libobjc系列1 - 类的结构

做iOS开发也挺久了，对语言本身的需要好好总结一下。 就基于objc4-723源码(苹果源码 )做个笔记。

类：

从public header来看以下的关系

id && objc_object

在<objc.h>可以看到如下定义

/// A pointer to an instance of a class.
typedef struct objc_object *id;

可以看出来id类型实际对应objc_object结构体指针，

/// Represents an instance of a class.
struct objc_object {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
};

对外开放的唯一属性是Class类型的isa。

Class && objc_class

/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

Class类型即objc_class结构体指针类型。

相关实现

其实上面的代码被

#if !OBJC_TYPES_DEFINED
我是代码
#endif

包裹。 所以上面这些定义是没用的。不过还是可以作为参考。

在<objc-private.h>中，也有一样的定义

typedef struct objc_class *Class;
typedef struct objc_object *id;

先看 objc_object的实现

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
};

上面的代码，其实是一大段函数的声明，所以我们是看不出来objc_object到底是什么结构的。
所以，我们换个角度这个问题，oc对象的初始化时二段式构造，由此来看它的alloc方法。

+ (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];
}

一个有意思的地方 fastpath和slowpath，他们是实际使用GCC 提供了内建函数

long __builtin_expect(long exp, long c);

来帮助程序员处理分支预测，这里slowpath指小概率进入分支，fastpath指大概率进入。
在<objc-runtime-new.h>中

#if !__LP64__

...

#elif 1

...

#else
// Leaks-incompatible version that steals lots of bits.

// class is a Swift class
#define FAST_IS_SWIFT           (1UL<<0)
// class's instances requires raw isa
#define FAST_REQUIRES_RAW_ISA   (1UL<<1)
// class or superclass has .cxx_destruct implementation
//   This bit is aligned with isa_t->hasCxxDtor to save an instruction.
#define FAST_HAS_CXX_DTOR       (1UL<<2)
// data pointer
#define FAST_DATA_MASK          0x00007ffffffffff8UL
// class or superclass has .cxx_construct implementation
#define FAST_HAS_CXX_CTOR       (1UL<<47)
// class or superclass has default alloc/allocWithZone: implementation
// Note this is is stored in the metaclass.
#define FAST_HAS_DEFAULT_AWZ    (1UL<<48)
// class or superclass has default retain/release/autorelease/retainCount/
//   _tryRetain/_isDeallocating/retainWeakReference/allowsWeakReference
#define FAST_HAS_DEFAULT_RR     (1UL<<49)
// summary bit for fast alloc path: !hasCxxCtor and 
//   !instancesRequireRawIsa and instanceSize fits into shiftedSize
#define FAST_ALLOC              (1UL<<50)
// instance size in units of 16 bytes
//   or 0 if the instance size is too big in this field
//   This field must be LAST
#define FAST_SHIFTED_SIZE_SHIFT 51

// FAST_ALLOC means
//   FAST_HAS_CXX_CTOR is set
//   FAST_REQUIRES_RAW_ISA is not set
//   FAST_SHIFTED_SIZE is not zero
// FAST_ALLOC does NOT check FAST_HAS_DEFAULT_AWZ because that 
// bit is stored on the metaclass.
#define FAST_ALLOC_MASK  (FAST_HAS_CXX_CTOR | FAST_REQUIRES_RAW_ISA)
#define FAST_ALLOC_VALUE (0)

#endif

这里有个#elif 1，所以现在其实FAST_ALLOC 是没有定义的，所以一定会调用class_createInstance创建对象。

所以最后基本就走到了

id obj = class_createInstance(cls, 0);