Releasing an object is causing my app to crash.

[rpierich]

In one of my controllers I have an array called videoGameItems (an instance variable) that contains dictionary objects. In my viewWillAppear method I call another method to load this list in from a sqlite database. In my load db method I

1. [videoGameItems release];

2. videoGameItems = [[NSMutableArray alloc] initWithCapacity: 100];

3. read in rows from the database and populate videoGameItems

The first time viewWillAppear is called everything works correctly. When I switch to a different view and then pop that view off the stack to return to my videogamelist view the application crashes on the [videoGameItems release]; line.

Does this sound like an error from memory management or something else?

Thank You,
Ruben

[rpierich]

Also, I am using a UINavigationController to push the view onto the stack and then pop it off. Could that corrupt the videoGameItems variable (an instance variable)?

Originally Posted by rpierich

In one of my controllers I have an array called videoGameItems (an instance variable) that contains dictionary objects. In my viewWillAppear method I call another method to load this list in from a sqlite database. In my load db method I

1. [videoGameItems release];

2. videoGameItems = [[NSMutableArray alloc] initWithCapacity: 100];

3. read in rows from the database and populate videoGameItems

The first time viewWillAppear is called everything works correctly. When I switch to a different view and then pop that view off the stack to return to my videogamelist view the application crashes on the [videoGameItems release]; line.

Does this sound like an error from memory management or something else?

Thank You,
Ruben

[Robby]

Why are you calling Release before you alloc the object?
Release should only be called after alloc, and once you no longer need the memory.
Rob.

[smasher]

Originally Posted by Robby

Why are you calling Release before you alloc the object?
Release should only be called after alloc, and once you no longer need the memory.
Rob.

I think he's releasing the old list, and then creating a new one. That seems reasonable. On the first call videoGameItems is probably nil, and releasing nil has no effect.

If you get a crash when you release an object, it usually means that the object has been released too many times - you're releasing an object that has already been destroyed. Is there any other place where you're releasing videoGameItems?

Addendum: and no, pushing the view onto a stack should not change the instance variables.

[Kalimba]

I also think that it sounds like the crash is due to over-releasing the videoGameItems member. If you are releasing this member somewhere else in your code, setting it to 'nil' after releasing it will probably get you fixed up.

[Kalimba]

Originally Posted by Robby

Why are you calling Release before you alloc the object?
Release should only be called after alloc, and once you no longer need the memory.
Rob.

It's easy to see how you might be confused by the posted code, since nearly all memory-management articles describe an "alloc then release" strategy, e.g. if you alloc/init it, you should later release it. I can't think of a single Obj-C/Cocoa memory management discussion that I've seen that doesn't take this same approach.

But in practice, the "release the old thing, then alloc the new thing" strategy is quite common. The basic thinking here is that the first time through the code, the "old thing" is nil, so releasing it does absolutely nothing, then the alloc/init (or the 'malloc', in plain C) makes the initial "new thing" assignment. Subsequent trips through the code will release the current member, then assign a new one.

One caveat of this "release then alloc" approach is that there really needs to be another release somewhere in the code to release the member when it's no longer needed, or the memory may be held onto longer than necessary and might possibly never be released. A problem arises when this alternate release doesn't put things into the proper state for the next trip through the "release then alloc" code. Specifically, if the member is released but is allowed to remain pointing at the released object, the app will crash the next time through the "release then alloc" code. This is because the member is again released, but it's not actually pointing to a valid object. Setting the member to nil after the alternate release properly fixes the member so the release then alloc code works properly without crashing.