NSObject isa

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title: NSObject isa

一直对NSObject isa理解不是很好，看到个大牛写的博客后，理解很多，在此记录下学习成果，方便日后查看。博客地址 http://xiongzenghuidegithub.github.io/blog/2016/05/17/runtime-part7-object-getclass-yu-object-class-de-qu-bie/

isa指向：对象isa->类,类isa->元类,元类isa->根元类,根元类isa->根元类自己，形成回路
super class:对象super class指向对象super class。元类super class指向super class父类。根元类meta root class指向类NSObject

     // 对象
     @interface NSObject <NSObject> {
     Class isa  OBJC_ISA_AVAILABILITY; // 指向类
     }
     
     // 类
     struct objc_class {
     Class isa  OBJC_ISA_AVAILABILITY; // 指向元类
     
     #if !__OBJC2__
     Class super_class                                        OBJC2_UNAVAILABLE;
     const char *name                                         OBJC2_UNAVAILABLE;
     long version                                             OBJC2_UNAVAILABLE;
     long info                                                OBJC2_UNAVAILABLE;
     long instance_size                                       OBJC2_UNAVAILABLE;
     struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
     struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
     struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
     struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
     #endif
     
     } OBJC2_UNAVAILABLE;
     
     // 元类
     struct objc_class {
     Class isa  OBJC_ISA_AVAILABILITY; // 指向root元类
     
     #if !__OBJC2__
     Class super_class                                        OBJC2_UNAVAILABLE;
     const char *name                                         OBJC2_UNAVAILABLE;
     long version                                             OBJC2_UNAVAILABLE;
     long info                                                OBJC2_UNAVAILABLE;
     long instance_size                                       OBJC2_UNAVAILABLE;
     struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
     struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
     struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
     struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
     #endif
     
     } OBJC2_UNAVAILABLE;
     
     // root元类
     struct objc_class {
     Class isa  OBJC_ISA_AVAILABILITY; // 指向自己
     
     #if !__OBJC2__
     Class super_class                                        OBJC2_UNAVAILABLE;
     const char *name                                         OBJC2_UNAVAILABLE;
     long version                                             OBJC2_UNAVAILABLE;
     long info                                                OBJC2_UNAVAILABLE;
     long instance_size                                       OBJC2_UNAVAILABLE;
     struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
     struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
     struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
     struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
     #endif
     
     } OBJC2_UNAVAILABLE;
     
    Person *person = [[Person alloc] init];
    NSLog(@"person class:%p", [person class]); // 获取到类 objc_class结构体
    NSLog(@"person meta class:%p", object_getClass(person)); // 获取到类 objc_class结构体
    NSLog(@"person meta class:%p", [Person class]); // 获取到元类 objc_class结构体
    NSLog(@"person meta class:%p", object_getClass([person class])); // 获取到元类 objc_class结构体