大话设计模式C++版——代理模式

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mb5fdb1365b75a0 2017-05-08 16:12:00

文章标签 mysql 数据库代理模式设计模式代理类 文章分类 C/C++ 后端开发

代理模式是一种比較简单但却有用的设计模式。他能够灵活的更换代理的对象，但保证功能的完整性。就如卖衣服的代理商，他能够代理美特斯邦威的衣服，假设美特斯邦威的衣服被大家吐槽不好卖了，他还能够换去代理卖佐丹奴的，但无论怎么更换，还是能满足大家的需求——买衣服。

以下以大话设计模式书中的样例为例，设计一个代理帮小明送花给小红。

1、根据接口编程，设计代理对象的接口

class IPursuit
{
public:
  virtual ~IPursuit() {}

  virtual void  SendFlowers() = 0;
};

2、代理类。也继承代理对象类，保持接口一致

class CProxy : public IPursuit
{
public:
  CProxy() : m_poIPursuit(NULL) {}
  ~CProxy()
  {
    if (m_poIPursuit)
    {
      delete  m_poIPursuit;
      m_poIPursuit = NULL;
    }
  }
  
  void  SetPursuit(IPursuit* poIPursuit)
  {
    //假设有旧的代理，要先删除，否则会造成内存泄漏
    if (m_poIPursuit)
    {
      delete  m_poIPursuit;
    }

    m_poIPursuit = poIPursuit;
  }

  void  SendFlowers()
  {
    if (m_poIPursuit)
    {
      printf("Proxy help ");
      m_poIPursuit->SendFlowers();
    }
  }

private:
  IPursuit* m_poIPursuit;
};

代理类实际上啥也没干，仅仅是对相同的函数调用了一手被代理的对象的相应函数，当了一回二传手的角色。这里要注意代理对象因为会在代理中被释放，所以代理的对象一律必须是new出来的。即需在堆上创建的。

3、被代理对象类

class CPursuit : public IPursuit
{
public:
  CPursuit(TString tstrName) : m_tstrName(tstrName) {}
  ~CPursuit() {}

  void  SendFlowers()
  {
    _tprintf(_T("%s sent flowers to Xiaohong\n"), m_tstrName.c_str());
  }

private:
  TString m_tstrName;
};

另附上TString宏

#ifdef  UNICODE
  #define TString std::wstring
#else
  #define TString std::string
#endif

4、測试演示样例

void  Test()
{
  IPursuit* poIXiaoMing = new CPursuit(_T("XiaoMing"));
  CProxy    oCProxy;
  
  oCProxy.SetPursuit(poIXiaoMing);
  oCProxy.SendFlowers();
}

5、代理类的应用

这个样例非常形象，但却非常难看出代理模式的应用和长处。实际上在《大话设计模式C++版——抽象工厂模式》中有一个操作数据库管理员工信息的样例，因为可能会在使用数据库的过程中切换数据库。如曾经用的MySql,可能某个客户要求支持Access。这时就得进行切换了。此时用代理模式一样能够实现。

5.1 代理模式实现员工数据库管理类对数据库的切换

typedef struct Employee 
{
  int   nID;
  TString tstrName;
};

class IEmployee
{
public:
  ~IEmployee()  {}
  
  virtual bool    InserttoDB(Employee& stEmployee) = 0;
  virtual Employee  GetEmployee(int nID) = 0;
};

class CProxy : public IEmployee
{
public:
public:
  CProxy() : m_poIEmployee(NULL)  {}
  ~CProxy()
  {
    if (m_poIEmployee)
    {
      delete  m_poIEmployee;
      m_poIEmployee = NULL;
    }
  }
  
  void    SetEmployee(IEmployee* poIEmployee)
  {
    if (m_poIEmployee)
    {
      delete  m_poIEmployee;
    }

    m_poIEmployee = poIEmployee;
  }

  bool    InserttoDB(Employee& stEmployee)
  {
    if (m_poIEmployee)
    {
      return  m_poIEmployee->InserttoDB(stEmployee);
    }
    
    return  false;
  }

  Employee  GetEmployee(int nID)
  {
    if (m_poIEmployee)
    {
      return  m_poIEmployee->GetEmployee(nID);
    }
    
    Employee  stEmployee;
    return  stEmployee;
  }

private:
  IEmployee*  m_poIEmployee;
};

class CEmployeefromMysql : public IEmployee
{
public:
  bool    InserttoDB(Employee& stEmployee)
  {
    _tprintf(_T("Insert employee %s into mysql\n"), stEmployee.tstrName.c_str());
    return  true;
  }
  
  Employee  GetEmployee(int nID)
  {
    Employee  stEmployee;
    printf("Get an employee from mysql by id %d\n", nID);
    return  stEmployee;
  }
};

class CEmployeefromAccess : public  IEmployee
{
public:
  bool    InserttoDB(Employee& stEmployee)
  {
    _tprintf(_T("Insert employee %s into access\n"), stEmployee.tstrName.c_str());
    return  true;
  }
  
  Employee  GetEmployee(int nID)
  {
    Employee  stEmployee;
    printf("Get an employee from access by id %d\n", nID);
    return  stEmployee;
  }
};

5.2 使用演示样例

void  DataBaseTest()
{
  IEmployee*  poIEmployee = new CEmployeefromMysql();
  CProxy    oCProxy;

  oCProxy.SetEmployee(poIEmployee);
  
  Employee  stEmployee;
  stEmployee.nID = 1;
  stEmployee.tstrName = _T("Jim");
  
  oCProxy.InserttoDB(stEmployee);
  
  //切换数据库对象
  poIEmployee = new CEmployeefromAccess();
  
  oCProxy.SetEmployee(poIEmployee);
  oCProxy.InserttoDB(stEmployee);
}