to the property. So, when the autorelease is actually done (when you get back to the main look, roughly), your object will have already had its retain count incremented by 1; then auto releasing it will not make its retain count go to 0, and the object will not be deallocated.

What you have to know clearly is that:

1. alloc will set the retain count to 1;

2. retain properties will increment the retain count when assigning to them;

3. autorelease is like a delayed release, so that in the mean time (before the release is actually done, which means in the rest of your method and callers up to the main loop) you can use the object safely and thereafter it will be released.