Objective C memory management example

Question

I'm trying to learn objective c and I'm still a bit confused with memory management. Yes I know, I should use ARC, but my project uses TouchXML that does not support it. Also there is a lot of documentation and threads about memory management that I have read but I still have some doubt that I hope you guys will help me to clarify.

I've learnt that who allocs an object is then responsible to free it. I've also learnt that "retain" increments the reference counter whereas "release" decrements it. When an object's reference counter reaches 0, it is automatically de-alloced. I've finally learnt that "autorelease" releases the object automatically at the end of current event cycle. That's fine.

Now please consider the following case:

I alloc an array that I need to use for the full lifecycle of my object. I'm responsible to release it when my object is deleted:

@implementation MyClass
-(id) init {
  myArray = [[NSMutableArray alloc] init]; // this is a @property
}

- (void) dealloc {
  [myArray release];
  [super dealloc];
}
@end

In this way, in dealloc method, myArray release also causes myArray o be deallocated. If I then instance a new object from myClass and retain myArray like this...

// MyOtherClass

MyClass *o = [[[MyClass alloc] init] autorelease];
NSMutableArray *retainedArray = [[o.myArray] retain];

...at the end of current event cycle, "o" will be automatically deallocated, whereas retainedArray (actually pointing to o.myArray) will not be deallocated until I'll call [retainedArray release]. Is this correct up to here?

If so, I also guess the same applies if I call something like:

NSArray *contents = [[NSFileManager defaultManager] contentsOfDirectoryAtPath:@"somePath" error:nil];

I don't need (actually I can't otherwise it will give a runtime error) call either release or autorelease for "contents" unless I retain it somewhere in my code. Correct?

If so, summing everything up, in the end I only have to call release if I call either alloc or retain. The balance of reference counts in my class should always be 0, where alloc / retains gives +1 and release gives -1. Correct?

You can put TouchXML into a separate target or even project and add that as a dependency to your project. This way, 3rd-party libs can use whatever they want while your code will be using ARC normally. — Alexei Sholik

Martin R Martin R · Accepted Answer · 2013-06-05T08:16:07

It is almost 100% correct what you said, but there are a few more cases where you get a (+1) retained object that you have to release.

The basic rules are (see "Basic Memory Management Rules"):

You must eventually release or autorelease objects that you own.
You own objects that you created using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy”
You own an object if you take the ownership using retain.

The "Static Analyzer" (Product -> Analyze in the Xcode menu) is quite good at finding violations to this rule, so I can only recommend to use that.

Objective C memory management example

3 Answers