NSDate retain message sent to deallocated instance - iphone

Question

I've been having some problems with NSDate and saving it in NSUserDefaults. It seams that every second time NSUserDefaults saves my NSDate, it can't because it is deallocated and shows this error in the log.
-[__NSDate retain]: message sent to deallocated instance 0x4c20c80
I know that NSDate allocs and deallocs in different ways to that of normal objects, but I was wondering if anyone knows if by using:
- (void)saveData
{
NSUserDefaults *data = [NSUserDefaults standardUserDefaults];
[data setObject:dateOpened forKey:#"dateOpened"];
[dData synchronize];
}
...or...
- (void)loadData
{
NSUserDefaults *data = [NSUserDefaults standardUserDefaults];
dateOpened = [data objectForKey:#"dateOpened"];
}
i am releasing my instance of NSDate and so giving it a retain count of 0 so my app cant save it again when it tries?
I am using:
#property (retain) NSDate *dateOpened;
Any idea's would be much grateful as I am going nuts trying to figure this out. I've only been learning for about 4 months or so and am so nearly finished my first app and this is a major spanner in the works!
Thanks a lot, and if you need any more code or information on what I'm doing, please let me know. :-D

Answer 1

NSDate does not behave any differently than any other object as far as memory management goes. What you may be referring to is that it is common to use convenience operators like [NSDate date] that return an autoreleased object (meaning the object will be deleted at the end of the main loop (or whenever the autorelease pool is released) unless another class calls retain on it. Since I cannot see all of your code I can only make an educated guess, but I believe that you are calling release on the object returned from [data objectForKey:] and this would be your mistake. That function returns an autoreleased object and therefore you do not have "ownership" of the object until you call retain on it. If you do not call retain on it, or allocate it explicitly, you should never be calling release on it (this goes for all objects).

Answer 2

The issue you are having is in your -loadData method.
dateOpened = [data objectForKey:#"dateOpened"];
Is the line above, you are accessing the ivar directly and not going thru the property which will retain. So you have one of two choice.
// First choice retain it yourself
dateOpened = [data objectForKey:#"dateOpened"];
[dateOpened retain];
Or
// Second choice have the #property do it for you
[self setDateOpened:[data objectForKey:#"dateOpened"]];
This is why it's a good idea to use underscores on your private ivars so you know when you are accessing them directly. You will have few mistakes. :)
// declaring it with underscore would have caught your mistake.
NSDate *_dateOpened;

Answer 3

The problem in loadData is that you are directly assigning dateOpened to an autoreleased value, which will be invalid once the event loop passes.
- (void)loadData
{
NSUserDefaults *data = [NSUserDefaults standardUserDefaults];
// dateOpened = [data objectForKey:#"dateOpened"];
// try:
self.dateOpened = [data objectForKey:#"dateOpened"];
// which is basically the same as:
// [self setDateOpened:[data objectForKey:#"dateOpened"]];
// the following 2 lines could also work:
// [dateOpened release];
// dateOpened = [[data objectForKey:#"dateOpened"] retain];
}
If you have #synthesized dateOpened, then you can imagine that the following 2 methods have been added to your class:
- (NSDate *)dateOpened {
return dateOpened;
}
- (void)setDateOpened:(NSDate *)aDate {
[aDate retain];
[dateOpened release];
dateOpened = aDate;
}