1 interface IUnknown
2 {
3     virtual HRESULT __stdcall QueryInterface(const IID & iid, void ** ppv) = 0;
4     virtual ULONG _stdcall AddRef() = 0;
5     virtual ULONG _stdcall Release() = 0;
6 };

 1 void foo(IUnknown * pI)
 2 {
 3     IX * pIX = NULL;
 4     HRESULT hr = pI->QueryInterface(IID_IX, (void **)&pIX);
 5     if (SUCCEEDED(hr))
 6     {
 7         pIX->Fx();
 8     }
 9     else
10         ...
11 }

 1 HRESULT __stdcall CA::QueryInterface(const IID & iid, void ** ppv)
 2 {
 3     if (iid == IID_IUnknown)
 4         *ppv = static_cast<IUnknown *>this;
 5     else if (iid == IID_IX)
 6         *ppv = static_cast<IX *>this;
 7     else if (iid == IID_IY)
 8         *ppv = static_cast<IY *>this;
 9     else
10         *ppv = NULL;
11 
12     static_cast<IUnknown *>(*ppv)->AddRef();
13     return S_OK;
14 }