Question

So I modified Apple’s PageControl example to dynamically load various navigation controllers (along with their root view controllers) into the scroll view. I also added a technique that attempts to unload a navigation controller when it’s no longer needed. I’ve only been at ObjC for a little over a month, so I’m not sure if I’m doing the unloading correctly. Please see my code below, followed by my questions.

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First I create a mutable array and fill it with nulls, just like Apple does:

// Create dummy array for viewControllers array, fill it with nulls, and assign to viewControllers
NSMutableArray *array = [[NSMutableArray alloc] init];
for (unsigned i = 0; i <= kNumberOfPages; i++)
{
    [array addObject:[NSNull null]];
}

self.viewControllers = array;
[array release];

…Later, I fill the array with UINavigationController objects like so (this is just partial code, please excuse the missing parts…the main idea is that I alloc a couple of things, assign them and then release):

id controller = [[classForViewController alloc] initWithNibName:NSStringFromClass(classForViewController) bundle:nil];
    navController = [[UINavigationController alloc] initWithRootViewController:controller];
    [controller release];
    [self.viewControllers replaceObjectAtIndex:page withObject:navController];
    [navController release];

…Finally, if a page doesn’t need to be loaded anymore I do this:

[self.viewControllers replaceObjectAtIndex:i withObject:[NSNull null]];

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Questions:

1. My understanding is that once I replace the navigation controller in my viewControllers array with null, the array releases the navigation controller. Thus the navigation controller’s retain count hits zero and it no longer takes up memory. Is this correct?

2. What about the root view controller inside the navigation controller? Do I need to do anything with it or does it get released automatically once the navigation controller’s retain count hit zero?

Thanks!

Answer 1

1. Yes. Any object put into a collection is sent a retain message. Likewise any object removed from a collection is sent a release message, the cause of the removal is irrelevant.
2. Yes, all objects will release all the objects it owns when they are released.

This all boils down to the simple principle of ownership that Cocoa defines:

• You own the object if you received it as return value by calling a method that:
  1. Is named alloc or new.
  2. Contains the word copy, such as copy and mutableCopy.
• You own the object if you call retain.
• You may only call release and autorelease on objects you own.
• You must release all owned objects in your dealloc methods.

There is just one exception; delegates are never owned. This is to avoid circular references and the memory leaks they cause.

As a side effect this also means that when you yourself are implementing a method, you must return an auto released object unless you are implementing new, or a method with copy in it’s name. Objects returned as out arguments are always autoreleased.

Follow this strictly and Objective-C can be treated as if it is garbage collected 95% of the time.