Objc 对象的今生今世

ppsu 7年前
   <p><img src="https://simg.open-open.com/show/6b730b450f950c448875c21c8352a7f2.png"></p>    <h3><strong>前言</strong></h3>    <p>在面向对象编程中,我们每天都在创建对象,用对象描述着整个世界,然而对象是如何从孕育到销毁的呢?</p>    <h3><strong>目录</strong></h3>    <ul>     <li> <p>1.孕育对象</p> </li>     <li> <p>2.对象的出生</p> </li>     <li> <p>3.对象的成长</p> </li>     <li> <p>4.对象的销毁</p> </li>     <li> <p>5.总结</p> </li>    </ul>    <h3><strong>一.孕育对象</strong></h3>    <p style="text-align:center"><img src="https://simg.open-open.com/show/f57f0f7021776ada21af30f84115b1d7.jpg"></p>    <p>每天开发我们都在alloc对象,而alloc方法做了些什么呢?</p>    <pre>  <code class="language-objectivec">+ (id)alloc {      return _objc_rootAlloc(self);  }</code></pre>    <p>所有对象alloc都会调用这个root的方法</p>    <pre>  <code class="language-objectivec">id _objc_rootAlloc(Class cls)    {      return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);  }</code></pre>    <p>这个方法又会去调用callAlloc方法</p>    <pre>  <code class="language-objectivec">static ALWAYS_INLINE id callAlloc(Class cls, bool checkNil, bool allocWithZone=false)    {      if (checkNil && !cls) return nil;    #if __OBJC2__      if (! 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 (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 (!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 (!obj) return callBadAllocHandler(cls);              return obj;          }      }  #endif        // No shortcuts available.      if (allocWithZone) return [cls allocWithZone:nil];      return [cls alloc];  }</code></pre>    <p>由于入参 checkNil = false,所以不会返回nil。</p>    <pre>  <code class="language-objectivec">    bool hasCustomAWZ() {          return ! bits.hasDefaultAWZ();      }</code></pre>    <p style="text-align:center"><img src="https://simg.open-open.com/show/9451e24410d9927a474ccf23c17f9ea7.png"></p>    <p>在 这张图 ,我们可以看到在对象的数据段data中,class_rw_t中有一个flags。</p>    <pre>  <code class="language-objectivec">    bool hasDefaultAWZ( ) {          return data()->flags & RW_HAS_DEFAULT_AWZ;      }    #define RW_HAS_DEFAULT_AWZ    (1<<16)</code></pre>    <p>RW_HAS_DEFAULT_AWZ 这个是用来标示当前的class或者是superclass是否有默认的alloc/allocWithZone:。值得注意的是,这个值会存储在metaclass 中。</p>    <p>hasDefaultAWZ( )方法是用来判断当前class是否有默认的allocWithZone。</p>    <p>如果cls->ISA()->hasCustomAWZ()返回YES,意味着有默认的allocWithZone方法,那么就直接对class进行allocWithZone,申请内存空间。</p>    <pre>  <code class="language-objectivec">    if (allocWithZone) return [cls allocWithZone:nil];</code></pre>    <p>allocWithZone会去调用rootAllocWithZone</p>    <pre>  <code class="language-objectivec">+ (id)allocWithZone:(struct _NSZone *)zone {      return _objc_rootAllocWithZone(self, (malloc_zone_t *)zone);  }</code></pre>    <p>接下来就仔细看看_objc_rootAllocWithZone的具体实现</p>    <pre>  <code class="language-objectivec">id _objc_rootAllocWithZone(Class cls, malloc_zone_t *zone)    {      id obj;    #if __OBJC2__      // allocWithZone under __OBJC2__ ignores the zone parameter      (void)zone;      obj = class_createInstance(cls, 0);  #else      if (!zone || UseGC) {          obj = class_createInstance(cls, 0);      }      else {          obj = class_createInstanceFromZone(cls, 0, zone);      }  #endif        if (!obj) obj = callBadAllocHandler(cls);      return obj;  }</code></pre>    <p>在__OBJC2__中,直接调用class_createInstance(cls, 0);方法去创建对象。