a) instance的isa指向class
1) 当调用对象方法时,通过instance的isa找到class,最后找到对象方法的实现进行调用;
2) 那么为什么不直接拿到class对象,来调用这个对象方法?
答:OC语法设计,达到面向对象的目的
b) class的isa指向meta-class
1) 当调用类方法时,通过class的isa找到meta-class,最后找到类方法的实现进行调用;
c) meta-class的isa指向基类的meta-class

a) superclass(继承)
b) 类对象的superclass指针指向的是它父类的类对象
c) 当Student的instance对象调用Person的对象方法时,会先通过isa找到Student的class,然后通过superclass找到Person的class,最后找到对象方法的实现进行调用

3.meta-class对象的superclass指针
a) superclass(继承)
b) 元类对象的superclass指针指向的是它父类的元类对象
c) 当Student的class对象调用Person的类方法时,会先通过isa找到Student的meta-class,然后通过superclass找到Person的meta-class,最后找到类方法的实现进行调用

a) instance的isa指向class
b) class的isa指向meta-class
c) meta-class的isa指向基类的meta-class
d) class的superclass指向父类的class(如果没有父类,superclass指针为nil)
e) meta-class的superclass指向父类的meta-class(基类的meta-class的superclass指向基类的class)
f) instance调用对象方法的轨迹
1) isa找到class,方法存在就调用,不存在,就通过superclass找到父类
g) class调用类方法的轨迹
1) isa找到meta-class,方法存在就调用,不存在,就通过superclass找到父类,如果基类的meta-class也没有会通过superclass找到基类的class然后找有没有

5.isa指针地址&superclass指针地址问题
struct mj_objc_class {
Class isa;
Class superclass;
};
int main(int argc, const char * argv[]) {
@autoreleasepool {
struct mj_objc_class *personClass = (__bridge struct mj_objc_class *)([MJPerson class]);
struct mj_objc_class *studentClass = (__bridge struct mj_objc_class *)([MJStudent class]);
NSLog(@"1111");
}
return 0;
}
a) 在以前类对象的isa指针的地址指向的就是元类对象的地址,在64位以后,isa需要进行一次位运算,才能计算出真实地址
b) supercalss(继承)是直接指向的不存在这个规则
c) ISA_MASK
# if __arm64__ (iOS平台)
# define ISA_MASK 0x0000000ffffffff8ULL
# elif __x86_64__ (Mac平台)
# define ISA_MASK 0x00007ffffffffff8ULL
#endif

a) 类对象还是元类对象都是Class类型即都是struct objc_class结构体,所以存放的东西都是一样的
b) c++的结构体跟oc的类除了作用域是public还是private其余基本是没区别的
c) c++的结构体不用带struct
d) iOS里面要兼容c++的代码把文件名后缀改成.mm即可
e) struct objc_class的底层结构
#import <Foundation/Foundation.h>
#ifndef MJClassInfo_h
#define MJClassInfo_h
# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# elif __x86_64__
# define ISA_MASK 0x00007ffffffffff8ULL
# endif
#if __LP64__
typedef uint32_t mask_t;
#else
typedef uint16_t mask_t;
#endif
typedef uintptr_t cache_key_t;
struct bucket_t {
cache_key_t _key;
IMP _imp;
};
struct cache_t {
bucket_t *_buckets;
mask_t _mask;
mask_t _occupied;
};
struct entsize_list_tt {
uint32_t entsizeAndFlags;
uint32_t count;
};
struct method_t {
SEL name;
const char *types;
IMP imp;
};
struct method_list_t : entsize_list_tt {
method_t first;
};
struct ivar_t {
int32_t *offset;
const char *name;
const char *type;
uint32_t alignment_raw;
uint32_t size;
};
struct ivar_list_t : entsize_list_tt {
ivar_t first;
};
struct property_t {
const char *name;
const char *attributes;
};
struct property_list_t : entsize_list_tt {
property_t first;
};
struct chained_property_list {
chained_property_list *next;
uint32_t count;
property_t list[0];
};
typedef uintptr_t protocol_ref_t;
struct protocol_list_t {
uintptr_t count;
protocol_ref_t list[0];
};
struct class_ro_t {
uint32_t flags;
uint32_t instanceStart;
uint32_t instanceSize;
#ifdef __LP64__
uint32_t reserved;
#endif
const uint8_t * ivarLayout;
const char * name;
method_list_t * baseMethodList;
protocol_list_t * baseProtocols;
const ivar_list_t * ivars;
const uint8_t * weakIvarLayout;
property_list_t *baseProperties;
};
struct class_rw_t {
uint32_t flags;
uint32_t version;
const class_ro_t *ro;
method_list_t * methods;
property_list_t *properties;
const protocol_list_t * protocols;
Class firstSubclass;
Class nextSiblingClass;
char *demangledName;
};
#define FAST_DATA_MASK 0x00007ffffffffff8UL
struct class_data_bits_t {
uintptr_t bits;
public:
class_rw_t* data() {
return (class_rw_t *)(bits & FAST_DATA_MASK);
}
};
struct mj_objc_object {
void *isa;
};
struct mj_objc_class : mj_objc_object {
Class superclass;
cache_t cache;
class_data_bits_t bits;
public:
class_rw_t* data() {
return bits.data();
}
mj_objc_class* metaClass() {
return (mj_objc_class *)((long long)isa & ISA_MASK);
}
};
#endif
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a) 成员变量的具体值,存放在instance对象
b) 对象方法、属性、成员变量、协议信息,存放在class对象中
c) 类方法,存放在meta-class对象中