Re: self = [super init] debate.

FROM : Jens Alfke
DATE : Sun Feb 10 20:45:00 2008

On 10 Feb '08, at 9:18 AM, Scott Andrew wrote:

>  I see Will, and a few others, say no [super init] will initialize 
> self in its code there is no reason to assign self.


No, you do [sometimes] need to assign self. 'self' is just a local 
variable (actually a parameter, one declared for you by the compiler.) 
The superclass's init method can't affect its value for you. However, 
in some cases the superclass method can decide to return a different 
object, and in that case unless you reassign self, you won't get the 
object the superclass initialized.

Why would an init method return a different object? Usually because 
it's part of a "class cluster" design pattern, where the public class 
is abstract, and behind the scenes some concrete subclasses are used. 
Most of the Cocoa collection classes work this way. When you call "-
[[NSArray alloc] init] the result isn't an instance of NSArray, it's 
usually an NSCFArray, an internal class that manages the bridging to 
CoreFoundation's CFArray.

Another reason is to take advantage of variable-sized objects. You 
could have an array-like class that stores its array in itself, past 
the end of its regular instance variables. To do that, you have to 
call NSAllocateObject passing in the number of extra bytes to reserve. 
But you don't know that size until your -initWithSize: is called, 
after you've been allocated. So you make your -initWithSize call [self 
release], then NSAllocateObject with extra bytes, and assign that to 
self. The result is that -initWithSize returns a different instance 
than it was called on.

Any subclass that calls [super initWithSize] and doesn't reassign self 
to the result now has a 'self' variable that points to an obsolete 
dealloced object, and is now very likely to either crash immediately 
or (more likely) corrupt the heap and cause a later crash. Bad news.

-init methods that return a new object are rare. In most cases you 
don't need to add "self=" in front of the "[super init]" call. But in 
those few cases the consequences are grim, so it's better to train 
yourself to do it every time. It's only five more characters to type, 
and it should only compile into one or two extra machine instructions.

—Jens_______________________________________________

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