[super class]原理

@interface Person : NSObject
- (void)run;
@end

@implementation Person
- (void)run
{
    NSLog(@"%s", __func__);
}
@end

@interface Student : Person

@end

@implementation Student
//重写run方法
- (void)run
{
// super调用的receiver仍然是Student对象
 [super run];
 //底层实现   
//    struct objc_super arg = {self, [Person class]};
//    objc_msgSendSuper(arg, @selector(run));

}
@end

 [super message]的底层实现
 1.消息接收者仍然是子类对象
 2.从父类开始查找方法的实现
//结构
struct objc_super {
    __unsafe_unretained _Nonnull id receiver; // 消息接收者
    __unsafe_unretained _Nonnull Class super_class; // 消息接收者的父类
}
无论是[self class]还是[super class]找到的class都是在NSObject里面  只不过[self class]是从当前类对象开始查找 [super class]是从当前类对象的父类开始查找

Student调用init方法
- (instancetype)init
{
    if (self = [super init]) {
        //objc_msgSend(self, @selector(class));
        NSLog(@"[self class] = %@", [self class]); // Student
        NSLog(@"[self superclass] = %@", [self superclass]); // Person

        NSLog(@"--------------------------------");

        // objc_msgSendSuper({self, [Person class]}, @selector(class));
        //看起来是[super class],但是方法调用者还是self,
        NSLog(@"[super class] = %@", [super class]); // Student
        
        //当前类型是Student 父类是Person
        NSLog(@"[super superclass] = %@", [super superclass]); // Person
    }
    return self;
}

NSObject的实现大概
@implementation NSObject

//返回值取决于self即消息接受者，谁调用就返回谁的类型
- (Class)class
{
    return object_getClass(self);
}

//把self传进去  获取当前类型  再告诉当前类型的父类
- (Class)superclass
{
    return class_getSuperclass(object_getClass(self));
}

@end