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分类: WINDOWS

2008-05-06 10:15:12

class CMyComponent : public CUnknown, public ISomeInterface
{
public:
    DECLARE_IUNKNOWN;
    STDMETHODIMP NonDelegatingQueryInterface(REFIID riid, void **ppv)
    {
        if( riid == IID_ISomeInterface )
        {
            return GetInterface((ISomeInterface*)this, ppv);
        }
        return CUnknown::NonDelegatingQueryInterface(riid, ppv);
    }
 
    CMyComponent(TCHAR *tszName, LPUNKNOWN pUnk, HRESULT *phr) 
        : CUnknown(tszName, pUnk, phr)
    { 
        /* Other initializations */ 
    };
    // More declarations will be added later.
};
 
调用QueryInterface和NonDelegatingQueryInterface的几种形式:
this-> QueryInterface
this-> NonDelegatingQueryInterface
(INonDelegatingUnknown *)this-> NonDelegatingQueryInterface
(IUnknown *)(INonDelegatingUnknown *)this-> QueryInterface
注意:最后一种实际上调用的是NonDelegatingQueryInterface
下面看看CMyComponent类如何查询接口:
CMyComponent *pCMyComponent = new CMyComponent (L”MyComponent”,
NULL,NULL);
ISomeInterface *pSomeInterface = NULL;
pMyComponent->QueryInterface(IID_ISomeInterface,
(void**)&pSomeInterface)
在这里使用的是“this-> QueryInterface”方式调用QueryInterface函数。
 
对象要查询接口,首先调用QueryInterface成员函数进行查询, CMyComponent类通过DECLARE_IUNKNOWN宏定义了QueryInterface成员函数,DECLARE_IUNKNOWN的定义如下:
 
#define DECLARE_IUNKNOWN                                        \
    STDMETHODIMP QueryInterface(REFIID riid, __deref_out void **ppv) {      \
        return GetOwner()->QueryInterface(riid,ppv);            \
    };                                                          \
    STDMETHODIMP_(ULONG) AddRef() {                             \
        return GetOwner()->AddRef();                            \
    };                                                          \
    STDMETHODIMP_(ULONG) Release() {                            \
        return GetOwner()->Release();                           \
    };
 
所以MyComponent.QueryInterface(IID_ISomeInterface,
(void**)&pSomeInterface)
产生的结果等价于:
GetOwner()->QueryInterface(IID_ISomeInterface,
(void**)&pSomeInterface)
 
下面看GetOwner()函数:
LPUNKNOWN GetOwner() const {
        return m_pUnknown;
};
The utility function CUnknown::GetOwner retrieves a pointer to the IUnknown interface of the component that owns this component. For an aggregated component, the owner is the outer component. Otherwise, the component owns itself. Include the DECLARE_IUNKNOWN macro in the public section of your class definition.
CMyComponent::CMyComponent(IUnknown *pOuterUnkown)
{
    if (pOuterUnknown == NULL)
        m_pUnknown = (IUnknown *)(INonDelegatingUnknown *)this;
    else
        m_pUnknown = pOuterUnknown;
    [ ... more constructor code ... ]
}
 
GetOwner获取一个指向该对象(CMyComponent所属对象的IUnknown接口的指针,如果该对象(CMyComponent)是聚合对象,它的拥有者是外部对象(m_pUnknown = pOuterUnknown),否则,此对象拥有自己(m_pUnknown = (IUnknown *)(INonDelegatingUnknown *)this)。
 
注意m_pUnknown为IUnknown *类型,这一点很重要。
m_pUnknown = (IUnknown *)(INonDelegatingUnknown *)this;也将this转换成了IUnknown *对象。
 
STDMETHODIMP QueryInterface(REFIID riid, __deref_out void **ppv) {      \
        return GetOwner()->QueryInterface(riid,ppv);            \
    };
可以看出QueryInterface函数可以改写成下面的形式:
HRESULT QueryInterface(REFIID iid, void **ppv) 
{
    return m_pUnknown->QueryInterface(iid, ppv);
}
在这里使用的是“(IUnknown *)(INonDelegatingUnknown *)this-> QueryInterface”方式调用,实际上调用的是NonDelegatingQueryInterface。
因为m_pUnknown是通过语句m_pUnknown = (IUnknown *)(INonDelegatingUnknown *)this;转换过来的。
 
注意:这里不是“this-> QueryInterface”,而是(IUnknown *)(INonDelegatingUnknown *)this-> QueryInterface”,这两者有本质的区别:
如果是“this-> QueryInterface”,这样调用的是DECLARE_IUNKNOWN宏中定义的QueryInterface, 如下所示:                                     
    STDMETHODIMP QueryInterface(REFIID riid, __deref_out void **ppv) {     
        return GetOwner()->QueryInterface(riid,ppv);           
};     
这样将导致死循环。 
 
