Question

我有一个共享基类CMyBase，它进一步分为子类CMyFirst和CMySecond。如何为客户端实现工厂接口，以便他们不需要知道使用CComObjects创建了哪些子项？

基本上我要做的是：

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

但是如何创建子COM对象的实例呢？

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;
}

我明白为什么会收到此错误，但我怎么能这样做呢？

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

Answer 1

首先创建派生类，然后在返回之前将其强制转换为基础。

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;
}

CComObjects的工厂模式

1 个答案: