引言：这篇文章旨在从runtime源码中分析出 引用计数 值本身的保存位置，适合对底层原理有兴趣的朋友，或者面试造火箭的同学（比如百度的面试官非常喜欢问底层原理：好，我知道你说了深浅复制的区别一大堆，如果我让你自己实现一个copy，你能实现吗？如果我让你实现引用计数的功能，你有思路吗？）。因而本文并 不适用于 专注业务层快速开发的同学，因为这里将贴有大量的源码。没有耐心的同学可以先收藏暂时回避一下，日后造火箭造飞机的时候再来。
核心问题
iOS开发者都知道OC里面的内存管理是通过对象的引用计数来管理的，或手动MRC，或自动ARC，有些操作可以让引用计数加1，有些可以减1，一旦一个对象的引用计数为0，就回收内存了。
可是，你仅仅知道这里就行了吗？指望你能造火箭造飞机的面试官可不这么想了，比如问你一句，一个对象的 引用计数本身 保存在哪里？？不关注底层的面试者，这时候可能会懵逼。很多介绍内存管理的文章对此也含糊不清，例如：
研究方式
这篇文章不同于其它文章通过 clang编译 一个类文件以查看它的实现原理（笔者曾用clang编译分析Block的原理，传送门），而是直接通过下载runtime的源码来查看分析。
依据版本
苹果开源了runtime的代码，查看的方式既可以通过 在线网页版 预览，也可以 下载归档文件 到本地查看。本篇文件讨论的版本是 objc4-723。
目录
-
- 类与对象
- 1.1 对象 -- Object
- 1.2 对象 -- NSObject
- 1.3 对象 -- objc_object
- 1.4 类 -- objc_class
- 1.5 NSObject，objc_object，objc_class 三者的关系
-
- 手动引用对引用计数的影响 -- retain操作
- 2.1 两种对象：NSObject与Object的引用增加
- 2.2 归根结底 -- NSObject对象的rootRetain()
-
- isa与Tagged Pointer
- 3.1 NSObject的唯一成员变量 -- isa
- 3.2 isa_t联合体里面的数据含义
- 3.3 isa_t联合体里面的宏
- 3.4 是否Tagged Pointer的判断
- 3.5 与isa类型有关的宏
- 3.6 怎么判断是否支持优化的isa指针？-- 看设备、自己设置。
- 3.7 怎么判断是否Tagged Pointer的对象？-- 看对象、自己设置
- 3.8 引用计数的存储形式 -- 散列表
-
- 散列表
- 4.1 增加引用计数 -- sidetable_retain()
- 4.2 增加引用计数 -- sidetable_tryRetain()
- 4.3 获取散列表 -- SideTable()
-
- 设置变量导致的引用计数变化 -- objc_retain操作
- 5.1 情况1
- 5.2 情况2
- 5.3 objc_storeStrong导致的retain
-
- 新建对象（分配内存与初始化）导致的引用计数变化 -- alloc 和 init 操作
- 6.1 分配内存 -- alloc
- 6.2 初始化 -- init
-
- 获取引用计数
-
- 结论
-
- 拓展阅读
1. 类与对象
下载完工程，打开查看
module.modulemap
头文件描述文件
module ObjectiveC [system] [extern_c] {
umbrella "."
export *
module * {
export *
}
module NSObject {
requires objc
header "NSObject.h"
export *
}
#if defined(BUILD_FOR_OSX)
module List {
// Uses @defs, which does not work in ObjC++ or non-ARC.
requires objc, !objc_arc, !cplusplus
header "List.h"
export *
}
module Object {
requires objc
header "Object.h"
export *
}
module Protocol {
requires objc
header "Protocol.h"
export *
}
#endif
#if !defined(BUILD_FOR_OSX)
// These file are not available outside macOS.
exclude header "hashtable.h"
exclude header "hashtable2.h"
#endif
}
这里的Module本质上是一个描述文件，用来描述Module中包涵的内容，每个Module中必须包涵一个umbrella头文件，这个文件用来#import所有这个Module下的文件，比如#import 这个UIKit.h就是一个umbrella文件。关于Module更多参考 这篇文章。
从#if defined(BUILD_FOR_OSX)这句逻辑判断可知， Object是针对macOS的，iOS开发暂时只关心NSObject即可。
1.1 对象 -- Object
Object.mm
Object
#include "objc-private.h"
#undef id
#undef Class
typedef struct objc_class *Class;
typedef struct objc_object *id;
#if __OBJC2__
__OSX_AVAILABLE(10.0)
__IOS_UNAVAILABLE __TVOS_UNAVAILABLE
__WATCHOS_UNAVAILABLE __BRIDGEOS_UNAVAILABLE
OBJC_ROOT_CLASS
@interface Object {
Class isa;
}
@end
@implementation Object
+ (id)initialize
{
return self;
}
+ (id)class
{
return self;
}
-(id) retain
{
return _objc_rootRetain(self);
}
-(void) release
{
_objc_rootRelease(self);
}
-(id) autorelease
{
return _objc_rootAutorelease(self);
}
+(id) retain
{
return self;
}
+(void) release
{
}
+(id) autorelease
{
return self;
}
@end
1.2 对象 -- NSObject
NSObject.h
NSObject
#ifndef _OBJC_NSOBJECT_H_
#define _OBJC_NSOBJECT_H_
#if __OBJC__
#include
#include
@class NSString, NSMethodSignature, NSInvocation;
@protocol NSObject
- (BOOL)isEqual:(id)object;
@property (readonly) NSUInteger hash;
@property (readonly) Class superclass;
- (Class)class OBJC_SWIFT_UNAVAILABLE("use 'type(of: anObject)' instead");
- (instancetype)self;
- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
- (BOOL)isProxy;
- (BOOL)isKindOfClass:(Class)aClass;
- (BOOL)isMemberOfClass:(Class)aClass;
- (BOOL)conformsToProtocol:(Protocol *)aProtocol;
- (BOOL)respondsToSelector:(SEL)aSelector;
- (instancetype)retain OBJC_ARC_UNAVAILABLE;
- (oneway void)release OBJC_ARC_UNAVAILABLE;
- (instancetype)autorelease OBJC_ARC_UNAVAILABLE;
- (NSUInteger)retainCount OBJC_ARC_UNAVAILABLE;
- (struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
@property (readonly, copy) NSString *description;
@optional
@property (readonly, copy) NSString *debugDescription;
@end
OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0)
OBJC_ROOT_CLASS
OBJC_EXPORT
@interface NSObject {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
Class isa OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}
+ (void)load;
+ (void)initialize;
- (instancetype)init
#if NS_ENFORCE_NSOBJECT_DESIGNATED_INITIALIZER
NS_DESIGNATED_INITIALIZER
#endif
;
+ (instancetype)new OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)allocWithZone:(struct _NSZone *)zone OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)alloc OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
- (void)dealloc OBJC_SWIFT_UNAVAILABLE("use 'deinit' to define a de-initializer");
- (void)finalize OBJC_DEPRECATED("Objective-C garbage collection is no longer supported");
- (id)copy;
- (id)mutableCopy;
+ (id)copyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
+ (id)mutableCopyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
+ (BOOL)conformsToProtocol:(Protocol *)protocol;
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;
- (void)doesNotRecognizeSelector:(SEL)aSelector;
- (id)forwardingTargetForSelector:(SEL)aSelector OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
- (void)forwardInvocation:(NSInvocation *)anInvocation OBJC_SWIFT_UNAVAILABLE("");
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");
+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");
- (BOOL)allowsWeakReference UNAVAILABLE_ATTRIBUTE;
- (BOOL)retainWeakReference UNAVAILABLE_ATTRIBUTE;
+ (BOOL)isSubclassOfClass:(Class)aClass;
+ (BOOL)resolveClassMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
+ (BOOL)resolveInstanceMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
+ (NSUInteger)hash;
+ (Class)superclass;
+ (Class)class OBJC_SWIFT_UNAVAILABLE("use 'aClass.self' instead");
+ (NSString *)description;
+ (NSString *)debugDescription;
@end
#endif
#endif
1.3 对象 -- objc_object
关键信息：
-
isa:isa_t类型的指针，详情可见下面3.2节。简单的说，它是这样的一个联合体，包含了bits（是一个uintptr_t类型的值，作为isa初始化列表中必初始化的值，可以用来获取isa结构体）和cls（该变量会指向对象所属的类的结构，在 64 位设备上会占用 8byte）。
objc-private.h
objc_object
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();
// initIsa() should be used to init the isa of new objects only.
