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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

Baza207
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  • In your sample code above where you are saving and load the data, the ivar say CLOSED (dateClosed) and the key say OPENED (dateOpened). But when you mention the @property line, you say the ivar is dateOpened (OPENED). Are you say you not mixing your dates up? – Black Frog Mar 24 '11 at 23:36
  • Oh sorry, forgot to change that. Just edited it to how it suposed to be. Dates are not being mixed up, just bad copy and pasting on my part. :) – Baza207 Mar 24 '11 at 23:41

3 Answers3

3

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).

drewag
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  • So release should never be called on an NSDate object if you want to keep it? Also if I'm understanding you correctly if I don't retain an instance of NSDate at all is it as bad as releasing it? – Baza207 Mar 24 '11 at 23:32
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    My friend, thank you, thank you, thank you, thank you! This has been driving me crazy for ages and now i understand. Every time I was getting the date i was just putting [NSDate date] and so not retaining it. So now changing that to [[NSDate date] retain] it clears everything up. Thanks for your help. :-D – Baza207 Mar 24 '11 at 23:49
  • your welcome :). If you want to keep an object around, and guarantee it will be around for a specific class you should always retain it (either through implicitly in an init call or with an explicit retain call). then when the class is done with it, it should always call release on it so it doesn't get leaked. – drewag Mar 25 '11 at 02:16
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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;
Black Frog
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  • What does adding an underscore do exactly? I noticed Xcode 4 recommended it to me for a few other things when I first got it, but I didn't really know what it was doing... – Baza207 Mar 25 '11 at 00:13
  • In "first choice retain it yourself", you should also release `dateOpened` beforehand, in case there is already something already there. Also, retain and the assignment can be done on the same line: `dateOpened = [[data objectForKey:@"dateOpened"] retain];` – user102008 Apr 19 '11 at 21:57
  • "use underscores on your private ivars so you know when you are accessing them directly. You will have few mistakes." This doesn't make sense at all. You "know" you are accessing a property because you write a dot (`.`) in the code; you "know" you are accessing an ivar because you don't have a dot. You always retain/release when you set an ivar (except delegates) that is an object, even if you happen to be using that ivar as a property. So you see, it's irrelevant whether it is used as a property; it only matters that you follow the rules to retain when setting instance variables. – user102008 Apr 19 '11 at 22:05
  • Yes, in proper memory management, you would [dataOpened release]. Also, if you want to make a comment about the coding style, it should be directed to the question. – Black Frog Apr 19 '11 at 22:07
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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;
}
NSGod
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