问题

以下代码输出结果是什么？

    BOOL res1 = [(id)[NSObject class] isKindOfClass:[NSObject class]];
    BOOL res2 = [(id)[NSObject class] isMemberOfClass:[NSObject class]];
    BOOL res3 = [(id)[Sark class] isKindOfClass:[Sark class]];
    BOOL res4 = [(id)[Sark class] isMemberOfClass:[Sark class]];
    BOOL res5 = [(id)[[NSObject alloc] init] isKindOfClass:[NSObject class]];
    BOOL res6 = [(id)[[NSObject alloc] init] isMemberOfClass:[NSObject class]];
    BOOL res7 = [(id)[[Sark alloc] init] isKindOfClass:[Sark class]];
    BOOL res8 = [(id)[[Sark alloc] init] isMemberOfClass:[Sark class]];
    NSLog(@"%d %d %d %d %d %d %d %d", res1, res2, res3, res4,res5,res6,res7,res8);
    //Sark继承于NSObject

现在我们来一步一步解析上述代码：

isKindOfClass & isMemberOfClass

苹果官方文档对isKindOfClass的解释是：

Returns a Boolean value that indicates whether the receiver is an instance of given class or an instance of any class that inherits from that class.

gnustep中给出的实现代码为:

/**
 * Returns YES if aClass is the NSObject class
 */
+ (BOOL) isKindOfClass: (Class)aClass
{
  if (aClass == [NSObject class])
    return YES;
  return NO;
}
/**
 * Returns YES if the class of the receiver is either the same as aClass
 * or is derived from (a subclass of) aClass.
 */
- (BOOL) isKindOfClass: (Class)aClass
{
  Class class = object_getClass(self);

  return GSObjCIsKindOf(class, aClass);
}

对isMemberOfClass的解释是：

Returns a Boolean value that indicates whether the receiver is an instance of a given class.

gnustep中给出的实现代码为:

/**
 * Returns YES if aClass is the same as the receiving class.
 */
+ (BOOL) isMemberOfClass: (Class)aClass
{
  return (self == aClass) ? YES : NO;
}
/**
 * Returns YES if the class of the receiver is aClass
 */
- (BOOL) isMemberOfClass: (Class)aClass
{
  return ([self class] == aClass) ? YES : NO;
}

其中class方法的实现为

/**
 * Returns the receiver.
 */
+ (Class) class
{
  return self;
}
/**
 * Returns the class of which the receiver is an instance.<br />
 * The default implementation returns the actual class that the
 * receiver is an instance of.<br />
 * NB.  When NSZombie is enabled (see NSDebug.h) this is changed
 * to be the NSZombie class upon object deallocation.
 */
- (Class) class
{
  return object_getClass(self);
}

看到这里我们应该对这两个函数的意思很明白了。

isKindOfClass 判断的是该对象是否是该类以及该类子类的实例。

isMemberOfClass 判断的是该对象是否是该类的实例。

现在我们来看看object和class的内部结构（有删减）：

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

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

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

struct objc_class {
        Class isa; // 指向metaclass
        
        Class super_class ; // 指向其父类
        const char *name ; // 类名
        long version ; // 类的版本信息，初始化默认为0，可以通过runtime函数class_setVersion和class_getVersion进行修改、读取
        long info; // 一些标识信息,如CLS_CLASS (0x1L) 表示该类为普通 class ，其中包含对象方法和成员变量;CLS_META (0x2L) 表示该类为 metaclass，其中包含类方法;
        long instance_size ; // 该类的实例变量大小(包括从父类继承下来的实例变量);
        struct objc_ivar_list *ivars; // 用于存储每个成员变量的地址
        struct objc_method_list **methodLists ; // 与 info 的一些标志位有关,如CLS_CLASS (0x1L),则存储对象方法，如CLS_META (0x2L)，则存储类方法;
        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; // 存储该类遵守的协议
    }

由此可知，在OC中，类也是一个对象都是一个含有isa指针的结构体。
其中isa指向所属Class， super_class指向父类别。