// If this object already has an isa, use changeIsa() for correctness.
// initInstanceIsa(): objects with no custom RR/AWZ
// initClassIsa(): class objects
// initProtocolIsa(): protocol objects
// initIsa(): other objects
void initIsa(Class cls /*nonpointer=false*/);
void initClassIsa(Class cls /*nonpointer=maybe*/);
void initProtocolIsa(Class cls /*nonpointer=maybe*/);
void initInstanceIsa(Class cls, bool hasCxxDtor);
// changeIsa() should be used to change the isa of existing objects.
// If this is a new object, use initIsa() for performance.
Class changeIsa(Class newCls);
bool hasNonpointerIsa();
bool isTaggedPointer();
bool isBasicTaggedPointer();
bool isExtTaggedPointer();
bool isClass();
// object may have associated objects?
bool hasAssociatedObjects();
void setHasAssociatedObjects();
// object may be weakly referenced?
bool isWeaklyReferenced();
void setWeaklyReferenced_nolock();
// object may have -.cxx_destruct implementation?
bool hasCxxDtor();
// Optimized calls to retain/release methods
id retain();
void release();
id autorelease();
// Implementations of retain/release methods
id rootRetain();
bool rootRelease();
id rootAutorelease();
bool rootTryRetain();
bool rootReleaseShouldDealloc();
uintptr_t rootRetainCount();
// Implementation of dealloc methods
bool rootIsDeallocating();
void clearDeallocating();
void rootDealloc();
private:
void initIsa(Class newCls, bool nonpointer, bool hasCxxDtor);
// Slow paths for inline control
id rootAutorelease2();
bool overrelease_error();
#if SUPPORT_NONPOINTER_ISA
// Unified retain count manipulation for nonpointer isa
id rootRetain(bool tryRetain, bool handleOverflow);
bool rootRelease(bool performDealloc, bool handleUnderflow);
id rootRetain_overflow(bool tryRetain);
bool rootRelease_underflow(bool performDealloc);
void clearDeallocating_slow();
// Side table retain count overflow for nonpointer isa
void sidetable_lock();
void sidetable_unlock();
void sidetable_moveExtraRC_nolock(size_t extra_rc, bool isDeallocating, bool weaklyReferenced);
bool sidetable_addExtraRC_nolock(size_t delta_rc);
size_t sidetable_subExtraRC_nolock(size_t delta_rc);
size_t sidetable_getExtraRC_nolock();
#endif
// Side-table-only retain count
bool sidetable_isDeallocating();
void sidetable_clearDeallocating();
bool sidetable_isWeaklyReferenced();
void sidetable_setWeaklyReferenced_nolock();
id sidetable_retain();
id sidetable_retain_slow(SideTable& table);
uintptr_t sidetable_release(bool performDealloc = true);
uintptr_t sidetable_release_slow(SideTable& table, bool performDealloc = true);
bool sidetable_tryRetain();
uintptr_t sidetable_retainCount();
#if DEBUG
bool sidetable_present();
#endif
};
1.4 类 -- objc_class
关键信息：
-
isa: 继承于objc_object -
superclass: 指向自己父类的指针 -
cache: 方法缓存 -
bits: 它是一个class_data_bits_t类型的指针。作为本类的实例方法链表。
注意区别：
这里的bits是class_data_bits_t类型的，上一节objc_object的isa_t类型数据中也有一个uintptr_t类型的bits，但是这是两种结构。
class_data_bits_t
由此可见，objc_class 继承于 objc_object， 所以也是包含一个isa的类。在OC里，不只是对象的实例包含一个isa，这个对象的类本身也有这么一个isa，类本身也是一个对象。
objc-runtime-new.h
objc_class
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);
}
void setInfo(uint32_t set) {
assert(isFuture() || isRealized());
data()->setFlags(set);
}
void clearInfo(uint32_t clear) {
assert(isFuture() || isRealized());
data()->clearFlags(clear);
}
// set and clear must not overlap
void changeInfo(uint32_t set, uint32_t clear) {
assert(isFuture() || isRealized());
assert((set & clear) == 0);
data()->changeFlags(set, clear);
}
bool hasCustomRR() {
return ! bits.hasDefaultRR();
}
void setHasDefaultRR() {
assert(isInitializing());
bits.setHasDefaultRR();
}
void setHasCustomRR(bool inherited = false);
void printCustomRR(bool inherited);
bool hasCustomAWZ() {
return ! bits.hasDefaultAWZ();
}
void setHasDefaultAWZ() {
assert(isInitializing());
bits.setHasDefaultAWZ();
}
void setHasCustomAWZ(bool inherited = false);
void printCustomAWZ(bool inherited);
bool instancesRequireRawIsa() {
return bits.instancesRequireRawIsa();
}
void setInstancesRequireRawIsa(bool inherited = false);
void printInstancesRequireRawIsa(bool inherited);
bool canAllocNonpointer() {
assert(!isFuture());
return !instancesRequireRawIsa();
}
bool canAllocFast() {
assert(!isFuture());
return bits.canAllocFast();
}
bool hasCxxCtor() {
// addSubclass() propagates this flag from the superclass.
assert(isRealized());
return bits.hasCxxCtor();
}
void setHasCxxCtor() {
bits.setHasCxxCtor();
}
bool hasCxxDtor() {
// addSubclass() propagates this flag from the superclass.
assert(isRealized());
return bits.hasCxxDtor();
}
void setHasCxxDtor() {
bits.setHasCxxDtor();
}
bool isSwift() {
return bits.isSwift();
}
// Return YES if the class's ivars are managed by ARC,
// or the class is MRC but has ARC-style weak ivars.
bool hasAutomaticIvars() {
return data()->ro->flags & (RO_IS_ARC | RO_HAS_WEAK_WITHOUT_ARC);
}
// Return YES if the class's ivars are managed by ARC.
bool isARC() {
return data()->ro->flags & RO_IS_ARC;
}
#if SUPPORT_NONPOINTER_ISA
// Tracked in non-pointer isas; not tracked otherwise
#else
bool instancesHaveAssociatedObjects() {
// this may be an unrealized future class in the CF-bridged case
assert(isFuture() || isRealized());
return data()->flags & RW_INSTANCES_HAVE_ASSOCIATED_OBJECTS;
}
void setInstancesHaveAssociatedObjects() {
// this may be an unrealized future class in the CF-bridged case
assert(isFuture() || isRealized());
setInfo(RW_INSTANCES_HAVE_ASSOCIATED_OBJECTS);
}
#endif
bool shouldGrowCache() {
return true;
}
void setShouldGrowCache(bool) {
// fixme good or bad for memory use?
