通过OC对象本质及isa结构分析我们已经知道，实例对象的isa指向类。我们也知道在OC中对象包括实例对象和类对象，即然是对象，那么类对象肯定也有isa指针，那类对象的isa指针指向谁呢？

isa指针指向

首先创建一个LGPerson类，并初始化一个对象

LGPerson *person = [LGPerson alloc];

然后lldb调试

1.x/4gx person 以十六进制打印四段person的内存分布情况

其中0x001d80010000220d就是0x001d80010000220d对象的isa指针地址

2.p/x 0x001d80010000220d & 0x00007ffffffffff8 以十六进制打印得到对象的isa的shiftcls,即类信息LGPerson的地址

这里我们可以知道，对象person的isa指针指向类LGPerson

3.指针地址0x0000000100002208就是LGPerson的首地址,即LGPerson类对象的isa指针，我们可以通过这个isa指针，打印出此isa指向什么

x/4gx 0x0000000100002208 打印LGPerson类对象的isa指针指向的内存情况

4.其中的0x00000001000021e0就是类对象LGPerson的isa指针指向，可以通过它来打印出，isa的确定指向

p/x 0x0000000100002208 & 0x00007ffffffffff8拿到 shiftcls并打印 po 0x0000000100002208

5.这里打印的居然又是LGPerson,什么情况呢？其实我们细节对比一下发现，上面那个LGPerson的isa地址是0x0000000100002208,下面这个LGPerson的isa地址是0x00000001000021e0.难道存在两个LGPerson。很明显，这是不可能的。那么后面这个LGPerson是什么呢？其实是LGPerson的元类。 这里我们验证了，类对象的isa指向元类

6.接着，我们继续打印LGPerson元类的isa指针指向的内存信息

这里打印的是NSObject,说明LGPerson元类的isa指向了NSObject

7.接着，我们再打印NSObjcet的isa指向

通过打印可以发现，NSObjcet的isa指向了NSObjcet,即根元类

通过上面的分析，我们可以总结isa的指向为：

对象的isa指向类；

类对象的isa指向类的元类；

元类的isa指向根元类；

根元类的isa指向自已.

即下面这张经典的图

从这张图我们可以知道：

类继承自父类，父类继承于NSObject,NSObject继承于nil;

isa指向为：对象->类对象->元类对象->根元类; 根元类的isa指向自己，且根元类继承自NSObject;

类的实例对象和父类的实例对象没有直接关系。

类的结构分析

我们知道，所有的类都继承自NSObject，即所有的类结构都是以NSObject为模板创建的，所以我们只需要研究NSObject的类结构即可。

首先打开一份OC底层原码，通过NSObject进去查看它的结构体的具体实现

NSObject的结构体只有一个Class类型的isa，于是我们就去查看Class的结构体 ,我们发现Class是objc_class类型的结构体指针.

objc_class结构体实现如下：

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() const {
        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);
    }

#if FAST_HAS_DEFAULT_RR
    bool hasCustomRR() const {
        return !bits.getBit(FAST_HAS_DEFAULT_RR);
    }
    void setHasDefaultRR() {
        bits.setBits(FAST_HAS_DEFAULT_RR);
    }
    void setHasCustomRR() {
        bits.clearBits(FAST_HAS_DEFAULT_RR);
    }
#else
    bool hasCustomRR() const {
        return !(bits.data()->flags & RW_HAS_DEFAULT_RR);
    }
    void setHasDefaultRR() {
        bits.data()->setFlags(RW_HAS_DEFAULT_RR);
    }
    void setHasCustomRR() {
        bits.data()->clearFlags(RW_HAS_DEFAULT_RR);
    }
#endif