</p>    <pre>  <code class="language-objectivec">id  class_createInstance(Class cls, size_t extraBytes)    {      return _class_createInstanceFromZone(cls, extraBytes, nil);  }</code></pre>    <p>关于_class_createInstanceFromZone方法这里先不详细分析,下面再详细分析,先理清程序脉络。</p>    <p>在objc的老版本中要先去看看zone是否有空间,是否用了垃圾回收,如果没有空间,或者用了垃圾回收,就会调用class_createInstance(cls, 0)方法获取对象,否则调用class_createInstanceFromZone(cls, 0, zone);获取对象。</p>    <pre>  <code class="language-objectivec">id class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone)    {      return _class_createInstanceFromZone(cls, extraBytes, zone);  }</code></pre>    <p>可以看到,创建对象最终调用的函数都是_class_createInstanceFromZone,不管objc的版本是新版还是旧版。</p>    <p>如果创建成功就返回objc,如果创建失败,就会调用callBadAllocHandler方法。</p>    <pre>  <code class="language-objectivec">static id callBadAllocHandler(Class cls)    {      // fixme add re-entrancy protection in case allocation fails inside handler      return (*badAllocHandler)(cls);  }    static id(*badAllocHandler)(Class) = &defaultBadAllocHandler;    static id defaultBadAllocHandler(Class cls)    {      _objc_fatal("attempt to allocate object of class '%s' failed",                   cls->nameForLogging());  }</code></pre>    <p>创建对象失败后,最终会调用_objc_fatal输出"attempt to allocate object of class failed"创建对象失败。</p>    <p>到此就完成了callAlloc中hasCustomAWZ( )返回YES的情况。那么hasCustomAWZ( )函数返回NO,情况是怎么样的呢?</p>    <pre>  <code class="language-objectivec">    if (! 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 (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 (!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 (!obj) return callBadAllocHandler(cls);              return obj;          }      }</code></pre>    <p>这一段是hasCustomAWZ( )返回NO的情况,对应的是当前class没有默认的allocWithZone的情况。</p>    <p>在没有默认的allocWithZone的情况下,还需要再次判断当前的class是否支持快速alloc。如果可以,直接调用calloc函数,申请1块bits.fastInstanceSize()大小的内存空间,如果创建失败,也会调用callBadAllocHandler函数。</p>    <p>如果创建成功,就去初始化Isa指针和dtor。</p>    <pre>  <code class="language-objectivec">    bool hasCxxDtor() {          return data()->flags & RW_HAS_CXX_DTOR;      }    // class or superclass has .cxx_destruct implementation  #define RW_HAS_CXX_DTOR       (1<<17)</code></pre>    <p>dtor是用来判断当前class或者superclass是否有.cxx_destruct函数的实现。</p>    <p>如果当前的class不支持快速alloc,那么就乖乖的去调用class_createInstance(cls, 0);方法去创建一个新的对象。</p>    <p>小结一下:</p>    <p><img src="https://simg.open-open.com/show/9d3cffe0131d4dba6f81a2265bac3212.png"></p>    <p>经过上面的一系列判断,“孕育对象”的过程最终落在了_class_createInstanceFromZone函数上了。</p>    <pre>  <code class="language-objectivec">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->canAllocIndexed();        size_t size = cls->instanceSize(extraBytes);      if (outAllocatedSize) *outAllocatedSize = size;        id obj;      if (!UseGC  &&  !zone  &&  fast) {          obj = (id)calloc(1, size);          if (!obj) return nil;          obj->initInstanceIsa(cls, hasCxxDtor);      }       else {  #if SUPPORT_GC          if (UseGC) {              obj = (id)auto_zone_allocate_object(gc_zone, size,                                                  AUTO_OBJECT_SCANNED, 0, 1);          } else   #endif          if (zone) {              obj = (id)malloc_zone_calloc ((malloc_zone_t *)zone, 1, size);          } else {              obj = (id)calloc(1, size);          }          if (!obj) return nil;            // Use non-indexed 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;  }</code></pre>    <p>ctor 和 dtor 分别是什么呢?