如果是(INonDelegatingUnknown *)this-> QueryInterface,将导致编译错误,因为INonDelegatingUnknown接口没有QueryInterface函数。
语 句(IUnknown *)(INonDelegatingUnknown *)this先将this转换成INonDelegatingUnknown指针,因为this和INonDelegatingUnknown之间存在继 承关系,所以经过转换得到的指针将指向this物体中的INonDelegatingUnknown部分,然后再将 INonDelegatingUnknown指针转换成IUnknown指针,因为INonDelegatingUnknown和IUnknown之间不 存在继承关系,所以指针实际上还是指向this物体中的INonDelegatingUnknown部分,而这样做的目的是用一条语句使对于聚合和非聚合 都适应,如果是非聚合,m_pUnknown 等于(IUnknown *)(INonDelegatingUnknown *)this,所以m_pUnknown-> QueryInterface将调用
自己INonDelegatingUnknown接口的 NonDelegatingQueryInterface,而如果是聚合m_pUnknown等于外部对象IUnknown接口指针,则m_pUnknown-> QueryInterface调用的是外部对象IUnknown接口的QueryInterface
 
来看NonDelegatingQueryInterface的定义:
STDMETHODIMP NonDelegatingQueryInterface(REFIID riid, void **ppv)
    {
        if( riid == IID_ISomeInterface )
        {
            return GetInterface((ISomeInterface*)this, ppv);
        }
        return CUnknown::NonDelegatingQueryInterface(riid, ppv);
}
 
再来看GetInterface函数的定义:
STDAPI GetInterface(LPUNKNOWN pUnk, void **ppv)
{
    CheckPointer(ppv, E_POINTER);
    *ppv = pUnk;
    pUnk->AddRef();
    return NOERROR;
}
注意:这里pUnk->AddRef();调用的是
STDMETHODIMP_(ULONG) AddRef() {                             \
        return GetOwner()->AddRef();                            \
};
GetOwner()->AddRef();m_pUnknown-> AddRef();调用的是NonDelegatingAddRef();
 
 
class CMyFilter : public CSource
{
……
}
 
class CMyPin : public CSourceStream,IKsPropertySet,IAMStreamConfig
{
   DECLARE_IUNKNOWN;
   STDMETHODIMP NonDelegatingQueryInterface(REFIID riid, void ** ppv);
   ……
}
 
 
HRESULT   __stdcall   CB::NondelegatingQueryInterface(const   IID&   iid,  
                                                                                                      void**   ppv)  
  {    
  if   (iid   ==   IID_IUnknown)  
  {  
  //   !!!   CAST   IS   VERY   IMPORTANT   !!!  
  *ppv   =   static_cast(this)   ;     //   @N  
  }  
  else   if   (iid   ==   IID_IY)  
  {  
  *ppv   =   static_cast(this)   ;   //  
注意这一行
 
  }  
  else  
  {  
  *ppv   =   NULL   ;  
  return   E_NOINTERFACE   ;  
  }  
  reinterpret_cast(*ppv)->AddRef()   ;  
  return   S_OK   ;  
  }  
 
如果查询IID_IUnknown接口,那么先是调用*ppv   =   static_cast(this); 经过static_cast的转换,*ppv指向INondelegatingUnknown,而INondelegatingUnknown并不是IUnknown派生得到的,那么语句reinterpret_cast(*ppv)->AddRef()为什么要转化为指针(IUnknown *),明明INondelegatingUnknown不是继承自IUnknown?这样写是为了使代码对聚合和不聚合都适用。语句reinterpret_cast(*ppv)->AddRef()调用的实际上还是INondelegatingUnknown::AddRef();
为什么调用的是INondelegatingUnknown::AddRef()呢?
函数的调用实际是汇编 call  函数地址.对于一个类里定义的函数实际是一个一个的地址偏移.如果地址偏移是一样而且参数个数与排列都一样,函数叫什么名字是没有关系的。
   
  ((IUnknown   *)(*ppv))->AddRef()   ;
只是把编译器给骗了,好让它通过.   不过因为偏移与参数都是一样(没参数)   所以就会有那样的结果罗.   你可以把INondelegatingUnknownAddRefRelease位置换一下看看是不是就调用了Release()
 
如果查询IID_IY接口,那么先是调用*ppv   =   static_cast(this); 经过static_cast的转换,*ppv指向IY,而IY又是从IUnknown派生来的,所以语句reinterpret_cast(*ppv)->AddRef()调用的是IUnknownAddRef()
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