Correct [super dealloc]

Question

Does the order of statements in the dealloc method matter? Does the [super dealloc] need to be at the top of the method? Does it matter?

Also in e.g. viewDidLoad. Should [super viewDidLoad] be at the top of the method?

Cementation

Duplicate of - #4206656 and #7901802 – Audio

Answer 1

It ABSOLUTELY matters.

What you do depends on whether you're using Automatic Reference Counting (ARC) or manual reference counting.

Using Manual Release-Retain

Manual Release-Retain (MRR) is default memory management for all versions of Mac OS X, and the only way to handle memory until Xcode 4.2.

With MRR, [super dealloc] should be at the end of your method.

So your code should look like this:

- (void)dealloc
{
    [member release];
    [super dealloc];
}

The super dealloc actually frees the memory. Think about that. If you access an instance variable after that, like this:

[super dealloc];
[member release];

...it means that the instance variable is potentially invalid. Between the call to super dealloc and the call to member release, theoretically the bytes that stores the member pointer could have been changed to something else!

As Apple explains it in the Memory Management Programming Guide:

The role of the dealloc method is to free the object's own memory, and dispose of any resources it holds, including ownership of any object instance variables.

You do this by disposing of any resources your object holds, and calling [super dealloc]. Which disposes any objects it holds, and calls its super. And so on, and so on, until eventually the root object marks the memory used by the instance itself as free. By the time [super dealloc] returns, your instance has been freed. (Of course, the pointers in it are probably valid, but that's an implementation detail you shouldn't rely on.)

Generally, when constructing (or loading) let the super do the work first. When tearing things down, do your work first.

See also:

• dealloc, NSObject Class Reference, Mac OS X Developer Library
• Deallocating an Object, Object Ownership and Disposal, Memory Management Programming Guide, Mac OS X Developer Library

With Automatic Reference Counting

Automatic Reference Counting (ARC) is the new way of doing memory management introduced in Xcode 4.2. With ARC, the compiler adds memory management code when compiling your application. It has some wrinkles you'll want to read more about before using it (mostly, limited compatibility with older OS versions).

With ARC, you don't (and can't) call [super dealloc] at all. Instead, [super dealloc] is called when your dealloc finishes.

See also:

• LLVM Automatic Reference Counting dealloc

Tatianna

Objective-c doesn't have "member variables". Other than that, dead on correct. – Explicative

So... [super dealloc] actually causes the memory allocated for this derived-class instance to become corrupt, or available for reuse by the heap? The reason I ask is just to understand the difference. In C++ the memory for an object is not freed by the heap (or whoever owns it) until the destructor has returned. – Aerospace

Will: I've updated the answer with more details, but yes. The root object's dealloc frees the instance itself. – Tatianna

Answer 2

Does the order of statements in the dealloc method matter? Does the [super dealloc] need to be at the top of the method? Does it matter?

It should go at the end. The idea is to say "I've torn down all the bits I've done, so now I'll let my parent class do the same" (recursively)

Also in e.g. viewDidLoad. Should [super viewDidLoad] be at the top of the method?

It should go at the top. The parent class should do what it needs to do to load its view before the subclass loads its parts, because it might rely on something the parent class needs to set up first.

Sharronsharyl

I think of it as "all the startup stuff goes before me and the finishing up stuff goes after me." – Chartulary