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1、在实际的运用中我们也会用到如下的类关系：

class B；class A{int i;B  *lpb;}class B{int i;A* lpa;}

注意：一般来说，两者的定义，至少有一方是使用指针，或两者都使用指针，但是决不能两者都定义实体对象。

这样的实现，通俗地来说是 “你中有我，我中有你 ”，那么在具体运用时，我们可以将两个类所实现的含义联系起来，比如我们说A类的对象为一个一个的请求，B类对象表示会话（会话有相应的操作接口），此时只要A类和B类拥有对方的对象指针，如上代码所示，我们就可以将一个请求和一个会话，一一对应起来，当我们得到一个请求后，我们也就能得到这个请求对应的会话操作。

2、测试用例

class EventDownloadSession;class OpenModelEventDownload      //该类我们可以提供给外部使用{public:	OpenModelEventDownload(): bSynchronization(true)       {                   }	virtual ~OpenModelEventDownload()        {        }	virtual void dealloc()        {             delete  this;        }	static OpenModelEventDownload * alloc()        {            OpenModelEventDownload * lpValue = new OpenModelEventDownload();		lpValue->AddRef();		return lpValue;       }	inline void setFilePath(const char * value)	{		filePath = value;	}	inline const char * getFilePath()	{		return filePath.c_str();	}	inline void setFileSize(size_t value)	{		fileSize = value;	}	inline size_t getFileSize()	{		return fileSize;	}	inline void setFileSeek(size_t value)	{		fileSeek = value;	}	inline size_t getFileSeek()	{		return fileSeek;	}	inline void setDownloadUrl(const char * value)	{		downloadUrl = value;	}	inline const char * getDownloadUrl()	{		return downloadUrl.c_str();	}	inline void setDownloadProcess(int32_t value)	{		downloadProcess = value;	}	inline int32_t getDownloadProcess()	{		return downloadProcess;	}	inline void setSynchronization(bool value)	{		bSynchronization = value;	}	inline bool getSynchronization()	{		return bSynchronization;	}//注意如下的两个接口	inline OpenCloud::EventDownloadSession * getEventDownloadSession()	{		return downloadSession;	}	inline void setEventDownloadSession(OpenCloud::EventDownloadSession *lpSession)	{		downloadSession = lpSession;	}private:	std::string   filePath;	size_t        fileSize;	size_t        fileSeek;	bool          bSynchronization;		std::string            downloadUrl;	int32_t                downloadProcess; //[0,100]	EventDownloadSession * downloadSession;     //下一个对象指针};//基类class  EventDownloadSession       {public:	EventDownloadSession(OpenCloud::OpenModelEventDownload *lpOpenModelEventDownload)		:kOpenModelEventDownload(lpOpenModelEventDownload)	{}	virtual ~EventDownloadSession()	{}	virtual int stopSession() = 0;public:	OpenCloud::OpenModelEventDownload *kOpenModelEventDownload;    //上一个类的对象指针};//派生类class EventOperationDownloadSession : public EventDownloadSession{public:	EventOperationDownloadSession(OpenCloud::OpenModelEventDownload *lpOpenModelEventDownload):EventDownloadSession(lpOpenModelEventDownload)	{			}	virtual ~EventOperationDownloadSession()	{}	virtual void dealloc(void * ref)	{		delete this;	}	static EventOperationDownloadSession * alloc(OpenCloud::OpenModelEventDownload *lpOpenModelEventDownload)	{		EventOperationDownloadSession *lpEventOperationDownloadSession = new EventOperationDownloadSession(lpOpenModelEventDownload);		return lpEventOperationDownloadSession;	}	virtual int Init(void * conf)	{             //do something	     return 0;	}	virtual int Close()	{            //do something             return 0;	}	virtual int stopSession()	{              //do something	     return 0;	}public:	};int main(){     int iSsuccess  =0;     OpenModelEventDownload *lpOpenModelEventDownload = OpenModelEventDownload::alloc();     lpOpenModelEventDownload->setFilePath("/tmp/123456.mp4");          EventOperationDownloadSession *lpEventOperationDownloadSession =  EventOperationDownloadSession::alloc(lpOpenModelEventDownload );	download->setEventDownloadSession(lpEventOperationDownloadSession);    iSsuccess = lpEventOperationDownloadSession->Init(nullptr);       //do something  return 0;}

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