Favor Objective-C factory methods (or remember to release)

When Java came out and I began to spend virtually all my time building Java applications, I naively figured the days of manual memory management were gone (after all, Java has garbage collection!). Fast forward to now. The popularity of the iPhone and iPad have propelled C (really, Objective-C) back into the spotlight. And with Objective-C, the return of manual memory management (along with borderline extreme code verbosity).

Manual memory management isn’t too terribly difficult as it turns out — it just requires you pay attention, especially when you alloc objects. That is, alloc calls, for the most part, should usually be followed somewhere, in scope, with a release call.

Objective-C, however, has quite a few handy factory methods that handle memory aspects like retain counts nicely. Accordingly, you don’t have to explicitly release factory created objects — in fact, if you do, you’ll get nasty errors later when some section of code attempts to use it!

For example, in the code below, I’m creating two objects — an instance of NSMutableArray and <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSDictionary_Class/Reference/Reference.html%23//apple_ref/occ/cl/NSDictionary">NSDictionary</a> — both are alloc‘ed and init‘ed. In the ensuing code, the array instance is added to the NSDictionary and then that dictionary is passed along to NSNotificationCenter (code left out). Consequently, I must remember to release them (and thereby decrement their retain counts) after the handoff to NSNotificationCenter.

NSMutableArray *array = [[NSMutableArray alloc]init];
//...fill the array with objects
NSDictionary *dictionary = [[NSDictionary alloc] init];
[dictionary setObject:array forKey:APP_LIST_ARRAY_KEY];
//....add to NSNotificationCenter
[array release];
[dictionary release];

I can, however, make my life a bit easier by leveraging some of NSDictionary‘s factory methods, such as dictionaryWithObject:forKey, which initializes a new instance of NSDictionary containing one item (there’s a corresponding method to add multiple items too).

NSMutableArray *array = [[NSMutableArray alloc]init];
//...fill the array with objects
NSDictionary * dictionary =
  [NSDictionary dictionaryWithObject:array forKey:APP_LIST_ARRAY_KEY];
//....add to NSNotificationCenter
[array release];

Note in the code above, I’m freed from the responsibility to release my dictionary instance. Many core Objective-C classes have these factory-like methods, such as <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/Reference/NSString.html">NSString</a>‘s initWithFormat.

Using these methods can reduce the burden of manually managing memory, but certainly doesn’t obviate it. Can you dig it, man?