通过将OC编译成C++ 一探究竟（边学编写，帮忙点评）

只说重点

找到类得信息，如下：

struct _class_t {struct _class_t *isa;struct _class_t *superclass;void *cache;void *vtable;struct _class_ro_t *ro;
};struct _class_ro_t {unsigned int flags;unsigned int instanceStart;unsigned int instanceSize;const unsigned char *ivarLayout;const char *name;const struct _method_list_t *baseMethods;const struct _objc_protocol_list *baseProtocols;const struct _ivar_list_t *ivars;const unsigned char *weakIvarLayout;const struct _prop_list_t *properties;
};

_class_ro_t是类得其他信息，在里面我们可以找到我们平时用到得一些东西，如类名，方法列表、属性列表、继承的协议列表等

ro表示read only的意思，即只读，包含名称，方法，协议以及实例变量信息，因为是只读的，所以也就是WWDC说的clean memory，而clean memory在加载到内存后是不会发生改变的。

分类

分类信息如下：

struct _category_t {const char *name;struct _class_t *cls;const struct _method_list_t *instance_methods;const struct _method_list_t *class_methods;const struct _protocol_list_t *protocols;const struct _prop_list_t *properties;
};

上面得分类信息，也就是我们在写分类得时候可以添加什么，实例方法、类方法、协议、属性，成员变量是不能添加得，因为category得底层数据结构根本不支持，当然不排除你用别的方式来实现，实现可以参考这个文章：iOS-Category添加成员变量 Objective-C如何在category里加入成员变量

以下是本人实现得过程，仅做参考：

#import "Person.h"NS_ASSUME_NONNULL_BEGIN@interface Person (Category)@property (nonatomic, copy) NSString *address;
@property (nonatomic, copy) NSString *addressName;
@property (nonatomic, copy) NSString *bigoo;- (void) eat;
@endNS_ASSUME_NONNULL_END#import "Person+Category.h"
#import <objc/runtime.h>static void *strKey = &strKey;
static const char *kFriendsPropertyKey = "kFriendsPropertyKey";
static NSString *addressName_;
//
NSMutableDictionary *bigoos_;@implementation Person (Category)+ (void)load {bigoos_ = [NSMutableDictionary dictionary];
}- (void)eat {NSLog(@"I eat");
}//
- (NSString *)address {return objc_getAssociatedObject(self, kFriendsPropertyKey);
}- (void)setAddress:(NSString *)address {objc_setAssociatedObject(self, kFriendsPropertyKey, address, OBJC_ASSOCIATION_COPY_NONATOMIC);
}//
- (void)setAddressName:(NSString *)addressName {addressName_ = addressName;
}- (NSString *) addressName {return addressName_;
}//
- (void)setBigoo:(NSString *)bigoo {bigoos_[[NSString stringWithFormat:@"%p", self]] = bigoo;
}-(NSString *)bigoo {return bigoos_[[NSString stringWithFormat:@"%p", self]];
}
@end

上面几种方法可以试试。

通过这两个地方我们再来理理objc源码里面是怎么写得，参考如下：

struct objc_class {Class _Nonnull isa  OBJC_ISA_AVAILABILITY;#if !__OBJC2__Class _Nullable super_class                              OBJC2_UNAVAILABLE;const char * _Nonnull name                               OBJC2_UNAVAILABLE;long version                                             OBJC2_UNAVAILABLE;long info                                                OBJC2_UNAVAILABLE;long instance_size                                       OBJC2_UNAVAILABLE;struct objc_ivar_list * _Nullable ivars                  OBJC2_UNAVAILABLE;struct objc_method_list * _Nullable * _Nullable methodLists                    OBJC2_UNAVAILABLE;struct objc_cache * _Nonnull cache                       OBJC2_UNAVAILABLE;struct objc_protocol_list * _Nullable protocols          OBJC2_UNAVAILABLE;
#endif} OBJC2_UNAVAILABLE;

上面得#if !__OBJC2__告诉我们是objc2的话，我们不能用？？？

好吧，我找到得是以下内容：

struct objc_class : objc_object {// Class ISA;Class superclass;cache_t cache;             // formerly cache pointer and vtableclass_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flagsclass_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 overlapvoid 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 isSwiftStable() {return bits.isSwiftStable();}bool isSwiftLegacy() {return bits.isSwiftLegacy();}bool isAnySwift() {return bits.isAnySwift();}// 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
#elsebool instancesHaveAssociatedObjects() {// this may be an unrealized future class in the CF-bridged caseassert(isFuture()  ||  isRealized());return data()->flags & RW_INSTANCES_HAVE_ASSOCIATED_OBJECTS;}void setInstancesHaveAssociatedObjects() {// this may be an unrealized future class in the CF-bridged caseassert(isFuture()  ||  isRealized());setInfo(RW_INSTANCES_HAVE_ASSOCIATED_OBJECTS);}
#endifbool 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 metaclassClass 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 hereassert(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<uint32_t *>(&data()->ro->instanceSize) = newSize;}bits.setFastInstanceSize(newSize);}void chooseClassArrayIndex();void setClassArrayIndex(unsigned Idx) {bits.setClassArrayIndex(Idx);}unsigned classArrayIndex() {return bits.classArrayIndex();}};

不能向编译后得到的类中增加实例变量：因为编译后的类已经注册在 runtime 中,类结构体中的 objc_ivar_list 实例变量的链表和 instance_size 实例变量的内存大小已经确定，同时runtime会调用 class_setvarlayout 或 class_setWeaklvarLayout 来处理strong weak 引用.所以不能向存在的类中添加实例变量，可以看看文章开头编译成C++ 得到得类的实际情况。

能向运行时创建的类中添加实例变量：运行时创建的类是可以添加实例变量，调用class_addIvar函数. 但是的在调用 objc_allocateClassPair 之后，objc_registerClassPair 之前,原因同上

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通过将OC编译成C++ 一探究竟（边学编写，帮忙点评）

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