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In the Mac and iOS platforms, memory leaks are often caused by unreleased pointers. Traditionally, it has always been of utmost importance to check your allocs, copies and retains to make sure each has a corresponding release message.

The toolchain that comes with Xcode 4.2 introduces automatic reference counting (ARC) with the latest version of the LLVM compiler, that totally does away with this problem by getting the compiler to memory-manage your stuff for you. That's pretty cool, and it does cut lots of unnecessary, mundane development time and prevent a lot of careless memory leaks that are easy to fix with proper retain/release balance. Even autorelease pools need to be managed differently when you enable ARC for your Mac and iOS apps (as you shouldn't allocate your own NSAutoreleasePools anymore).

But what other memory leaks does it not prevent that I still have to watch out for?

As a bonus, what are the differences between ARC on Mac OS X and iOS, and garbage collection on Mac OS X?

My guess is that the compiler version you’re using follows the memory management rules for declared properties, too — more specifically, for declared properties’ accessors:

You take ownership of an object if you create it using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy”.

A property named newTitle, when synthesised, yields a method called -newTitle, hence the warning/error. -newTitle is supposed to be a getter method for the newTitle property, however naming conventions state that a method whose name begins with new returns an object that’s owned by the caller, which is not the case of getter methods.

You can solve this by:

  1. Renaming that property:

    @property (strong, nonatomic) NSString *theNewTitle;
  2. Keeping the property name and specifying a getter name that doesn’t begin with one of the special method name prefixes:

    @property (strong, nonatomic, getter=theNewTitle) NSString *newTitle;
  3. Keeping both the property name and the getter name, and telling the compiler that, even though the getter name starts with new, it belongs to the none method family as opposed to the new method family:

    #ifndef __has_attribute
    #define __has_attribute(x) 0  // Compatibility with non-clang compilers
    #if __has_attribute(objc_method_family)
    #define BV_OBJC_METHOD_FAMILY_NONE __attribute__((objc_method_family(none)))
    @interface ViewController : UIViewController
    @property (strong, nonatomic) NSString *newTitle;
    - (NSString *)newTitle BV_OBJC_METHOD_FAMILY_NONE;

    Note that even though this solution allows you to keep newTitle as both the property name and the getter name, having a method called -newTitle that doesn’t return an object owned by the caller can be confusing for other people reading your code.

For the record, Apple have published Transitioning to ARC Release Notes, in which they state:

You cannot give a property a name that begins with new or copy.

They’ve already been notified that their statement is not quite accurate: the culprit is the getter method name, not the property name.

Edit 17 Jan 2015: I’ve just noticed a recent commit to Clang that suggests option 3 above (using objc_method_family(none)), including a fix-it, for the general case where a property name matches one of the special method family prefixes. Xcode will likely incorporate this change eventually.

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