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- instance的isa指向class
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 - class的isa指向meta-class
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
class对象的superclass指针
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当Student的instance对象要调用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
meta-class对象的superclass指针
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当Student的class要调用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
isa、superclass总结
- instance的isa指向class
- class的isa指向meta-class
- meta-class的isa指向基类的meta-class
- class的superclass指向父类的class
如果没有父类，superclass指针为nil - meta-class的superclass指向父类的meta-class
基类的meta-class的superclass指向基类的class - instance调用对象方法的轨迹
isa找到class，方法不存在，就通过superclass找父类 - class调用类方法的轨迹
isa找meta-class，方法不存在，就通过superclass找父类
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从64bit开始，isa需要进行一次位运算，才能计算出真实地址,如下图:
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# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# elif __x86_64__
# define ISA_MASK 0x00007ffffffffff8ULL
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(lldb) p/x person->isa
(Class) $5 = 0x001d8001000014d1 LQPerson//isa指针的地址
(lldb) p/x 0x001d8001000014d1 & 0x00007ffffffffff8
(long) $6 = 0x00000001000014d0 //class对象的地址
struct objc_class的结构
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源码中的定义如下：
struct objc_class {
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
#if !__OBJC2__
Class _Nullable super_class OBJC2_UNAVAILABLE;
const char * _Nonnull name OBJC2_UNAVAILABLE;
long version OBJC2_UNAVAILABLE;
long info OBJC2_UNAVAILABLE;
long instance_size OBJC2_UNAVAILABLE;
struct objc_ivar_list * _Nullable ivars OBJC2_UNAVAILABLE;
struct objc_method_list * _Nullable * _Nullable methodLists OBJC2_UNAVAILABLE;
struct objc_cache * _Nonnull cache OBJC2_UNAVAILABLE;
struct objc_protocol_list * _Nullable protocols OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;
struct objc_class : objc_object {
// Class ISA;
Class superclass;
cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
class_rw_t *data() {
return bits.data();
}
void setData(class_rw_t *newData) {
bits.setData(newData);
}
......
struct class_rw_t {
// Be warned that Symbolication knows the layout of this structure.
uint32_t flags;
uint32_t version;
const class_ro_t *ro;
method_array_t methods;
property_array_t properties;
protocol_array_t protocols;
Class firstSubclass;
Class nextSiblingClass;
char *demangledName;
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;
method_list_t *baseMethods() const {
return baseMethodList;
}
};
ro--readonly
rw--readwrite
struct lq_objc_class {
Class isa;
Class superclass;
};
LQPerson *person = [[LQPerson alloc] init];
Class personClass = [LQPerson class];
打印结果如下：
(lldb) p/x person->isa
(Class) $0 = 0x001d8001000014c9 LQPerson
(lldb) p/x personClass
(Class) $1 = 0x00000001000014c8 LQPerson
(lldb) p/x 0x001d8001000014c9 & 0x0000000ffffffff8
(long) $2 = 0x00000001000014c8
由此可见，实例对象的的isa确实指向class对象
也可以通过如下方式验证superClass
(lldb) p/x studentClass->superclass
(Class) $3 = 0x00000001000014c8 LQPerson
QA
- 对象的isa指针指向哪里？
instance对象的isa指向class对象
class对象的isa指向meta-class对象
meta-class对象的isa指向基类的meta-class对象
- OC的类信息存放在哪里？
对象方法、属性、成员变量、协议信息，存放在class对象中
类方法，存放在meta-class对象中
成员变量的具体值，存放在instance对象
