COM聚合时候对IUnknown有两个不同的实现

[gooore]

COM聚合的时候对IUnknown有两个不同的实现，请教书上的这段话该怎么用vtbl来理解？

You might expect that with two implementations of IUnknown in one object, naming collisions would occur. You can avoid this problem by giving the second implementation of IUnknown a new name: INoAggregationUnknown. Because COM+ is a binary standard, it is not concerned with the name given to an interface—only with its v-table layout. Therefore, the INoAggregationUnknown interface is defined with a v-table structure identical to that of the real IUnknown:

interface INoAggregationUnknown
{
    virtual HRESULT __stdcall QueryInterface_NoAggregation(
        REFIID riid, void** ppv)=0;
    virtual ULONG __stdcall AddRef_NoAggregation()=0;
    virtual ULONG __stdcall Release_NoAggregation()=0;
};

[jameshooo]

虚表只有一个，但三个方法要实现两套，换个名称即可，例如 Outer_QueryInterface/Outer_AddRef/Outer_Release，原始的那套方法要判断自己是不是被聚合了，选择性把调用转移到第二套方法里。

[gooore]

不理解，主要是下面的代码中m_pUnknownInner->QueryInterface(riid, ppv);会调用
QueryInterface_NoAggregation(REFIID riid, void** ppv)，系统是怎么知道该运行这个函数的？

HRESULT CAggregator::QueryInterface(REFIID riid, void** ppv)
{
	if(riid == IID_IUnknown)
	{
		cout << "Container: CAggregator::QueryInterface() for IUnknown returning " << this << endl;
		*ppv = (IUnknown*)this;
	}
	else if(riid == IID_ISum)
	{
		cout << "Container: CAggregator::QueryInterface() for ISum calling inside object" << endl;
		return m_pUnknownInner->QueryInterface(riid, ppv);
	}
	else if(riid == IID_IMultiply)
	{
		cout << "Container: CAggregator::QueryInterface() for ISum returning " << this << endl;
		*ppv = (IMultiply*)this;
	}
	else 
	{
		*ppv = NULL;
		return E_NOINTERFACE;
	}
	AddRef();
	return S_OK;
}

[gooore]

http://blog.csdn.net/nobugtodebug/archive/2009/10/12/4659351.aspx

好文章，看完弄懂了，希望也能帮到有类似问题的朋友。