小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
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;
/* Use `Class` instead of `struct objc_class *` */
我们发现OC2.0 此种结构已经过时了。 c++的结构体和类基本没什么区别
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();
}
bool isMetaClass() {
assert(this);
assert(isRealized());
return data()->ro->flags & RO_META;
}
// NOT identical to this->ISA when this is a metaclass
Class getMeta() {
if (isMetaClass()) return (Class)this;
else return this->ISA();
}
bool isRootClass() {
return superclass == nil;
}
bool isRootMetaclass() {
return ISA() == (Class)this;
}
size_t instanceSize(size_t extraBytes) {
size_t size = alignedInstanceSize() + extraBytes;
// CF requires all objects be at least 16 bytes.
if (size < 16) size = 16;
return size;
}
};
struct objc_object {
private:
isa_t isa;
public:
// ISA() assumes this is NOT a tagged pointer object
Class ISA();
// getIsa() allows this to be a tagged pointer object
Class getIsa();
bool isClass();
// object may have associated objects?
id retain();
void release();
id autorelease();
// Implementations of retain/release methods
id rootRetain();
bool rootRelease();
id rootAutorelease();
bool rootTryRetain();
bool rootReleaseShouldDealloc();
uintptr_t rootRetainCount();
void rootDealloc();
};
综合两者：
struct objc_class {
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();
}
};
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;
#if SUPPORT_INDEXED_ISA
uint32_t index;
#endif
};
struct class_ro_t {
uint32_t flags;
uint32_t instanceStart;
uint32_t instanceSize; // 成员变量大小
#ifdef __LP64__
uint32_t reserved;
#endif
const uint8_t * ivarLayout;
const char * name; // 类名
method_list_t * baseMethodList;
protocol_list_t * baseProtocols;
const ivar_list_t * ivars; // 成员变量
const uint8_t * weakIvarLayout;
property_list_t *baseProperties;
method_list_t *baseMethods() const {
return baseMethodList;
}
};
简化：struct objc_class的结构
struct objc_class{
Class isa;\
Class superclass;\
cache_t cache; // 方法缓存
class_data_bits_t bits;// 用于获取具体的类信息
}；
bits & FAST_DATA_MASK
获取具体的类信息。
struct class_rw_t{
uint32_t flags;\
uint32_t version;\
const class_ro_t *ro; // 方法列表\
property_list_t *properties; //属性列表
const protocol_list_t *protocols; //协议列表
Class firstSubclass；
Class nextSiblingClass；
char *demangleName;\
};
struct class_ro_t {
uint32_t flags;
uint32_t instanceStart;
uint32_t instanceSize; //instance对象占用的内存控件
#ifdef __LP64__
uint32_t reserved;
#endif
const uint8_t * ivarLayout;
const char * name; // 类名
method_list_t * baseMethodList;
protocol_list_t * baseProtocols;
const ivar_list_t * ivars; // 成员变量列表
const uint8_t * weakIvarLayout;
property_list_t *baseProperties;
};
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指针指向基类的meta-class对象。
OC的类信息存放在那里？
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