</p>    <pre>  <code class="language-objectivec">    bool hasCxxCtor() {          // addSubclass() propagates this flag from the superclass.          assert(isRealized());          return bits.hasCxxCtor();      }        bool hasCxxCtor() {          return data()->flags & RW_HAS_CXX_CTOR;      }    #define RW_HAS_CXX_CTOR       (1<<18)</code></pre>    <p>ctor是判断当前class或者superclass 是否有.cxx_construct构造方法的实现。</p>    <pre>  <code class="language-objectivec">    bool hasCxxDtor() {          // addSubclass() propagates this flag from the superclass.          assert(isRealized());          return bits.hasCxxDtor();      }        bool hasCxxDtor() {          return data()->flags & RW_HAS_CXX_DTOR;      }    #define RW_HAS_CXX_DTOR       (1<<17)</code></pre>    <p>dtor是判断判断当前class或者superclass 是否有.cxx_destruct析构方法的实现。</p>    <pre>  <code class="language-objectivec">    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;      }        uint32_t alignedInstanceSize() {          return word_align(unalignedInstanceSize());      }        uint32_t unalignedInstanceSize() {          assert(isRealized());          return data()->ro->instanceSize;      }</code></pre>    <p>实例大小 instanceSize会存储在类的 isa_t结构体中,然后经过对齐最后返回。</p>    <p>注意:Core Foundation 需要所有的对象的大小都必须大于或等于 16 字节。</p>    <p>在获取对象大小之后,直接调用calloc函数就可以为对象分配内存空间了。</p>    <p>关于calloc函数</p>    <p>The calloc( ) function contiguously allocates enough space for count objects that are size bytes of memory each and returns a pointer to the allocated memory. The allocated memory is filled with bytes of value zero.</p>    <p>这个函数也是为什么我们申请出来的对象,初始值是0或者nil的原因。因为这个calloc( )函数会默认的把申请出来的空间初始化为0或者nil。</p>    <p>申请完内存空间之后,还需要再初始化Isa指针。</p>    <pre>  <code class="language-objectivec">obj->initInstanceIsa(cls, hasCxxDtor);    obj->initIsa(cls);</code></pre>    <p>初始化Isa指针有这上面两个函数。</p>    <pre>  <code class="language-objectivec">inline void  objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)    {      assert(!UseGC);      assert(!cls->requiresRawIsa());      assert(hasCxxDtor == cls->hasCxxDtor());        initIsa(cls, true, hasCxxDtor);  }      inline void  objc_object::initIsa(Class cls)    {      initIsa(cls, false, false);  }</code></pre>    <p>从上述源码中,我们也能看出,最终都是调用了initIsa函数,只不过入参不同。</p>    <pre>  <code class="language-objectivec">inline void  objc_object::initIsa(Class cls, bool indexed, bool hasCxxDtor)    {       assert(!isTaggedPointer());             if (!indexed) {          isa.cls = cls;      } else {          assert(!DisableIndexedIsa);          isa.bits = ISA_MAGIC_VALUE;          // isa.magic is part of ISA_MAGIC_VALUE          // isa.indexed is part of ISA_MAGIC_VALUE          isa.has_cxx_dtor = hasCxxDtor;          isa.shiftcls = (uintptr_t)cls >> 3;      }  }</code></pre>    <p>初始化的过程就是对isa_t结构体初始化的过程。</p>    <pre>  <code class="language-objectivec"># if __arm64__  #   define ISA_MASK        0x0000000ffffffff8ULL  #   define ISA_MAGIC_MASK  0x000003f000000001ULL  #   define ISA_MAGIC_VALUE 0x000001a000000001ULL      struct {          uintptr_t indexed           : 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)      };</code></pre>    <p><strong>将当前地址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。