#if FAST_CACHE_HAS_DEFAULT_AWZ
    bool hasCustomAWZ() const {
        return !cache.getBit(FAST_CACHE_HAS_DEFAULT_AWZ);
    }
    void setHasDefaultAWZ() {
        cache.setBit(FAST_CACHE_HAS_DEFAULT_AWZ);
    }
    void setHasCustomAWZ() {
        cache.clearBit(FAST_CACHE_HAS_DEFAULT_AWZ);
    }
#else
    bool hasCustomAWZ() const {
        return !(bits.data()->flags & RW_HAS_DEFAULT_AWZ);
    }
    void setHasDefaultAWZ() {
        bits.data()->setFlags(RW_HAS_DEFAULT_AWZ);
    }
    void setHasCustomAWZ() {
        bits.data()->clearFlags(RW_HAS_DEFAULT_AWZ);
    }
#endif

#if FAST_CACHE_HAS_DEFAULT_CORE
    bool hasCustomCore() const {
        return !cache.getBit(FAST_CACHE_HAS_DEFAULT_CORE);
    }
    void setHasDefaultCore() {
        return cache.setBit(FAST_CACHE_HAS_DEFAULT_CORE);
    }
    void setHasCustomCore() {
        return cache.clearBit(FAST_CACHE_HAS_DEFAULT_CORE);
    }
#else
    bool hasCustomCore() const {
        return !(bits.data()->flags & RW_HAS_DEFAULT_CORE);
    }
    void setHasDefaultCore() {
        bits.data()->setFlags(RW_HAS_DEFAULT_CORE);
    }
    void setHasCustomCore() {
        bits.data()->clearFlags(RW_HAS_DEFAULT_CORE);
    }
#endif

#if FAST_CACHE_HAS_CXX_CTOR
    bool hasCxxCtor() {
        ASSERT(isRealized());
        return cache.getBit(FAST_CACHE_HAS_CXX_CTOR);
    }
    void setHasCxxCtor() {
        cache.setBit(FAST_CACHE_HAS_CXX_CTOR);
    }
#else
    bool hasCxxCtor() {
        ASSERT(isRealized());
        return bits.data()->flags & RW_HAS_CXX_CTOR;
    }
    void setHasCxxCtor() {
        bits.data()->setFlags(RW_HAS_CXX_CTOR);
    }
#endif

#if FAST_CACHE_HAS_CXX_DTOR
    bool hasCxxDtor() {
        ASSERT(isRealized());
        return cache.getBit(FAST_CACHE_HAS_CXX_DTOR);
    }
    void setHasCxxDtor() {
        cache.setBit(FAST_CACHE_HAS_CXX_DTOR);
    }
#else
    bool hasCxxDtor() {
        ASSERT(isRealized());
        return bits.data()->flags & RW_HAS_CXX_DTOR;
    }
    void setHasCxxDtor() {
        bits.data()->setFlags(RW_HAS_CXX_DTOR);
    }
#endif

#if FAST_CACHE_REQUIRES_RAW_ISA
    bool instancesRequireRawIsa() {
        return cache.getBit(FAST_CACHE_REQUIRES_RAW_ISA);
    }
    void setInstancesRequireRawIsa() {
        cache.setBit(FAST_CACHE_REQUIRES_RAW_ISA);
    }
#elif SUPPORT_NONPOINTER_ISA
    bool instancesRequireRawIsa() {
        return bits.data()->flags & RW_REQUIRES_RAW_ISA;
    }
    void setInstancesRequireRawIsa() {
        bits.data()->setFlags(RW_REQUIRES_RAW_ISA);
    }
#else
    bool instancesRequireRawIsa() {
        return true;
    }
    void setInstancesRequireRawIsa() {
        // nothing
    }
#endif
    void setInstancesRequireRawIsaRecursively(bool inherited = false);
    void printInstancesRequireRawIsa(bool inherited);

    bool canAllocNonpointer() {
        ASSERT(!isFuture());
        return !instancesRequireRawIsa();
    }

    bool isSwiftStable() {
        return bits.isSwiftStable();
    }

    bool isSwiftLegacy() {
        return bits.isSwiftLegacy();
    }

    bool isAnySwift() {
        return bits.isAnySwift();
    }

    bool isSwiftStable_ButAllowLegacyForNow() {
        return bits.isSwiftStable_ButAllowLegacyForNow();
    }

    bool isStubClass() const {
        uintptr_t isa = (uintptr_t)isaBits();
        return 1 <= isa && isa < 16;
    }

