Retain and Release rules in Objective-C

I've seen lots of people asking questions about how to manage memory properly in Objective-C so I thought I'd list out some of the rules I follow when developing for the iPhone.

Objective-C is interesting because it introduces a few additional gotchas when compared to traditional C. In C, you write free() when you write malloc() and that's pretty much it. In Objective-C it's easy to forget things. So here are the rules I try to follow:

• When adding a new member variable to a class, add a call to release in your dealloc method as your immediate next action.

• When assigning to a member variable outside of init* or loadView etc. release it first. If you are assigning in a setter method, check the new value for equality first.

• When releaseing a member variable outside of dealloc, immediately assign a new value, or nil.

• When you type alloc for a local variable, immediately add the appropriate release or autorelease code. Prefer autorelease.

• Move your dealloc method to immediately below your @synthesize lines. This reminds you to check your release calls and you can easily see any missing ones.

• If you implement your own NSAutoReleasePool block, wrap it in a @try...@finally:

  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
  
  @try {
          // do stuff here
  
  } @finally {
          [pool release];
  }
  

• If you implement a delegate property, don't retain the delegate. See this excellent post on how to avoid retain cycles (circular references): Rules to avoid retain cycles.