描述:
但从概念上偶明白它的实现机制，但看com原理与应用 给出的代码时反而又让我有些糊涂了，请帮忙解惑：
//CB为聚合类，CA为被聚合类，以下仅列出部分代码
HRESULT CB::Init()
{
IUnknown *pUnknownOuter = (IUnknown *)this;
HRESULT result = ::CoCreateInstance(CLSID_CompA, pUnknownOuter,
CLSCTX_INPROC_SERVER,
IID_IUnknown, (void **)& m_pUnknownInner) ;
if (FAILED(result))
return E_FAIL;
result = m_pUnknownInner->QueryInterface(IID_SomeInterface, (void **)&m_pSomeInterface);
if (FAILED(result))
{
m_pUnknownInner->Release();
return E_FAIL;
}
pUnknownOuter->Release();
return S_OK;
}
HRESULT CA::QueryInterface(const IID& iid, void **ppv)
{
if ( m_pUnknownOuter != NULL )
return m_pUnknownOuter->QueryInterface(iid, ppv);
else
return NondelegationQueryInterface(iid, ppv);
}
HRESULT CA::NondelegationQueryInterface(const IID& iid, void **ppv)
{
if ( iid == IID_IUnknown )
{
*ppv = (INondelegatingUnknown *) this ;
((IUnknown *)(*ppv))->AddRef() ;
} else if ( iid == IID_SomeInterface )
{
*ppv = (ISomeInterface *) this ;
((ISomeInterface *)(*ppv))->AddRef() ;
}
else
{
*ppv = NULL;
return E_NOINTERFACE ;
}
return S_OK;
}
我跟踪了一下代码执行，在CB的init中 m_pUnknownInner->QueryInterface(IID_SomeInterface, (void **)&m_pSomeInterface); 这句居然直接调用到了CA的NondelegationQueryInterface，我非常不明白为什么会这样，为什么不是调用CA::QueryInterface。