    // Swift stable ABI built for old deployment targets looks weird.
    // The is-legacy bit is set for compatibility with old libobjc.
    // We are on a "new" deployment target so we need to rewrite that bit.
    // These stable-with-legacy-bit classes are distinguished from real
    // legacy classes using another bit in the Swift data
    // (ClassFlags::IsSwiftPreStableABI)

    bool isUnfixedBackwardDeployingStableSwift() {
        // Only classes marked as Swift legacy need apply.
        if (!bits.isSwiftLegacy()) return false;

        // Check the true legacy vs stable distinguisher.
        // The low bit of Swift's ClassFlags is SET for true legacy
        // and UNSET for stable pretending to be legacy.
        uint32_t swiftClassFlags = *(uint32_t *)(&bits + 1);
        bool isActuallySwiftLegacy = bool(swiftClassFlags & 1);
        return !isActuallySwiftLegacy;
    }

    void fixupBackwardDeployingStableSwift() {
        if (isUnfixedBackwardDeployingStableSwift()) {
            // Class really is stable Swift, pretending to be pre-stable.
            // Fix its lie.
            bits.setIsSwiftStable();
        }
    }

    _objc_swiftMetadataInitializer swiftMetadataInitializer() {
        return bits.swiftMetadataInitializer();
    }

    // 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;
    }


    bool forbidsAssociatedObjects() {
        return (data()->flags & RW_FORBIDS_ASSOCIATED_OBJECTS);
    }

#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() const {
        return !isStubClass() && (data()->flags & RW_REALIZED);
    }

    // Returns true if this is an unrealized future class.
    // Locking: To prevent concurrent realization, hold runtimeLock.
    bool isFuture() const {
        return data()->flags & RW_FUTURE;
    }

    bool isMetaClass() {
        ASSERT(this);
        ASSERT(isRealized());
#if FAST_CACHE_META
        return cache.getBit(FAST_CACHE_META);
#else
        return data()->flags & RW_META;
#endif
    }

    // Like isMetaClass, but also valid on un-realized classes
    bool isMetaClassMaybeUnrealized() {
        static_assert(offsetof(class_rw_t, flags) == offsetof(class_ro_t, flags), "flags alias");
        static_assert(RO_META == RW_META, "flags alias");
        return data()->flags & RW_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 needsLock);
    const char *nameForLogging();

    // May be unaligned depending on class's ivars.
    uint32_t unalignedInstanceStart() const {
        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() const {
        return word_align(unalignedInstanceStart());
    }

    // May be unaligned depending on class's ivars.
    uint32_t unalignedInstanceSize() const {
        ASSERT(isRealized());
        return data()->ro()->instanceSize;
    }

    // Class's ivar size rounded up to a pointer-size boundary.
    uint32_t alignedInstanceSize() const {
        return word_align(unalignedInstanceSize());
    }

    size_t instanceSize(size_t extraBytes) const {
        if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
            return cache.fastInstanceSize(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());
        ASSERT(data()->flags & RW_REALIZING);
        auto ro = data()->ro();
        if (newSize != ro->instanceSize) {
            ASSERT(data()->flags & RW_COPIED_RO);
            *const_cast<uint32_t *>(&ro->instanceSize) = newSize;
        }
        cache.setFastInstanceSize(newSize);
    }

    void chooseClassArrayIndex();

    void setClassArrayIndex(unsigned Idx) {
        bits.setClassArrayIndex(Idx);
    }

    unsigned classArrayIndex() {
        return bits.classArrayIndex();
    }
}

可以看到objc_class包含了大量的信息，我们先只看最前面的部分

首先我们来分析一下：

objc_class继承于objc_object;

