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I'm trying to complete the puzzle.

__strong is the default for all Objective-C retainable object pointers like NSObject, NSString, etc.. It's a strong reference. ARC balances it with a -release at the end of the scope.

__unsafe_unretained equals the old way. It's used for a weak pointer without retaining the retainable object.

__weak is like __unsafe_unretained except that it's an auto-zeroing weak reference meaning that the pointer will be set to nil as soon as the referenced object is deallocated. This eliminates the danger of dangling pointers and EXC_BAD_ACCESS errors.

But what exactly is __autoreleasing good for? I'm having a hard time finding practical examples on when I need to use this qualifier. I believe it's only for functions and methods which expect a pointer-pointer such as:

- (BOOL)save:(NSError**);

or

NSError *error = nil;
[database save:&error];

which under ARC has to be declared this way:

- (BOOL)save:(NSError* __autoreleasing *);

But this is too vague and I'd like to fully understand why. The code snippets I find place the __autoreleasing inbetween the two stars, which looks weird to me. The type is NSError** (a pointer-pointer to NSError), so why place __autoreleasing inbetween the stars and not simply in front of NSError**?

Also, there might be other situations in which I must rely on __autoreleasing.