</strong> 绝大多数机器的架构都是byte-addressable的,但是对象的内存地址必须对齐到字节的倍数,这样可以提高代码运行的性能,在 iPhone5s中虚拟地址为33位,所以用于对齐的最后三位比特为000,我们只会用其中的30位来表示对象的地址。</p>    <p>至此,孕育对象的过程就完成了。</p>    <h3><strong>二.对象的出生</strong></h3>    <p style="text-align:center"><img src="https://simg.open-open.com/show/039d51165e45a4d6260ea7cf506d2c70.jpg"></p>    <p>一旦当我们调用init方法的时候,对象就会“出生”了。</p>    <pre>  <code class="language-objectivec">- (id)init {      return _objc_rootInit(self);  }</code></pre>    <p>init会调用_objc_rootInit方法。</p>    <pre>  <code class="language-objectivec">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;  }</code></pre>    <p>而_objc_rootInit方法的作用也仅仅就是返回了当前对象而已。</p>    <h3><strong>三.对象的生长</strong></h3>    <p style="text-align:center"><img src="https://simg.open-open.com/show/936054cd596d0caf8bd82a45dea20905.png"></p>    <p>关于对象的生长,其实是想谈谈对象初始化之后,访问它的属性和方法,它们在内存中的样子。</p>    <pre>  <code class="language-objectivec">#import <Foundation/Foundation.h>    @interface Student : NSObject  @property (strong , nonatomic) NSString *name;  +(void)study;  -(void)run;  @end      #import "Student.h"  @implementation Student    +(void)study  {      NSLog(@"Study");   }    -(void)run  {      NSLog(@"Run");  }  @end</code></pre>    <p>这里我们新建一个Student类,来举例说明。这个类很简单,只有一个name的属性,加上一个类方法,和一个实例方法。</p>    <pre>  <code class="language-objectivec">        Student  *stu = [[Student alloc]init];                    NSLog(@"Student's class is %@", [stu class]);          NSLog(@"Student's meta class is %@", object_getClass([stu class]));          NSLog(@"Student's meta class's superclass is %@", object_getClass(object_getClass([stu class])));                    Class currentClass = [Student class];          for (int i = 1; i < 5; i++)          {              NSLog(@"Following the isa pointer %d times gives %p %@", i, currentClass,currentClass);              currentClass = object_getClass(currentClass);          }                    NSLog(@"NSObject's class is %p", [NSObject class]);          NSLog(@"NSObject's meta class is %p", object_getClass([NSObject class]));</code></pre>    <p>写出上述的代码,分析一下结构。</p>    <p>输出如下:</p>    <pre>  <code class="language-objectivec">Student's class is Student    Student's meta class is Student    Student's meta class's superclass is NSObject    Following the isa pointer 1 times gives 0x100004d90 Student    Following the isa pointer 2 times gives 0x100004d68 Student    Following the isa pointer 3 times gives 0x7fffba0b20f0 NSObject    Following the isa pointer 4 times gives 0x7fffba0b20f0 NSObject    NSObject's class is 0x7fffba0b2140    NSObject's meta class is 0x7fffba0b20f0</code></pre>    <p>经过上面的打印结果,我们可以知道,一个类的实例的isa是指向它的class,如下图:</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/6fb4d5877c7f11943ecfd20f3cbd5de9.jpg"></p>    <p>一个类的实例,虚线指向灰色的区域,灰色的区域是一个Class pair,里面包含两个东西,一个是类,另一个是meta-class。类的isa指向meta-class。由于student是继承NSObject,所以Student的class的meta-class的superclass是NSObject。</p>    <p>为了弄清楚这3个东西里面分别存了些什么,我们进一步的打印一些信息。