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

问题描述

回答此问题时，我注意到现代的Objective-C运行时使用标记的指针. 文章由Mike Ash和其注释.请注意，它们用于某些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.

这让我考虑了不同平台的完整方案表:

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

OSX/iOS 32/64位Objective-C运行时在何处使用标记的指针?

推荐答案

OS X和iOS都使用64位代码中的带标记的指针对象.当前都没有使用32位代码中的任何带标记的指针对象，尽管原则上并非没有可能.优化类和优化值的特定集合经常更改.开源objc4/runtime/objc-internal.h描述了在至少一个OS版本中使用的以下类集:

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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