}
bool isInitializing() {
return getMeta()->data()->flags & RW_INITIALIZING;
}
void setInitializing() {
assert(!isMetaClass());
ISA()->setInfo(RW_INITIALIZING);
}
bool isInitialized() {
return getMeta()->data()->flags & RW_INITIALIZED;
}
void setInitialized();
bool isLoadable() {
assert(isRealized());
return true; // any class registered for +load is definitely loadable
}
IMP getLoadMethod();
// Locking: To prevent concurrent realization, hold runtimeLock.
bool isRealized() {
return data()->flags & RW_REALIZED;
}
// Returns true if this is an unrealized future class.
// Locking: To prevent concurrent realization, hold runtimeLock.
bool isFuture() {
return data()->flags & RW_FUTURE;
}
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;
}
const char *mangledName() {
// fixme can't assert locks here
assert(this);
if (isRealized() || isFuture()) {
return data()->ro->name;
} else {
return ((const class_ro_t *)data())->name;
}
}
const char *demangledName(bool realize = false);
const char *nameForLogging();
// May be unaligned depending on class's ivars.
uint32_t unalignedInstanceStart() {
assert(isRealized());
return data()->ro->instanceStart;
}
// Class's instance start rounded up to a pointer-size boundary.
// This is used for ARC layout bitmaps.
uint32_t alignedInstanceStart() {
return word_align(unalignedInstanceStart());
}
// May be unaligned depending on class's ivars.
uint32_t unalignedInstanceSize() {
assert(isRealized());
return data()->ro->instanceSize;
}
// Class's ivar size rounded up to a pointer-size boundary.
uint32_t alignedInstanceSize() {
return word_align(unalignedInstanceSize());
}
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;
}
void setInstanceSize(uint32_t newSize) {
assert(isRealized());
if (newSize != data()->ro->instanceSize) {
assert(data()->flags & RW_COPIED_RO);
*const_cast(&data()->ro->instanceSize) = newSize;
}
bits.setFastInstanceSize(newSize);
}
void chooseClassArrayIndex();
void setClassArrayIndex(unsigned Idx) {
bits.setClassArrayIndex(Idx);
}
unsigned classArrayIndex() {
return bits.classArrayIndex();
}
};
1.5 NSObject，objc_object，objc_class 三者的关系
1）NSObject与objc_class
NSObject有一个Class类型，名为isa成员变量
NSObject有一个Class类型，名为isa成员变量
继续查看Class的本质，可以发现Class 其实就是 C 语言定义的结构体类型(struct objc_class)的指针，这个声明说明 Objective-C 的 类 实际上就是 struct objc_class。
Class的本质
另外，第二个定义是经常遇到的 id 类型，这里可以看出 id 类型是 C 语言定义的结构体类型(struct objc_object)的指针，我们知道我们可以用 id 来声明一个对象，所以这也说明了 Objective-C 的 对象 实际上就是 struct objc_object。
2）objc_object与objc_class
继续查看objc_class的本质，可以发现objc_class是一个 继承 自objc_object的结构体。所以 Objective-C 中的 类 自身也是一个 对象，只是除了 objc_object 中定义的成员变量外，还有另外三个成员变量：superclass、cache 和 bits。
objc_class继承自objc_object
注意，这里面的 “结构体” 并非 C语言 里面的结构体，而是 C++语言 里面的结构体，而且这个概念仅限字面意思的结构体。严格来讲，其实struct关键字定义的是 类，跟class关键字定义的类除了默认访问权限的区别，没有区别。这一点，国内人写的C++书籍却很少有注意到。下面是比较权威的《C++ Primer》（第546页）一书关于这点的说明。
C++ Primer
3）知识补课
C++中的struct对C中的struct进行了扩充，它已经不再只是一个包含不同数据类型的数据结构了，它已经获取了太多的功能。下面简单列一下C++的struct跟C中的struct不一样的地方：
- struct能包含成员函数
- struct能继承
- struct能实现多态
2. 手动引用对引用计数的影响 -- retain操作
2.1 两种对象：NSObject与Object的引用增加
① NSObject的retain
NSObject.mm
retain
+ (id)retain {
return (id)self;
}
// Replaced by ObjectAlloc
- (id)retain {
return ((id)self)->rootRetain();
}
② Object的retain
Object.mm
retain
+(id) retain
{
return self;
}
-(id) retain
{
return _objc_rootRetain(self);
}
NSObject.mm
_objc_rootRetain(id obj)
id
_objc_rootRetain(id obj)
{
assert(obj);
return obj->rootRetain();
}
可见，无论是NSObject还是Object的 retain，归根结底，调用的都是 objc_object 的 rootRetain()。
2.2 归根结底 -- NSObject对象的rootRetain()
objc4/objc4-723/runtime/objc-object.h
objc_object::rootRetain()
ALWAYS_INLINE id
objc_object::rootRetain()
{
return rootRetain(false, false);
}
objc4/objc4-723/runtime/objc-object.h
objc_object::rootRetain(bool tryRetain, bool handleOverflow)
ALWAYS_INLINE id
objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
if (isTaggedPointer()) return (id)this;
bool sideTableLocked = false;
bool transcribeToSideTable = false;
isa_t oldisa;
isa_t newisa;
do {
transcribeToSideTable = false;
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
if (slowpath(!newisa.nonpointer)) {
ClearExclusive(&isa.bits);
if (!tryRetain && sideTableLocked) sidetable_unlock();
if (tryRetain) return sidetable_tryRetain() ? (id)this : nil;
else return sidetable_retain();
}
// don't check newisa.fast_rr; we already called any RR overrides
if (slowpath(tryRetain && newisa.deallocating)) {
ClearExclusive(&isa.bits);
if (!tryRetain && sideTableLocked) sidetable_unlock();
return nil;
}
uintptr_t carry;
newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++
if (slowpath(carry)) {
// newisa.extra_rc++ overflowed
if (!handleOverflow) {
ClearExclusive(&isa.bits);
return rootRetain_overflow(tryRetain);
}
// Leave half of the retain counts inline and
// prepare to copy the other half to the side table.