</p>    <pre>  <code class="language-objectivec">+ (NSArray *)instanceVariables {      unsigned int outCount;      Ivar *ivars = class_copyIvarList([self class], &outCount);      NSMutableArray *result = [NSMutableArray array];      for (unsigned int i = 0; i < outCount; i++) {          NSString *type = [NSString decodeType:ivar_getTypeEncoding(ivars[i])];          NSString *name = [NSString stringWithCString:ivar_getName(ivars[i]) encoding:NSUTF8StringEncoding];          NSString *ivarDescription = [NSString stringWithFormat:@"%@ %@", type, name];          [result addObject:ivarDescription];      }      free(ivars);      return result.count ? [result copy] : nil;  }</code></pre>    <p>从之前的打印信息我们能知道,0x100004d90是类的地址。0x100004d68是meta-class类的地址。</p>    <pre>  <code class="language-objectivec">po [0x100004d90 instanceVariables]    po [0x100004d68 instanceVariables]</code></pre>    <p>打印出来:</p>    <pre>  <code class="language-objectivec"><__NSSingleObjectArrayI 0x100302460>(      NSString* _name  )    nil</code></pre>    <p>从这里就知道了,属性这些是存储在类中。</p>    <p>接下来就是关于类方法和实例方法的认识,+号方法和-号方法的认识。</p>    <p>在内存中其实没有+号和-号方法的概念。做个试验:</p>    <pre>  <code class="language-objectivec">+ (NSArray *)ClassMethodNames  {      NSMutableArray * array = [NSMutableArray array];      unsigned int methodCount = 0;      Method * methodList = class_copyMethodList([self class], &methodCount);      unsigned int i;      for(i = 0; i < methodCount; i++) {          [array addObject: NSStringFromSelector(method_getName(methodList[i]))];      }            free(methodList);      return array;  }</code></pre>    <pre>  <code class="language-objectivec">po [0x100004d90 ClassMethodNames]    po [0x100004d68 ClassMethodNames]</code></pre>    <p>打印出来:</p>    <pre>  <code class="language-objectivec"><__NSArrayM 0x100303310>(    .cxx_destruct,  name,    setName:,    run    )    <__NSArrayM 0x100303800>(    study    )</code></pre>    <p>0x100004d90是类对象,里面存储的是-号方法,还有另外3个方法,getter,setter,还有.cxx_destruct方法</p>    <p>0x100004d68是meta-class,里面存储的是+号方法。</p>    <p>当然在runtime的meta-class有一处很特殊,那就是NSObject的meta-class,它的superclass是它自己本身。为了防止调用NSObject协议里面的减号方法可能会出现崩溃,比如copy的-号方法,于是在NSObject的meta-class里面把所有的NSObject的+号方法都重新实现了一遍,就是为了消息传递到这里,拦截了一遍。所以一般NSObject协议方法同一个方法都有+号和-号方法。</p>    <p>值得说明的是,class和meta-class都是单例。</p>    <p>关于对象,所有的对象在内存里面都有一个isa,isa就是一个小“雷达”,有了它,就可以在runtime下给一个对象发送消息了。</p>    <p>所以对象的实质:Objc中的对象是一个指向ClassObject地址的变量,即 id obj = &ClassObject 。</p>    <p>关于对象的属性实质是,void *ivar = &obj + offset(N)</p>    <pre>  <code class="language-objectivec">        NSString *myName = @"halfrost";                    id cls = [Student class];          NSLog(@"Student class = %@ 地址 = %p , 大小 = %lu", cls, &cls,sizeof(cls));                    void *obj = &cls;          NSLog(@"Void *obj = %@ 地址 = %p , 大小 = %lu", obj,&obj, sizeof(obj));                    NSLog(@"%p",((__bridge Student *)obj).name);</code></pre>    <p>输出</p>    <pre>  <code class="language-objectivec">Student class = Student 地址 = 0x7fff5fbff778 , 大小 = 8    Void *obj = <Student: 0x7fff5fbff778> 地址 = 0x7fff5fbff770 , 大小 = 8    halfrost</code></pre>    <p>从这个例子就可以说明,对象的实质就是指向类对象的地址变量,从上面例子里面obj就可以看出, id obj = &ClassObject ,cls是Student的类对象,所以obj是Student的对象。</p>    <p>类对象是在main函数执行之前就加载进内存的,可执行文件中和动态库所有的符号(Class,Protocol,Selector,IMP,…)都已经按格式成功加载到内存中,被 runtime 所管理,再这之后,runtime 的那些方法(动态添加 Class、swizzle 等等才能生效)</p>    <p>关于对象的属性,就是obj的地址加上偏移量,就可以访问到,上述的例子中,obj加上偏移量,就访问到了@“halfrost”字符串了。</p>    <h3><strong>四.对象的销毁</strong></h3>    <p style="text-align:center"><img src="https://simg.open-open.com/show/aacaeeb3569265a9e5b5d3d6e7a2ee94.jpg"></p>    <p>对象的销毁就是调用dealloc方法。