// Class ISA;告诉我们，objc_class的第一个元素是isa指针，这个isa指针来自objc_object的继承;

cache_t cache 先不用管；

class_data_bits_t bits，类的信息都存在这里面，包含方法列表、属性列表、协议列表等等

所以我们要想获取类的实际信息，只需要解析class_data_bits_t bits就可以了

在解析class_data_bits_t bits之前，我们先来了解一下指针偏移

执行以下代码

int arr[4] = {1,2,3,4};
NSLog(@"%p\n%p\n%p",arr,&arr[0],&arr[1]);
for (int i=0; i<4; i++) {
    int a = arr[i];
    NSLog(@"a ==== %d",a);
    int a1 = *(arr + i);
    NSLog(@"a1 ==== %d",a1);
}

打印结果如下

我们知道，数据的内存是连续的，即arr中的1、2、3、4在内存中是连续存储的，数据的首地址即为每0个元素的地址，数组的第1个元素的地址为数组的首地址偏移元素类型的长度，所以我们才能通过偏移，以int a1 = *(arr + i);的方式拿到数组中的元素.

同样的，结构体的内存也是连接的，我们同样可以通过指针偏移，拿到objc_class结构体的class_data_bits_t bits的内容

首先分析一下，要从objc_class结构体的首地址偏移多少，才能拿到class_data_bits_t bits的首地址

1.isa指针点8个字节；

2.Class superclass也是指针，也占8个字节；

3.cache_t cache 占多少字节呢？下面我们来分析一下

cache_t结构体结构如下：

struct cache_t {
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_OUTLINED
    explicit_atomic<struct bucket_t *> _buckets;
    explicit_atomic<mask_t> _mask;
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
    explicit_atomic<uintptr_t> _maskAndBuckets;
    mask_t _mask_unused;
    
    // How much the mask is shifted by.
    static constexpr uintptr_t maskShift = 48;
    
    // Additional bits after the mask which must be zero. msgSend
    // takes advantage of these additional bits to construct the value
    // `mask << 4` from `_maskAndBuckets` in a single instruction.
    static constexpr uintptr_t maskZeroBits = 4;
    
    // The largest mask value we can store.
    static constexpr uintptr_t maxMask = ((uintptr_t)1 << (64 - maskShift)) - 1;
    
    // The mask applied to `_maskAndBuckets` to retrieve the buckets pointer.
    static constexpr uintptr_t bucketsMask = ((uintptr_t)1 << (maskShift - maskZeroBits)) - 1;
    
    // Ensure we have enough bits for the buckets pointer.
    static_assert(bucketsMask >= MACH_VM_MAX_ADDRESS, "Bucket field doesn't have enough bits for arbitrary pointers.");
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
    // _maskAndBuckets stores the mask shift in the low 4 bits, and
    // the buckets pointer in the remainder of the value. The mask
    // shift is the value where (0xffff >> shift) produces the correct
    // mask. This is equal to 16 - log2(cache_size).
    explicit_atomic<uintptr_t> _maskAndBuckets;
    mask_t _mask_unused;

    static constexpr uintptr_t maskBits = 4;
    static constexpr uintptr_t maskMask = (1 << maskBits) - 1;
    static constexpr uintptr_t bucketsMask = ~maskMask;
#else
#error Unknown cache mask storage type.
#endif
    
#if __LP64__
    uint16_t _flags;
#endif
    uint16_t _occupied;

public:
    static bucket_t *emptyBuckets();
    
    struct bucket_t *buckets();
    mask_t mask();
    mask_t occupied();
    void incrementOccupied();
    void setBucketsAndMask(struct bucket_t *newBuckets, mask_t newMask);
    void initializeToEmpty();

    unsigned capacity();
    bool isConstantEmptyCache();
    bool canBeFreed();

#if __LP64__
    bool getBit(uint16_t flags) const {
        return _flags & flags;
    }
    void setBit(uint16_t set) {
        __c11_atomic_fetch_or((_Atomic(uint16_t) *)&_flags, set, __ATOMIC_RELAXED);
    }
    void clearBit(uint16_t clear) {
        __c11_atomic_fetch_and((_Atomic(uint16_t) *)&_flags, ~clear, __ATOMIC_RELAXED);
    }
#endif