if (!tryRetain && !sideTableLocked) sidetable_lock();
sideTableLocked = true;
transcribeToSideTable = true;
newisa.extra_rc = RC_HALF;
newisa.has_sidetable_rc = true;
}
} while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)));
if (slowpath(transcribeToSideTable)) {
// Copy the other half of the retain counts to the side table.
sidetable_addExtraRC_nolock(RC_HALF);
}
if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock();
return (id)this;
}
其中，手动retain对引用计数的影响关键在这么一句话：
newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++
对isa的 extra_rc 变量进行+1，前面说到isa会存很多东西。
3. isa与Tagged Pointer
3.1 NSObject的唯一成员变量 -- isa
NSObject.h
NSObject的isa
OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0)
OBJC_ROOT_CLASS
OBJC_EXPORT
@interface NSObject {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
Class isa OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}
其中，Class isa继续查看Class的定义：
objc-private.h
Class
typedef struct objc_class *Class;
typedef struct objc_object *id;
其中，objc_object类内部结构：
其中，私有的成员数据isa为isa_t类型的联合体：
objc-private.h
isa_t
union isa_t
{
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
#if SUPPORT_PACKED_ISA
// extra_rc must be the MSB-most field (so it matches carry/overflow flags)
// nonpointer must be the LSB (fixme or get rid of it)
// shiftcls must occupy the same bits that a real class pointer would
// bits + RC_ONE is equivalent to extra_rc + 1
// RC_HALF is the high bit of extra_rc (i.e. half of its range)
// future expansion:
// uintptr_t fast_rr : 1; // no r/r overrides
// uintptr_t lock : 2; // lock for atomic property, @synch
// uintptr_t extraBytes : 1; // allocated with extra bytes
# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# define ISA_MAGIC_MASK 0x000003f000000001ULL
# define ISA_MAGIC_VALUE 0x000001a000000001ULL
struct {
uintptr_t nonpointer : 1;
uintptr_t has_assoc : 1;
uintptr_t has_cxx_dtor : 1;
uintptr_t shiftcls : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
uintptr_t magic : 6;
uintptr_t weakly_referenced : 1;
uintptr_t deallocating : 1;
uintptr_t has_sidetable_rc : 1;
uintptr_t extra_rc : 19;
# define RC_ONE (1ULL<<45)
# define RC_HALF (1ULL<<18)
};
# elif __x86_64__
# define ISA_MASK 0x00007ffffffffff8ULL
# define ISA_MAGIC_MASK 0x001f800000000001ULL
# define ISA_MAGIC_VALUE 0x001d800000000001ULL
struct {
uintptr_t nonpointer : 1;
uintptr_t has_assoc : 1;
uintptr_t has_cxx_dtor : 1;
uintptr_t shiftcls : 44; // MACH_VM_MAX_ADDRESS 0x7fffffe00000
uintptr_t magic : 6;
uintptr_t weakly_referenced : 1;
uintptr_t deallocating : 1;
uintptr_t has_sidetable_rc : 1;
uintptr_t extra_rc : 8;
# define RC_ONE (1ULL<<56)
# define RC_HALF (1ULL<<7)
};
# else
# error unknown architecture for packed isa
# endif
// SUPPORT_PACKED_ISA
#endif
#if SUPPORT_INDEXED_ISA
# if __ARM_ARCH_7K__ >= 2
# define ISA_INDEX_IS_NPI 1
# define ISA_INDEX_MASK 0x0001FFFC
# define ISA_INDEX_SHIFT 2
# define ISA_INDEX_BITS 15
# define ISA_INDEX_COUNT (1 << ISA_INDEX_BITS)
# define ISA_INDEX_MAGIC_MASK 0x001E0001
# define ISA_INDEX_MAGIC_VALUE 0x001C0001
struct {
uintptr_t nonpointer : 1;
uintptr_t has_assoc : 1;
uintptr_t indexcls : 15;
uintptr_t magic : 4;
uintptr_t has_cxx_dtor : 1;
uintptr_t weakly_referenced : 1;
uintptr_t deallocating : 1;
uintptr_t has_sidetable_rc : 1;
uintptr_t extra_rc : 7;
# define RC_ONE (1ULL<<25)
# define RC_HALF (1ULL<<6)
};
# else
# error unknown architecture for indexed isa
# endif
// SUPPORT_INDEXED_ISA
#endif
};
其中，cls 变量会指向对象所属的类的结构，在 64 位设备上会占用 8byte。
另外，bits 变量保存着isa的唯一标志（可以根据bits获取isa），是一个类型为 uintptr_t 的数据， uintptr_t的定义：
typedef unsigned long uintptr_t;
知识回顾
不熟悉C++的朋友可能很难看出来bits会是如何初始化的，其实，这是一种与构造函数并列的初始化办法 -- 初始化列表。关于初始化列表的定义，截取百度百科的一段话：
初始化列表的用法
所以，再回过来看bits，bits以isa_t(uintptr_t value)中的value为初始化的值:
`bits`
例如isa初始化的API objc_object::initIsa(Class cls)中，有这样一句：
isa_t newisa(0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
//...
