关于DSHOW和COM学习一个疑点解析
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.
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:
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:
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(); \
};
而GetOwner()->AddRef();即
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<INondelegatingUnknown*>(this) ; // @N } else if (iid == IID_IY) { *ppv = static_cast<IY*>(this) ; // 注意这一行 } else { *ppv = NULL ; return E_NOINTERFACE ; } reinterpret_cast<IUnknown*>(*ppv)->AddRef() ; return S_OK ; } 如果查询IID_IUnknown接口,那么先是调用*ppv = static_cast<INondelegatingUnknown*>(this); 经过static_cast<INondelegatingUnknown*>的转换,*ppv指向INondelegatingUnknown,而INondelegatingUnknown并不是IUnknown派生得到的,那么语句reinterpret_cast<IUnknown*>(*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<IUnknown*>(*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INondelegatingUnknown的AddRef与Release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<IY*>(this); 经过static_cast<IY*>的转换,*ppv指向IY,而IY又是从IUnknown派生来的,所以语句reinterpret_cast<IUnknown*>(*ppv)->AddRef()调用的是IUnknown的AddRef()。
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