</p>    <pre>  <code class="language-objectivec">- (void)dealloc {      _objc_rootDealloc(self);  }</code></pre>    <p>dealloc方法会调用_objc_rootDealloc方法</p>    <pre>  <code class="language-objectivec">void _objc_rootDealloc(id obj)    {      assert(obj);        obj->rootDealloc();  }      inline void objc_object::rootDealloc()    {      assert(!UseGC);      if (isTaggedPointer()) return;        if (isa.indexed  &&            !isa.weakly_referenced  &&            !isa.has_assoc  &&            !isa.has_cxx_dtor  &&            !isa.has_sidetable_rc)      {          assert(!sidetable_present());          free(this);      }       else {          object_dispose((id)this);      }  }</code></pre>    <p>如果是TaggedPointer,直接return。</p>    <p>indexed是代表是否开启isa指针优化。weakly_referenced代表对象被指向或者曾经指向一个 ARC 的弱变量。has_assoc代表对象含有或者曾经含有关联引用。has_cxx_dtor之前提到过了,是析构器。has_sidetable_rc判断该对象的引用计数是否过大。</p>    <pre>  <code class="language-objectivec">id  object_dispose(id obj)    {      if (!obj) return nil;        objc_destructInstance(obj);        #if SUPPORT_GC      if (UseGC) {          auto_zone_retain(gc_zone, obj); // gc free expects rc==1      }  #endif        free(obj);        return nil;  }</code></pre>    <p>object_dispose会调用objc_destructInstance。</p>    <pre>  <code class="language-objectivec">/***********************************************************************  * objc_destructInstance  * Destroys an instance without freeing memory.   * Calls C++ destructors.  * Calls ARR ivar cleanup.  * Removes associative references.  * Returns `obj`. Does nothing if `obj` is nil.  * Be warned that GC DOES NOT CALL THIS. If you edit this, also edit finalize.  * CoreFoundation and other clients do call this under GC.  **********************************************************************/  void *objc_destructInstance(id obj)    {      if (obj) {          // Read all of the flags at once for performance.          bool cxx = obj->hasCxxDtor();          bool assoc = !UseGC && obj->hasAssociatedObjects();          bool dealloc = !UseGC;            // This order is important.          if (cxx) object_cxxDestruct(obj);          if (assoc) _object_remove_assocations(obj);          if (dealloc) obj->clearDeallocating();      }        return obj;  }</code></pre>    <p>销毁一个对象,靠的是底层的C++析构函数完成的。还需要移除associative的引用。</p>    <p>接下来就依次详细看看销毁对象的3个方法。</p>    <p><strong>1.object_cxxDestruct</strong></p>    <pre>  <code class="language-objectivec">void object_cxxDestruct(id obj)    {      if (!obj) return;      if (obj->isTaggedPointer()) return;      object_cxxDestructFromClass(obj, obj->ISA());  }    static void object_cxxDestructFromClass(id obj, Class cls)    {      void (*dtor)(id);        // Call cls's dtor first, then superclasses's dtors.        for ( ; cls; cls = cls->superclass) {          if (!cls->hasCxxDtor()) return;           dtor = (void(*)(id))              lookupMethodInClassAndLoadCache(cls, SEL_cxx_destruct);          if (dtor != (void(*)(id))_objc_msgForward_impcache) {              if (PrintCxxCtors) {                  _objc_inform("CXX: calling C++ destructors for class %s",                                cls->nameForLogging());              }              (*dtor)(obj);          }      }  }</code></pre>    <p>从子类开始沿着继承链一直找到父类,向上搜寻SEL_cxx_destruct 这个selector,找到函数实现(void (*)(id)(函数指针)并执行。