而这个bits值可以用来获取isa（注意区分左右两边的bits分别是两个东西）:
isa_t bits = LoadExclusive(&isa.bits);
其中，LoadExclusive根据平台的不同，实现体并不一样，这是__arm64__平台的实现体：
#if __arm64__
static ALWAYS_INLINE
uintptr_t
LoadExclusive(uintptr_t *src)
{
uintptr_t result;
asm("ldxr %x0, [%x1]"
: "=r" (result)
: "r" (src), "m" (*src));
return result;
}
对这个isa (这里是左边的bits，它是个isa，而非右边的uintptr_t) 的调用，比如获取引用计数的源代码中就有几处：
inline uintptr_t
objc_object::rootRetainCount()
{
if (isTaggedPointer()) return (uintptr_t)this;
sidetable_lock();
isa_t bits = LoadExclusive(&isa.bits);
ClearExclusive(&isa.bits);
if (bits.nonpointer) {
uintptr_t rc = 1 + bits.extra_rc;
if (bits.has_sidetable_rc) {
rc += sidetable_getExtraRC_nolock();
}
sidetable_unlock();
return rc;
}
sidetable_unlock();
return sidetable_retainCount();
}
调用的有：
bits.extra_rc
bits.nonpointer
bits.has_sidetable_rc
3.2 isa_t联合体里面struct的数据含义
nonpointer
该变量占用 1bit 内存空间，可以有两个值：0 和 1，分别代表不同的 isa_t 的类型：
0 表示 isa_t 没有开启指针优化，不使用 isa_t 中定义的结构体。访问 objc_object 的 isa 会直接返回 isa_t 结构中的
cls变量，cls变量会指向对象所属的类的结构，在 64 位设备上会占用 8byte。1 表示 isa_t 开启了指针优化，不能直接访问 objc_object 的 isa 成员变量 (因为 isa 已经不是一个合法的内存指针了，而是一个 Tagged Pointer )，从其名字 nonpointer 也可获知这个 isa 已经不是一个指针了。但是 isa 中包含了类信息、对象的引用计数等信息，在 64 位设备上充分利用了内存空间。
shiftcls
存储类指针的值。开启指针优化的情况下，在 arm64 架构中有 33 位用来存储类指针。
has_assoc
该变量与对象的关联引用有关，当对象有关联引用时，释放对象时需要做额外的逻辑。关联引用就是我们通常用 objc_setAssociatedObject 方法设置给对象的，这里对于关联引用不做过多分析，如果后续有时间写关联引用实现时再深入分析关联引用有关的代码。
has_cxx_dtor
表示该对象是否有 C++ 或者 Objc 的析构器，如果有析构函数，则需要做析构逻辑，如果没有，则可以更快的释放对象。
magic
用于判断对象是否已经完成了初始化，在 arm64 中 0x16 是调试器判断当前对象是真的对象还是没有初始化的空间(在 x86_64 中该值为 0x3b)。
weakly_referenced
标志对象是否被指向或者曾经指向一个 ARC 的弱变量，没有弱引用的对象可以更快释放。
deallocating
标志对象是否正在释放内存。
extra_rc
表示该对象的引用计数值，实际上是引用计数值减 1，例如，如果对象的引用计数为 10，那么 extra_rc 为 9。如果引用计数大于 10，则需要使用到下面的 has_sidetable_rc。
has_sidetable_rc
当对象引用计数大于 10 时，则has_sidetable_rc 的值为 1，那么引用计数会存储在一个叫 SideTable 的类的属性中，这是一个散列表。
ISA_MAGIC_MASK
通过掩码方式获取 magic 值。
ISA_MASK
通过掩码方式获取 isa 的类指针值。
RC_ONE 和 RC_HALF
用于引用计数的相关计算。
3.3 isa_t联合体里面的宏
SUPPORT_PACKED_ISA
表示平台是否支持在 isa 指针中插入除 Class 之外的信息。
- 如果支持就会将 Class 信息放入 isa_t 定义的 struct 内，并附上一些其他信息，例如上面的
nonpointer等等； - 如果不支持，那么不会使用 isa_t 内定义的 struct，这时 isa_t 只使用 cls(Class 指针)。
小结：在 iOS 以及 MacOSX 设备上，SUPPORT_PACKED_ISA 定义为 1。
__arm64__、__x86_64__
表示 CPU 架构，例如电脑一般是 __x86_64__ 架构，手机一般是 arm 结构，这里 64 代表是 64 位 CPU。上面只列出了 __arm64__ 架构的定义。
小结：iOS 设备上 __arm64__ 是 1。
SUPPORT_INDEXED_ISA
SUPPORT_INDEXED_ISA 表示 isa_t 中存放的 Class 信息是 Class 的地址，还是一个索引(根据该索引可在类信息表中查找该类结构地址)。可以看出，多了一个 uintptr_t indexcls : 15;。
小结：iOS 设备上 SUPPORT_INDEXED_ISA 是 0。
3.4 是否Tagged Pointer的判断
objc-object.h
objc_object::isTaggedPointer()
inline bool
objc_object::isTaggedPointer()
{
return _objc_isTaggedPointer(this);
}
objc-internal.h
_objc_isTaggedPointer(const void * _Nullable ptr)
static inline bool
_objc_isTaggedPointer(const void * _Nullable ptr)
{
return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}
3.5 与isa类型有关的宏
SUPPORT_NONPOINTER_ISA
用于标记是否支持优化的 isa 指针，其字面含义意思是 isa 的内容不再是类的指针了，而是包含了更多信息，比如引用计数，析构状态，被其他 weak 变量引用情况。下面看看SUPPORT_NONPOINTER_ISA及其相关宏的定义：
objc-config.h
SUPPORT_TAGGED_POINTERS
// Define SUPPORT_TAGGED_POINTERS=1 to enable tagged pointer objects
// Be sure to edit tagged pointer SPI in objc-internal.h as well.
#if !(__OBJC2__ && __LP64__)
# define SUPPORT_TAGGED_POINTERS 0
#else
# define SUPPORT_TAGGED_POINTERS 1
#endif
// Define SUPPORT_MSB_TAGGED_POINTERS to use the MSB
// as the tagged pointer marker instead of the LSB.
// Be sure to edit tagged pointer SPI in objc-internal.h as well.
#if !SUPPORT_TAGGED_POINTERS || !TARGET_OS_IPHONE
# define SUPPORT_MSB_TAGGED_POINTERS 0
#else
# define SUPPORT_MSB_TAGGED_POINTERS 1
#endif
// Define SUPPORT_INDEXED_ISA=1 on platforms that store the class in the isa
// field as an index into a class table.
// Note, keep this in sync with any .s files which also define it.
// Be sure to edit objc-abi.h as well.
#if __ARM_ARCH_7K__ >= 2
# define SUPPORT_INDEXED_ISA 1
#else
# define SUPPORT_INDEXED_ISA 0
#endif
// Define SUPPORT_PACKED_ISA=1 on platforms that store the class in the isa
// field as a maskable pointer with other data around it.
#if (!__LP64__ || TARGET_OS_WIN32 || TARGET_OS_SIMULATOR)
# define SUPPORT_PACKED_ISA 0
#else
# define SUPPORT_PACKED_ISA 1
#endif
// Define SUPPORT_NONPOINTER_ISA=1 on any platform that may store something
// in the isa field that is not a raw pointer.
#if !SUPPORT_INDEXED_ISA && !SUPPORT_PACKED_ISA
# define SUPPORT_NONPOINTER_ISA 0
#else
# define SUPPORT_NONPOINTER_ISA 1
#endif
3.6 怎么判断是否支持优化的isa指针？-- 看设备、自己设置。
已知iOS系统的
SUPPORT_PACKED_ISA为1，SUPPORT_INDEXED_ISA为0，根据4.5节中源代码的定义可知，iOS系统的SUPPORT_NONPOINTER_ISA为1。在环境变量中设置
OBJC_DISABLE_NONPOINTER_ISA。
即，iOS系统 支持 优化的isa指针。
在 64 位环境下，优化的 isa 指针并不是就一定会存储引用计数，毕竟用 19bit （iOS 系统）保存引用计数不一定够。需要注意的是这 19 位保存的是引用计数的值减一。
3.7 怎么判断是否Tagged Pointer的对象？-- 看对象、自己设置
可以启用Tagged Pointer的类对象有：NSDate、NSNumber、NSString。Tagged Pointer专门用来存储小的对象。
在环境变量中设置OBJC_DISABLE_TAGGED_POINTERS=YES强制不启用Tagged Pointer。
3.8 引用计数的存储形式 -- 散列表
下面对sidetable_retain进行分析。
4. 散列表
4.1 增加引用计数 -- sidetable_retain()
第2节的增加引用假设，以及后面第8节的获取引用计数会用到下面的API:
NSObject.mm
objc_object::sidetable_retain()
id
objc_object::sidetable_retain()
{
#if SUPPORT_NONPOINTER_ISA
assert(!isa.nonpointer);
#endif
SideTable& table = SideTables()[this];
table.lock();
size_t& refcntStorage = table.refcnts[this];
if (! (refcntStorage & SIDE_TABLE_RC_PINNED)) {
refcntStorage += SIDE_TABLE_RC_ONE;
}
table.unlock();
return (id)this;
}
4.2 增加引用计数 -- sidetable_tryRetain()
NSObject.mm
objc_object::sidetable_tryRetain()
bool
objc_object::sidetable_tryRetain()
{
#if SUPPORT_NONPOINTER_ISA
assert(!isa.nonpointer);
#endif
SideTable& table = SideTables()[this];
// NO SPINLOCK HERE
// _objc_rootTryRetain() is called exclusively by _objc_loadWeak(),
// which already acquired the lock on our behalf.
