CComObjects的工厂模式

Question

我有一个共享的基类CMyBase，它进一步分为CMyFirst和CMySecond的子类。我如何为客户实现一个工厂接口，以便他们不需要知道使用CComObjects创建了哪些子项？

基本上我想要做的是这样的：

CMyBase* CFactory::Create 
{ 
    CMyBase* pInst = NULL; 
    if (something) 
    { 
     pInst = new CMyFirst(); 
    } 
    else 
    { 
     pInst = new CMySecond(); 
    } 
    return pInst; 
} 

但我怎么能创建一个子COM对象的实例？

CComObject<CMyBase>* CFactory::Create 
{ 
    HRESULT hr = E_FAIL; 
    CComObject<CMyBase>* pInst = NULL; 

    if (something) 
    { 
     hr = CComObject<CMyFirst>::CreateInstance(&pInst); // compiler error (see below) 
    } 
    else 
    { 
     hr = CComObject<CMySecond>::CreateInstance(&pInst); // compiler error (see below) 
    } 

    if (SUCCEEDED(hr)) 
    { 
     pInst->AddRef(); 
    } 
    return pInst; 
} 

我明白为什么会出现此错误，但我该如何做到这一点呢？

error C2664: 'ATL::CComObject<Base>::CreateInstance' : cannot convert parameter 1 from 'ATL::CComObject<Base> *' to 'ATL::CComObject<Base> **' 

Answer 1

先创建派生类，然后在返回之前将其转回基地。

一个例子：

CComObject<CMyBase>* CFactory::Create 
{ 
    HRESULT hr = E_FAIL; 
    CComObject<CMyBase>* pInst = NULL; 

    if (something) 
    { 
     CComObject<CMyFirst>* pFirst = NULL; 
     hr = CComObject<CMyFirst>::CreateInstance(&pFirst); 
     pInst = (CComObject<CMyBase>*)pFirst; 
    } 
    else 
    { 
     CComObject<CMySecond>* pSecond = NULL; 
     hr = CComObject<CMySecond>::CreateInstance(&pSecond); 
     pInst = (CComObject<CMyBase>*)pSecond; 
    } 

    if (SUCCEEDED(hr)) 
    { 
     pInst->AddRef(); 
    } 
    return pInst; 
}