CODEWITHSUNDEEP
objective-cclassobjectmemory-management
Dmitry
Dmitry

Reputation: 14622

How can I check if object isn't deallocated on Objective-C?

How can I check if object isn't deallocated on Objective-C?

The following standard condition checks only if object is initialized:

NSObject *objectVariable = nil;
...
if (objectVariable) {...}

Upvotes: 0

Views: 2729

Answers (2)

newacct
newacct

Reputation: 122439

You can't check after-the-fact whether an object is already deallocated or not, because it is invalid to do anything with an object that is deallocated (once an object is deallocated, any pointer that used to point to it is now an invalid pointer, and dereferencing it is undefined behavior). And since you don't know whether it is deallocated or not, and it may be deallocated, you cannot meaningfully examine it in any way. And a deallocated object may well "look like" a valid object if you try to examine it (it is undefined behavior, so any behavior is possible).

If this is an object of your own class, the obvious thing would be to print something or perform some other indicator action in the class's -dealloc method. That will tell you when it is deallocated.

Even if this is an object that is not of your own class, you may be able to either 1) add -dealloc as a category method, if the class doesn't override -dealloc, or 2) swizzle the -dealloc method, if the class does override it. Then you can print something in your version.

You can also profile it in Instruments; the Allocations instrument can tell you how many objects of a given class have been allocated, and how many are alive. If you suspect that you are losing track of objects or there is a retain cycle, the Leaks instrument may be able to tell you that.

Upvotes: 2

BergQuester
BergQuester

Reputation: 6187

I'm in agreement with the comments, If you're doing memory management right, there should be no need for such a check. Nor am I aware of such a check, if the address gets populated with a new object, the check would pass but could still crash your app.

My suggestions are to:

  1. Read up on manual memory management rules, pay special attention to how blocks affect memory management, alloc/init methods, when to use assign, etc. Memory management rules should become second nature to you. Start with this Apple guide.
  2. Run static analysis on your app and fix any memory errors. Fix all the errors really, these are bugs in your app. (CMD+Shift+B or Product->Analyze in the menu)
  3. Reproduce the crash in Instruments using zombies. Read the retain/release report to find out where the object may have been over-released. (CMD+I or Product->Profile. Select Zombies in the window that appears)
  4. Consider converting to ARC. Converting to ARC doesn't get you completely off the hook for understanding ObjC memory management, but it will take a lot of the burden off of you.

Upvotes: 1

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