// fixme can't do this efficiently with os_lock_handoff_s
// if (table.slock == 0) {
// _objc_fatal("Do not call -_tryRetain.");
// }
bool result = true;
RefcountMap::iterator it = table.refcnts.find(this);
if (it == table.refcnts.end()) {
table.refcnts[this] = SIDE_TABLE_RC_ONE;
} else if (it->second & SIDE_TABLE_DEALLOCATING) {
result = false;
} else if (! (it->second & SIDE_TABLE_RC_PINNED)) {
it->second += SIDE_TABLE_RC_ONE;
}
return result;
}
4.3 获取散列表 -- SideTable()
NSObject.mm
SideTable
struct SideTable {
spinlock_t slock;
RefcountMap refcnts;
weak_table_t weak_table;
SideTable() {
memset(&weak_table, 0, sizeof(weak_table));
}
~SideTable() {
_objc_fatal("Do not delete SideTable.");
}
void lock() { slock.lock(); }
void unlock() { slock.unlock(); }
void forceReset() { slock.forceReset(); }
// Address-ordered lock discipline for a pair of side tables.
template
static void lockTwo(SideTable *lock1, SideTable *lock2);
template
static void unlockTwo(SideTable *lock1, SideTable *lock2);
};
其中，RefcountMap以及HaveOld，HaveNew的定义为：
// RefcountMap disguises its pointers because we
// don't want the table to act as a root for `leaks`.
typedef objc::DenseMap,size_t,true> RefcountMap;
// Template parameters.
enum HaveOld { DontHaveOld = false, DoHaveOld = true };
enum HaveNew { DontHaveNew = false, DoHaveNew = true };
llvm-DenseMap.h
DenseMap/DenseMapBase
位置
DenseMapBase
DenseMapBase
DenseMap
DenseMap
5. 设置变量导致的引用计数变化 -- objc_retain操作
5.1 情况1 -- strong
runtime.h
设置strong变量
/**
* Sets the value of an instance variable in an object.
*
* @param obj The object containing the instance variable whose value you want to set.
* @param ivar The Ivar describing the instance variable whose value you want to set.
* @param value The new value for the instance variable.
*
* @note Instance variables with known memory management (such as ARC strong and weak)
* use that memory management. Instance variables with unknown memory management
* are assigned as if they were strong.
* @note \c object_setIvar is faster than \c object_setInstanceVariable if the Ivar
* for the instance variable is already known.
*/
OBJC_EXPORT void
object_setIvarWithStrongDefault(id _Nullable obj, Ivar _Nonnull ivar,
id _Nullable value)
OBJC_AVAILABLE(10.12, 10.0, 10.0, 3.0, 2.0);
objc-class.mm
object_setIvarWithStrongDefault
void object_setIvarWithStrongDefault(id obj, Ivar ivar, id value)
{
return _object_setIvar(obj, ivar, value, true /*strong default*/);
}
objc-class.mm
_object_setIvar
static ALWAYS_INLINE
void _object_setIvar(id obj, Ivar ivar, id value, bool assumeStrong)
{
if (!obj || !ivar || obj->isTaggedPointer()) return;
ptrdiff_t offset;
objc_ivar_memory_management_t memoryManagement;
_class_lookUpIvar(obj->ISA(), ivar, offset, memoryManagement);
if (memoryManagement == objc_ivar_memoryUnknown) {
if (assumeStrong) memoryManagement = objc_ivar_memoryStrong;
else memoryManagement = objc_ivar_memoryUnretained;
}
id *location = (id *)((char *)obj + offset);
switch (memoryManagement) {
case objc_ivar_memoryWeak: objc_storeWeak(location, value); break;
case objc_ivar_memoryStrong: objc_storeStrong(location, value); break;
case objc_ivar_memoryUnretained: *location = value; break;
case objc_ivar_memoryUnknown: _objc_fatal("impossible");
}
}
NSObject.mm
objc_storeStrong
void
objc_storeStrong(id *location, id obj)
{
id prev = *location;
if (obj == prev) {
return;
}
objc_retain(obj);
*location = obj;
objc_release(prev);
}
5.2 情况2 -- weak
objc-class.mm
设置weak变量
object_setIvar(id _Nullable obj, Ivar _Nonnull ivar, id _Nullable value)
OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
objc-class.mm
object_setIvar
void object_setIvar(id obj, Ivar ivar, id value)
{
return _object_setIvar(obj, ivar, value, false /*not strong default*/);
}
- 可见，这里同样调用了
_object_setIvar，代码情况1，是同一个API。其中，不同于objc_storeStrong，走的是objc_storeWeak，下面分析一下：
NSObject.mm
objc_storeWeak
/**
* This function stores a new value into a __weak variable. It would
* be used anywhere a __weak variable is the target of an assignment.
*
* @param location The address of the weak pointer itself
* @param newObj The new object this weak ptr should now point to
*
* @return \e newObj
*/
id
objc_storeWeak(id *location, id newObj)
{
return storeWeak
(location, (objc_object *)newObj);
}
上面有一个storeWeak，它的代码有点长，核心的关键是更新了weak哈希表：->weak_table。读者可以从下面搜索一下这个关键词的位置。
// Update a weak variable.
// If HaveOld is true, the variable has an existing value
// that needs to be cleaned up. This value might be nil.
// If HaveNew is true, there is a new value that needs to be
// assigned into the variable. This value might be nil.
// If CrashIfDeallocating is true, the process is halted if newObj is
// deallocating or newObj's class does not support weak references.
// If CrashIfDeallocating is false, nil is stored instead.
enum CrashIfDeallocating {
DontCrashIfDeallocating = false, DoCrashIfDeallocating = true
};
template
static id
storeWeak(id *location, objc_object *newObj)
{
assert(haveOld || haveNew);
if (!haveNew) assert(newObj == nil);
Class previouslyInitializedClass = nil;
id oldObj;
SideTable *oldTable;
SideTable *newTable;
// Acquire locks for old and new values.
// Order by lock address to prevent lock ordering problems.
