Tagged pointers in Objective-C

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OS X and iOS both use tagged pointer objects in 64-bit code. Neither currently uses any tagged pointer objects in 32-bit code, though in principle it’s not impossible. The specific set of optimized classes and optimized values changes frequently. Open-source objc4/runtime/objc-internal.h describes this set of classes that was used in at least one OS version:

OBJC_TAG_NSAtom            = 0, 
OBJC_TAG_1                 = 1, 
OBJC_TAG_NSString          = 2, 
OBJC_TAG_NSNumber          = 3, 
OBJC_TAG_NSIndexPath       = 4, 
OBJC_TAG_NSManagedObjectID = 5, 
OBJC_TAG_NSDate            = 6, 
OBJC_TAG_7                 = 7

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