Objective-C 中的标记指针 [英] Tagged pointers in Objective-C

问题描述

虽然回答这个问题，但我注意到现代 Objective-C 运行时使用标记指针.文章由 Mike Ash 及其注释注意它们用于一些 NSNumber 和 NSDate 实例.

While answering this question I noted that modern Objective-C runtime uses tagged pointers. The article by Mike Ash and its comments note that they are used for some NSNumber and NSDate instances.

这让我想到了不同平台的完整场景表:

Which got me thinking about the complete table of scenarios for different platforms:

OSX/iOS 32/64 位 Objective-C 运行时在哪里使用标记指针?

推荐答案

OS X 和 iOS 都在 64 位代码中使用标记指针对象.目前两者都没有在 32 位代码中使用任何标记的指针对象，但原则上并非不可能.一组特定的优化类和优化值经常变化.开源 objc4/runtime/objc-internal.h 描述了这组至少在一个操作系统版本中使用过的类:

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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