</p>    <p>ARC actually creates a -.cxx_destruct method to handle freeing instance variables. This method was originally created for calling C++ destructors automatically when an object was destroyed.</p>    <p>和《Effective Objective-C 2.0》中提到的:</p>    <p>When the compiler saw that an object contained C++ objects, it would generate a method called .cxx_destruct. ARC piggybacks on this method and emits the required cleanup code within it.</p>    <p>可以了解到,.cxx_destruct方法原本是为了C++对象析构的,ARC借用了这个方法插入代码实现了自动内存释放的工作。</p>    <p>在ARC中dealloc方法在最后一次release后被调用,但此时实例变量(Ivars)并未释放, 父类的dealloc的方法将在子类dealloc方法返回后自动调用 。ARC下对象的实例变量在根类[NSObject dealloc]中释放(通常root class都是NSObject),变量释放顺序各种不确定(一个类内的不确定,子类和父类间也不确定,也就是说不用care释放顺序)</p>    <p>经过@sunnyxx文中的研究: 1.ARC下对象的成员变量于编译器插入的.cxx_desctruct方法自动释放。</p>    <p>2.ARC下[super dealloc]方法也由编译器自动插入。</p>    <p>至于.cxx_destruct方法的实现,还请看@sunnyxx 那篇文章里面详细的分析。</p>    <p><strong>2._object_remove_assocations</strong></p>    <pre>  <code class="language-objectivec">void _object_remove_assocations(id object) {        vector< ObjcAssociation,ObjcAllocator<ObjcAssociation> > elements;      {          AssociationsManager manager;          AssociationsHashMap &associations(manager.associations());          if (associations.size() == 0) return;          disguised_ptr_t disguised_object = DISGUISE(object);          AssociationsHashMap::iterator i = associations.find(disguised_object);          if (i != associations.end()) {              // copy all of the associations that need to be removed.              ObjectAssociationMap *refs = i->second;              for (ObjectAssociationMap::iterator j = refs->begin(), end = refs->end(); j != end; ++j) {                  elements.push_back(j->second);              }              // remove the secondary table.              delete refs;              associations.erase(i);          }      }      // the calls to releaseValue() happen outside of the lock.      for_each(elements.begin(), elements.end(), ReleaseValue());  }</code></pre>    <p>在移除关联对象object的时候,会先去判断object的isa_t中的第二位has_assoc的值,当object 存在并且object->hasAssociatedObjects( )值为1的时候,才会去调用_object_remove_assocations方法。</p>    <p>_object_remove_assocations方法的目的是删除第二张ObjcAssociationMap表,即删除所有的关联对象。删除第二张表,就需要在第一张AssociationsHashMap表中遍历查找。这里会把第二张ObjcAssociationMap表中所有的ObjcAssociation对象都存到一个数组elements里面,然后调用associations.erase( )删除第二张表。最后再遍历elements数组,把ObjcAssociation对象依次释放。</p>    <p>这里移除的方式和Associated Object关联对象里面的remove方法是完全一样的。</p>    <p><strong>3.clearDeallocating( )</strong></p>    <pre>  <code class="language-objectivec">inline void objc_object::clearDeallocating()    {      if (!isa.indexed) {          // Slow path for raw pointer isa.          sidetable_clearDeallocating();      }      else if (isa.weakly_referenced  ||  isa.has_sidetable_rc) {          // Slow path for non-pointer isa with weak refs and/or side table data.          clearDeallocating_slow();      }        assert(!sidetable_present());  }</code></pre>    <p>这里涉及到了2个clear函数,接下来一个个的看。</p>    <pre>  <code class="language-objectivec">void objc_object::sidetable_clearDeallocating()    {      SideTable& table = SideTables()[this];        // clear any weak table items      // clear extra retain count and deallocating bit      // (fixme warn or abort if extra retain count == 0 ?)      