ios OC swift run-time isa 指针

@interface Object { 
    Class isa; 
} 

#if !OBJC_TYPES_DEFINED
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

/// Represents an instance of a class.
struct objc_object {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
};

/// A pointer to an instance of a class.
typedef struct objc_object *id;
#endif

/// An opaque type that represents a method selector.
typedef struct objc_selector *SEL;

struct objc_class {
    Class isa  OBJC_ISA_AVAILABILITY;   //isa指针，指向metaclass（该类的元类）
 
#if !__OBJC2__
 
    Class super_class   //指向objc_class（该类）的super_class（父类）
 
    const char *name    //objc_class（该类）的类名
 
    long version        //objc_class（该类）的版本信息，初始化为0，可以通过runtime函数class_setVersion和class_getVersion进行修改和读取
 
    long info           //一些标识信息，如CLS_CLASS表示objc_class（该类）为普通类。ClS_CLASS表示objc_class（该类）为metaclass（元类）
 
    long instance_size  //objc_class（该类）的实例变量的大小
 
    struct objc_ivar_list *ivars    //用于存储每个成员变量的地址
 
    struct objc_method_list **methodLists   //方法列表，与info标识关联
 
    struct objc_cache *cache        //指向最近使用的方法的指针，用于提升效率
 
    struct objc_protocol_list *protocols    //存储objc_class（该类）的一些协议
#endif
 
} OBJC2_UNAVAILABLE;

MM * m = [[MM alloc] init];
        
object_setClass(m, objc_getClass("KK"));
        
 [m setValue:@(10) forKey:@"i"];

NSLog(@"%@", m);


==== console ==

KK -> description:10

bool isRootClass() {
       return superclass == nil;
}

/// 根 metaClass 的 isa 指向它自身
bool isRootMetaclass() {
      return ISA() == (Class)this;
}

bool isMetaClass() {
      return info & CLS_META;
}

/***********************************************************************
* objc_getMetaClass.  Return the id of the meta class the named class.
* Warning: doesn't work if aClassName is the name of a posed-for class's isa!
**********************************************************************/
Class objc_getMetaClass(const char *aClassName)
{
    Class cls;

    if (!aClassName) return Nil;

    cls = objc_getClass (aClassName);
    if (!cls)
    {
        _objc_inform ("class `%s' not linked into application", aClassName);
        return Nil;
    }

    return cls->ISA();
}

/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
/// A pointer to an instance of a class.
typedef struct objc_object *id;