#if FAST_CACHE_ALLOC_MASK
    bool hasFastInstanceSize(size_t extra) const
    {
        if (__builtin_constant_p(extra) && extra == 0) {
            return _flags & FAST_CACHE_ALLOC_MASK16;
        }
        return _flags & FAST_CACHE_ALLOC_MASK;
    }

    size_t fastInstanceSize(size_t extra) const
    {
        ASSERT(hasFastInstanceSize(extra));

        if (__builtin_constant_p(extra) && extra == 0) {
            return _flags & FAST_CACHE_ALLOC_MASK16;
        } else {
            size_t size = _flags & FAST_CACHE_ALLOC_MASK;
            // remove the FAST_CACHE_ALLOC_DELTA16 that was added
            // by setFastInstanceSize
            return align16(size + extra - FAST_CACHE_ALLOC_DELTA16);
        }
    }

    void setFastInstanceSize(size_t newSize)
    {
        // Set during realization or construction only. No locking needed.
        uint16_t newBits = _flags & ~FAST_CACHE_ALLOC_MASK;
        uint16_t sizeBits;

        // Adding FAST_CACHE_ALLOC_DELTA16 allows for FAST_CACHE_ALLOC_MASK16
        // to yield the proper 16byte aligned allocation size with a single mask
        sizeBits = word_align(newSize) + FAST_CACHE_ALLOC_DELTA16;
        sizeBits &= FAST_CACHE_ALLOC_MASK;
        if (newSize <= sizeBits) {
            newBits |= sizeBits;
        }
        _flags = newBits;
    }
#else
    bool hasFastInstanceSize(size_t extra) const {
        return false;
    }
    size_t fastInstanceSize(size_t extra) const {
        abort();
    }
    void setFastInstanceSize(size_t extra) {
        // nothing
    }
#endif

    static size_t bytesForCapacity(uint32_t cap);
    static struct bucket_t * endMarker(struct bucket_t *b, uint32_t cap);

    void reallocate(mask_t oldCapacity, mask_t newCapacity, bool freeOld);
    void insert(Class cls, SEL sel, IMP imp, id receiver);

    static void bad_cache(id receiver, SEL sel, Class isa) __attribute__((noreturn, cold));
}

通过分析我们发现，cache_t的有效元素只有四个,static修饰的内容存在静态存储区，不占用结构体的内存大小

explicit_atomic<struct bucket_t *> _buckets;占8个字节;

explicit_atomic<mask_t> _mask;即为typedef uint32_t mask_t;点4个字节;

uint16_t _flags;占2个字节；

uint16_t _occupied;点2个字节

或者

explicit_atomic<uintptr_t> _maskAndBuckets;uintptr_t即为typedef unsigned long uintptr_t;无符号long类型,占8个字节;

mask_t _mask_unused;即为typedef uint32_t mask_t;点4个字节;

uint16_t _flags;占2个字节；

uint16_t _occupied;点2个字节

由此得出cache_t所占的字节数是8+4+2+2=16个字节

从而得到从objc_class的首地址偏移到class_data_bits_t需要偏移32个字节

下面我们通过lldb调试来查看LGPerson的class_data_bits_t的信息

首先，打开OC底层源码781，创建LGPerson类,添加一个NSString类型的name属性列表和一个对象方法，并运行，断点

1.x/4gx LGPerson.class打印类信息,获取类的首地址

2.首地址偏移32位，得到class_data_bits_t的指针地址，并打印p (class_data_bits_t *)0x7fff93cdc128

3.打印class_data_bits_t的class_rw_t，得到类信息的存储地址

4.我们来查看一下class_rw_t的结构

struct class_rw_t {
    // Be warned that Symbolication knows the layout of this structure.
    uint32_t flags;
    uint16_t witness;
#if SUPPORT_INDEXED_ISA
    uint16_t index;
#endif

    explicit_atomic<uintptr_t> ro_or_rw_ext;

