isa_t Layout — ObjC Runtime

objc_object

private:

char isa_storage[sizeof(isa_t)]

↳ reinterpret_cast<isa_t*> 访问

↳ 旧版为 isa_t isa（public）

objc_class : objc_object

Class superclass

cache_t cache

class_data_bits_t bits

id / ordinary object

typedef objc_object * id

isa 位于 isa_storage 起始

NSObject instance

继承自 objc_object

isa 即 isa_storage 内存

位域颜色

1-bit 标志位

shiftcls（Class 指针）

magic 校验值

引用计数相关

unused / 预留

isa_t union · 8 bytes

uintptr_t bits

private: Class cls

↳ PAC 签名，不可直接访问

anonymous ISA_BITFIELD

↳ 按平台展开为位域 struct

setClass(cls, obj)

getClass(authenticated)

getDecodedClass(auth)

isDeallocating()

setDeallocating()

union 的含义

三个成员共享同一块内存

bits → 整数读法

cls → 指针读法（private）

ISA_BITFIELD → 位域读法

改其一，另外两个随之变化

arm64e PAC 64 bit

1 nonpointer bit 0

1 has_assoc bit 1

1 weakly_referenced bit 2

52 shiftcls_and_sig 3~54

1 has_sidetable_rc bit 55

8 extra_rc 56~63

has_cxx_dtor 移至 class_ro_t.flags

arm64 packed + simulator 64 bit

1 nonpointer bit 0

1 has_assoc bit 1

1 has_cxx_dtor bit 2

33 shiftcls 3~35

6 magic 36~41

1 weakly_referenced bit 42

1 unused bit 43

1 has_sidetable_rc bit 44

19 extra_rc 45~63

x86_64 packed 64 bit

1 nonpointer bit 0

1 has_assoc bit 1

1 has_cxx_dtor bit 2

44 shiftcls 3~46

6 magic 47~52

1 weakly_referenced bit 53

1 unused bit 54

1 has_sidetable_rc bit 55

8 extra_rc 56~63

indexed isa armv7k · arm64_32 32 bit

1 nonpointer bit 0

1 has_assoc bit 1

15 indexcls 2~16

4 magic 17~20

1 has_cxx_dtor bit 21

1 weakly_referenced bit 22

1 unused bit 23

1 has_sidetable_rc bit 24

7 extra_rc 25~31