Re: NSSound Reimplementation

I would go further and ask what the point of the _private array is?

By convention all the instance variables are private (that's what the leading underscore in the variable name means), so if all that's required is to inform developers that they should not use the ivars directly, there's no point in having the array, but maybe it would be worth using a @private declaration.

On the other hand, if the idea is to hide the implementation details so that ivar layouts won't need to change with future revisions (something I think we should all be doing), my preference would be to have a single pointer to a structure containing the required data.
eg.

struct  _NSSoundInternal;

@interface NSSound : NSObject <NSCoding, NSCopying>
{              
@private
 struct _NSSoundInternal       *_internal;
}

Then the designated initialiser allocates memory for the structure, clears it, and assigns the pointer when an instance is initialised, and -dealloc frees the memory at the end, and all internal references to ivars just go indirect from that pointer.  The implementation is completely hidden/private and can be changed in subsequent releases without breaking the ABI.