    Class firstSubclass;
    Class nextSiblingClass;

private:
    using ro_or_rw_ext_t = objc::PointerUnion<const class_ro_t *, class_rw_ext_t *>;

    const ro_or_rw_ext_t get_ro_or_rwe() const {
        return ro_or_rw_ext_t{ro_or_rw_ext};
    }

    void set_ro_or_rwe(const class_ro_t *ro) {
        ro_or_rw_ext_t{ro}.storeAt(ro_or_rw_ext, memory_order_relaxed);
    }

    void set_ro_or_rwe(class_rw_ext_t *rwe, const class_ro_t *ro) {
        // the release barrier is so that the class_rw_ext_t::ro initialization
        // is visible to lockless readers
        rwe->ro = ro;
        ro_or_rw_ext_t{rwe}.storeAt(ro_or_rw_ext, memory_order_release);
    }

    class_rw_ext_t *extAlloc(const class_ro_t *ro, bool deep = false);

public:
    void setFlags(uint32_t set)
    {
        __c11_atomic_fetch_or((_Atomic(uint32_t) *)&flags, set, __ATOMIC_RELAXED);
    }

    void clearFlags(uint32_t clear) 
    {
        __c11_atomic_fetch_and((_Atomic(uint32_t) *)&flags, ~clear, __ATOMIC_RELAXED);
    }

    // set and clear must not overlap
    void changeFlags(uint32_t set, uint32_t clear) 
    {
        ASSERT((set & clear) == 0);

        uint32_t oldf, newf;
        do {
            oldf = flags;
            newf = (oldf | set) & ~clear;
        } while (!OSAtomicCompareAndSwap32Barrier(oldf, newf, (volatile int32_t *)&flags));
    }

    class_rw_ext_t *ext() const {
        return get_ro_or_rwe().dyn_cast<class_rw_ext_t *>();
    }

    class_rw_ext_t *extAllocIfNeeded() {
        auto v = get_ro_or_rwe();
        if (fastpath(v.is<class_rw_ext_t *>())) {
            return v.get<class_rw_ext_t *>();
        } else {
            return extAlloc(v.get<const class_ro_t *>());
        }
    }

    class_rw_ext_t *deepCopy(const class_ro_t *ro) {
        return extAlloc(ro, true);
    }

    const class_ro_t *ro() const {
        auto v = get_ro_or_rwe();
        if (slowpath(v.is<class_rw_ext_t *>())) {
            return v.get<class_rw_ext_t *>()->ro;
        }
        return v.get<const class_ro_t *>();
    }

    void set_ro(const class_ro_t *ro) {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            v.get<class_rw_ext_t *>()->ro = ro;
        } else {
            set_ro_or_rwe(ro);
        }
    }

    const method_array_t methods() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>()->methods;
        } else {
            return method_array_t{v.get<const class_ro_t *>()->baseMethods()};
        }
    }

    const property_array_t properties() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>()->properties;
        } else {
            return property_array_t{v.get<const class_ro_t *>()->baseProperties};
        }
    }

    const protocol_array_t protocols() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>()->protocols;
        } else {
            return protocol_array_t{v.get<const class_ro_t *>()->baseProtocols};
        }
    }
}

我们发现

method_array_t methods() 函数就是方法列表；

property_array_t properties()函数就是属性列表；

protocol_array_t protocols()函数就是协议列表；

5.打印class_rw_t 的数据

6.properties()打印如下：

name不就是LGPerson的属性嘛

7.methods()打印如下：

"sayNB"不就是LGPerson的对象方法嘛

总结通过以上分析我们知道，OC的类都继承自NSObject,而NSObject的底层是objc_classs结构体，且objc_classs继承自objc_object,objc_object只有一个Class类型的isa。由此解释了为什么所有的对象、类和元类都有isa指针。通过类的结构的分析我们也验证了类的属性、方法和协议都在类信息中。

isa指针指向和类的结构分析

isa指针指向

首先创建一个LGPerson类，并初始化一个对象

然后lldb调试

类的结构分析