// Retry if the old value changes underneath us.
retry:
if (haveOld) {
oldObj = *location;
oldTable = &SideTables()[oldObj];
} else {
oldTable = nil;
}
if (haveNew) {
newTable = &SideTables()[newObj];
} else {
newTable = nil;
}
SideTable::lockTwo(oldTable, newTable);
if (haveOld && *location != oldObj) {
SideTable::unlockTwo(oldTable, newTable);
goto retry;
}
// Prevent a deadlock between the weak reference machinery
// and the +initialize machinery by ensuring that no
// weakly-referenced object has an un-+initialized isa.
if (haveNew && newObj) {
Class cls = newObj->getIsa();
if (cls != previouslyInitializedClass &&
!((objc_class *)cls)->isInitialized())
{
SideTable::unlockTwo(oldTable, newTable);
_class_initialize(_class_getNonMetaClass(cls, (id)newObj));
// If this class is finished with +initialize then we're good.
// If this class is still running +initialize on this thread
// (i.e. +initialize called storeWeak on an instance of itself)
// then we may proceed but it will appear initializing and
// not yet initialized to the check above.
// Instead set previouslyInitializedClass to recognize it on retry.
previouslyInitializedClass = cls;
goto retry;
}
}
// Clean up old value, if any.
if (haveOld) {
weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
}
// Assign new value, if any.
if (haveNew) {
newObj = (objc_object *)
weak_register_no_lock(&newTable->weak_table, (id)newObj, location,
crashIfDeallocating);
// weak_register_no_lock returns nil if weak store should be rejected
// Set is-weakly-referenced bit in refcount table.
if (newObj && !newObj->isTaggedPointer()) {
newObj->setWeaklyReferenced_nolock();
}
// Do not set *location anywhere else. That would introduce a race.
*location = (id)newObj;
}
else {
// No new value. The storage is not changed.
}
SideTable::unlockTwo(oldTable, newTable);
return (id)newObj;
}
5.3 objc_storeStrong导致的retain
上面第5.1节中有一个objc_storeStrong，这里继续分析它的原理。
NSObject.mm
objc_storeStrong(id *location, id obj)
void
objc_storeStrong(id *location, id obj)
{
id prev = *location;
if (obj == prev) {
return;
}
objc_retain(obj);
*location = obj;
objc_release(prev);
}
NSObject.mm
objc_retain(id obj)
/***********************************************************************
* Optimized retain/release/autorelease entrypoints
**********************************************************************/
#if __OBJC2__
__attribute__((aligned(16)))
id
objc_retain(id obj)
{
if (!obj) return obj;
if (obj->isTaggedPointer()) return obj;
return obj->retain();
}
__attribute__((aligned(16)))
void
objc_release(id obj)
{
if (!obj) return;
if (obj->isTaggedPointer()) return;
return obj->release();
}
__attribute__((aligned(16)))
id
objc_autorelease(id obj)
{
if (!obj) return obj;
if (obj->isTaggedPointer()) return obj;
return obj->autorelease();
}
// OBJC2
#else
// not OBJC2
id objc_retain(id obj) { return [obj retain]; }
void objc_release(id obj) { [obj release]; }
id objc_autorelease(id obj) { return [obj autorelease]; }
#endif
可知：
1）如果TaggedPointer，则返回本身。
2）如果非TaggedPointer，则由对象的retain()返回。
objc-object.h
objc_object::retain()
// Equivalent to calling [this retain], with shortcuts if there is no override
inline id
objc_object::retain()
{
assert(!isTaggedPointer());
if (fastpath(!ISA()->hasCustomRR())) {
return rootRetain();
}
return ((id(*)(objc_object *, SEL))objc_msgSend)(this, SEL_retain);
}
objc-object.h
objc_object::rootRetain()
// Base retain implementation, ignoring overrides.
// This does not check isa.fast_rr; if there is an RR override then
// it was already called and it chose to call [super retain].
//
// tryRetain=true is the -_tryRetain path.
// handleOverflow=false is the frameless fast path.
// handleOverflow=true is the framed slow path including overflow to side table
// The code is structured this way to prevent duplication.
ALWAYS_INLINE id
objc_object::rootRetain()
{
return rootRetain(false, false);
}
这里的rootRetain(false, false);正是上文第2.2节中介绍的，不再赘述。
6. 新建对象（分配内存与初始化）导致的引用计数变化 -- alloc 和 init 操作
首先，新建一个对象的典型写法：
NSObject *obj = [NSObject alloc] init];
6.1 分配内存 -- alloc
+ (id)alloc {
return _objc_rootAlloc(self);
}
// Base class implementation of +alloc. cls is not nil.
// Calls [cls allocWithZone:nil].
id
_objc_rootAlloc(Class cls)
{
return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}
// Call [cls alloc] or [cls allocWithZone:nil], with appropriate
// shortcutting optimizations.
static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
if (slowpath(checkNil && !cls)) return nil;
#if __OBJC2__
if (fastpath(!cls->ISA()->hasCustomAWZ())) {
// No alloc/allocWithZone implementation. Go straight to the allocator.
// fixme store hasCustomAWZ in the non-meta class and
// add it to canAllocFast's summary
if (fastpath(cls->canAllocFast())) {
// No ctors, raw isa, etc. Go straight to the metal.
bool dtor = cls->hasCxxDtor();
id obj = (id)calloc(1, cls->bits.fastInstanceSize());
if (slowpath(!obj)) return callBadAllocHandler(cls);
obj->initInstanceIsa(cls, dtor);
return obj;
}
else {
// Has ctor or raw isa or something. Use the slower path.
id obj = class_createInstance(cls, 0);
if (slowpath(!obj)) return callBadAllocHandler(cls);
return obj;
}
}
#endif
// No shortcuts available.
if (allocWithZone) return [cls allocWithZone:nil];
return [cls alloc];
}
分支1 -- obj->initInstanceIsa(cls, dtor);
inline void
objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
assert(!cls->instancesRequireRawIsa());
assert(hasCxxDtor == cls->hasCxxDtor());
initIsa(cls, true, hasCxxDtor);
}
inline void
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor)
{
assert(!isTaggedPointer());
if (!nonpointer) {
isa.cls = cls;
} else {
assert(!DisableNonpointerIsa);
assert(!cls->instancesRequireRawIsa());
isa_t newisa(0);
#if SUPPORT_INDEXED_ISA
assert(cls->classArrayIndex() > 0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
newisa.bits = ISA_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.shiftcls = (uintptr_t)cls >> 3;
#endif
// This write must be performed in a single store in some cases
// (for example when realizing a class because other threads
// may simultaneously try to use the class).