table.lock();      RefcountMap::iterator it = table.refcnts.find(this);      if (it != table.refcnts.end()) {          if (it->second & SIDE_TABLE_WEAKLY_REFERENCED) {              weak_clear_no_lock(&table.weak_table, (id)this);          }          table.refcnts.erase(it);      }      table.unlock();  }</code></pre>    <p>遍历SideTable,循环调用weak_clear_no_lock函数。</p>    <p>weakly <em>referenced代表对象被指向或者曾经指向一个 ARC 的弱变量。has</em> sidetable_rc判断该对象的引用计数是否过大。如果其中有一个为YES,则调用clearDeallocating_slow()方法。</p>    <pre>  <code class="language-objectivec">// Slow path of clearDeallocating()   // for objects with indexed isa  // that were ever weakly referenced   // or whose retain count ever overflowed to the side table.  NEVER_INLINE void objc_object::clearDeallocating_slow()    {      assert(isa.indexed  &&  (isa.weakly_referenced || isa.has_sidetable_rc));        SideTable& table = SideTables()[this];      table.lock();      if (isa.weakly_referenced) {          weak_clear_no_lock(&table.weak_table, (id)this);      }      if (isa.has_sidetable_rc) {          table.refcnts.erase(this);      }      table.unlock();  }</code></pre>    <p>clearDeallocating_slow也会最终调用weak_clear_no_lock方法。</p>    <pre>  <code class="language-objectivec">/**    * Called by dealloc; nils out all weak pointers that point to the    * provided object so that they can no longer be used.   *    * @param weak_table    * @param referent The object being deallocated.    */  void  weak_clear_no_lock(weak_table_t *weak_table, id referent_id)    {      objc_object *referent = (objc_object *)referent_id;        weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);      if (entry == nil) {          /// XXX shouldn't happen, but does with mismatched CF/objc          //printf("XXX no entry for clear deallocating %p\n", referent);          return;      }        // zero out references      weak_referrer_t *referrers;      size_t count;            if (entry->out_of_line) {          referrers = entry->referrers;          count = TABLE_SIZE(entry);      }       else {          referrers = entry->inline_referrers;          count = WEAK_INLINE_COUNT;      }            for (size_t i = 0; i < count; ++i) {          objc_object **referrer = referrers[i];          if (referrer) {              if (*referrer == referent) {                  *referrer = nil;              }              else if (*referrer) {                  _objc_inform("__weak variable at %p holds %p instead of %p. "                               "This is probably incorrect use of "                               "objc_storeWeak() and objc_loadWeak(). "                               "Break on objc_weak_error to debug.\n",                                referrer, (void*)*referrer, (void*)referent);                  objc_weak_error();              }          }      }            weak_entry_remove(weak_table, entry);  }</code></pre>    <p>这个函数会在weak_table中,清空引用计数表并清除弱引用表,将所有weak引用指nil。</p>    <h3><strong>总结</strong></h3>    <p><img src="https://simg.open-open.com/show/ef49527be503dac57e91cf6c79005fc1.jpg"></p>    <p>这篇文章详细的分析了objc对象 从 出生 到 最终销毁,它的今生今世全部在此。还请大家多多指点。</p>    <p> </p>    <p> </p>    <p>来自:http://www.halfrost.com/objc_life/</p>    <p> </p>