// fixme use atomics here to guarantee single-store and to
// guarantee memory order w.r.t. the class index table
// ...but not too atomic because we don't want to hurt instantiation
isa = newisa;
}
}
上述代码中，newisa.bits = ISA_MAGIC_VALUE; 是为了对 isa 结构赋值一个初始值，通过对 isa_t 的结构分析，我们可以知道此次赋值只是对 nonpointer 和 magic 部分进行了赋值。
newisa.shiftcls = (uintptr_t)cls >> 3; 是将类的地址存储在对象的 isa 结构中。这里右移三位的主要原因是用于将 Class 指针中无用的后三位清除减小内存的消耗，因为类的指针要按照字节（8 bits）对齐内存，其指针后三位都是没有意义的 0。关于类指针对齐的详细解析可参考：从 NSObject 的初始化了解 isa 。
分支2 -- id obj = class_createInstance(cls, 0);
id
class_createInstance(Class cls, size_t extraBytes)
{
return _class_createInstanceFromZone(cls, extraBytes, nil);
}
/***********************************************************************
* class_createInstance
* fixme
* Locking: none
**********************************************************************/
static __attribute__((always_inline))
id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
bool cxxConstruct = true,
size_t *outAllocatedSize = nil)
{
if (!cls) return nil;
assert(cls->isRealized());
// Read class's info bits all at once for performance
bool hasCxxCtor = cls->hasCxxCtor();
bool hasCxxDtor = cls->hasCxxDtor();
bool fast = cls->canAllocNonpointer();
size_t size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
if (!zone && fast) {
obj = (id)calloc(1, size);
if (!obj) return nil;
obj->initInstanceIsa(cls, hasCxxDtor);
}
else {
if (zone) {
obj = (id)malloc_zone_calloc ((malloc_zone_t *)zone, 1, size);
} else {
obj = (id)calloc(1, size);
}
if (!obj) return nil;
// Use raw pointer isa on the assumption that they might be
// doing something weird with the zone or RR.
obj->initIsa(cls);
}
if (cxxConstruct && hasCxxCtor) {
obj = _objc_constructOrFree(obj, cls);
}
return obj;
}
其中，有个 obj->initIsa(cls);，初始化isa的操作：
inline void
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor)
{
assert(!isTaggedPointer());
if (!nonpointer) {
isa.cls = cls;
} else {
assert(!DisableNonpointerIsa);
assert(!cls->instancesRequireRawIsa());
isa_t newisa(0);
#if SUPPORT_INDEXED_ISA
assert(cls->classArrayIndex() > 0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
newisa.bits = ISA_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.shiftcls = (uintptr_t)cls >> 3;
#endif
// This write must be performed in a single store in some cases
// (for example when realizing a class because other threads
// may simultaneously try to use the class).
// fixme use atomics here to guarantee single-store and to
// guarantee memory order w.r.t. the class index table
// ...but not too atomic because we don't want to hurt instantiation
isa = newisa;
}
}
可见，alloc的时候会初始化isa，并通过newisa(0)的初始化列表办法生成一个isa，并根据是否支持indexed isa分别设置.bits的值。
6.2 初始化 -- init
- (id)init {
return _objc_rootInit(self);
}
id
_objc_rootInit(id obj)
{
// In practice, it will be hard to rely on this function.
// Many classes do not properly chain -init calls.
return obj;
}
7. 获取引用计数
NSObject.mm
retainCount
- (NSUInteger)retainCount {
return ((id)self)->rootRetainCount();
}
objc-object.h
objc_object::rootRetainCount()
inline uintptr_t
objc_object::rootRetainCount()
{
if (isTaggedPointer()) return (uintptr_t)this;
sidetable_lock();
isa_t bits = LoadExclusive(&isa.bits);
ClearExclusive(&isa.bits);
if (bits.nonpointer) {
uintptr_t rc = 1 + bits.extra_rc;
if (bits.has_sidetable_rc) {
rc += sidetable_getExtraRC_nolock();
}
sidetable_unlock();
return rc;
}
sidetable_unlock();
return sidetable_retainCount();
}
可见，获取引用计数的关键在这么一句话：
uintptr_t rc = 1 + bits.extra_rc;
bits.extra_rc表示引用计数减1。当然，这只针对情况1，即bits.nonpointer为1（开启了指针优化），且bits.has_sidetable_rc为0（表示不存储在散列表Side Table中，而存储在extra_rc中）。
- 情况0 -- TaggedPointer
直接返回isa值本身。
- 情况1 -- 非TaggedPointer，开启了指针优化，且存储在
extra_rc中
objc-os.h
LoadExclusive(uintptr_t *src)
static ALWAYS_INLINE
uintptr_t
LoadExclusive(uintptr_t *src)
{
return *src;
}
- 情况2 -- 非TaggedPointer，开启指针优化，且存储在散列表中
NSObject.mm
objc_object::sidetable_getExtraRC_nolock()
size_t
objc_object::sidetable_getExtraRC_nolock()
{
assert(isa.nonpointer);
SideTable& table = SideTables()[this];
RefcountMap::iterator it = table.refcnts.find(this);
if (it == table.refcnts.end()) return 0;
else return it->second >> SIDE_TABLE_RC_SHIFT;
}
可见，其逻辑就是先从 SideTable 的静态方法获取当前实例对应的 SideTable 对象，其 refcnts 属性就是之前说的存储引用计数的散列表，这里将其类型简写为 RefcountMap：
typedef objc::DenseMap RefcountMap;
然后在引用计数表中用迭代器查找当前实例对应的键值对，获取引用计数值，并在此基础上 +1 并将结果返回。这也就是为什么之前说引用计数表存储的值为实际引用计数减一。
需要注意的是为什么这里把键值对的值做了向右移位操作（it->second >> SIDE_TABLE_RC_SHIFT）:
// The order of these bits is important.
#define SIDE_TABLE_WEAKLY_REFERENCED (1UL<<0)
#define SIDE_TABLE_DEALLOCATING (1UL<<1) // MSB-ward of weak bit
#define SIDE_TABLE_RC_ONE (1UL<<2) // MSB-ward of deallocating bit
#define SIDE_TABLE_RC_PINNED (1UL<<(WORD_BITS-1))
#define SIDE_TABLE_RC_SHIFT 2
#define SIDE_TABLE_FLAG_MASK (SIDE_TABLE_RC_ONE-1)
可以看出值的第一个 bit 表示该对象是否有过 weak 对象，如果没有，在析构释放内存时可以更快；第二个 bit 表示该对象是否正在析构。从第三个 bit 开始才是存储引用计数数值的地方。所以这里要做向右移两位的操作，而对引用计数的 +1 和 -1 可以使用 SIDE_TABLE_RC_ONE，还可以用 SIDE_TABLE_RC_PINNED 来判断是否引用计数值有可能溢出。
- 情况3 -- 默认值 -- 非TaggedPointer，没有开启指针优化
NSObject.mm
objc_object::sidetable_retainCount()
uintptr_t
objc_object::sidetable_retainCount()
{
SideTable& table = SideTables()[this];
size_t refcnt_result = 1;
table.lock();
RefcountMap::iterator it = table.refcnts.find(this);
if (it != table.refcnts.end()) {
// this is valid for SIDE_TABLE_RC_PINNED too
refcnt_result += it->second >> SIDE_TABLE_RC_SHIFT;
}
table.unlock();
return refcnt_result;
}
8. 结论
- 如果有些对象支持使用 TaggedPointer：
- 苹果会直接将对象的指针值作为引用计数返回。
- 如果另外一些对象不支持使用 TaggedPointer：
- 如果当前设备是 64 位环境并且使用 Objective-C 2.0，那么会使用对象的 isa 指针 的 一部分空间 （
bits.extra_rc）来存储它的引用计数； - 否则 Runtime 会使用一张 散列表 （
SideTables()）来管理引用计数。
9. 拓展阅读
weak表
https://www.jianshu.com/p/13c4fb1cedea