The next example shows how the same object is declared and initialized using the C++ nested class approach. As in the C example, the nested class declaration includes one data structure for each interface and four private data members: an object-level reference count, two interface pointers, and a pointer to the enclosing object. The private implementations of the IUnknown methods are called by the implementations declared for the derived interfaces. For each interface implementation, there is a structure containing a public constructor and destructor, private declarations of the interface methods, a private pointer to CObj, and an interface-level reference counter for debugging purposes. To allow the nested interface classes to access the private members of the outer class, each interface class is made a friend of the outer class.
The benefits of implementing with C++ nested classes lie in the ability to include initialization code and method implementation inline. However, inline declaration is for the convenience of illustration and is not required.
class CObj {
private:
ULONG m_ObjRefCount;
LPSTORAGE m_pStg;
LPOLEOBJECT m_pOleObj;
CDOC * m_pCDoc;
public:
CObj();
~CObj();
struct CUnknown : IUnknown
{
private:
ULONG m_RefCount;
CObj * m_pCObj;
public:
CUnknown(CObj (pCObj)
{ m_pCObj = pCObj; m_RefCount = 0; }
HRESULT QueryInterface(REFIID riid, LPVOID * ppvObj)
ULONG AddRef(void) { return ++m_ObjRefCount; }
ULONG Release(void);
}
friend CUnknown;
CUnknown m_Unknown;
struct InterfaceA : InterfaceA
{
private:
ULONG m_RefCount;
CObj * m_pCObj;
public:
CInterfaceA(CObj *pCObj)
{ m_pCObj = pCObj; m_RefCount = 0; }
HRESULT QueryInterface(REFIID riid, LPVOID * ppvObj)
ULONG AddRef(void) { return ++m_ObjRefCount; }
ULONG Release(void);
HRESULT MethodA1(LPVOID * ppvObj);
HRESULT MethodA2(DWORD dwArg);
}
friend CInterfaceA;
CInterfaceA m_InterfaceA;
struct InterfaceB : InterfaceB
{
private:
ULONG m_RefCount;
CObj * m_pCObj;
public:
CInterfaceB(CObj *pCObj)
{ m_pCObj = pCObj; m_RefCount = 0; }
HRESULT QueryInterface(REFIID riid, LPVOID * ppvObj)
ULONG AddRef(void) { return ++m_ObjRefCount; }
ULONG Release(void);
HRESULT MethodB1(void);
HRESULT MethodB2(DWORD dwArg1, DWORD dwArg2);
}
friend CInterfaceB;
CInterfaceB m_InterfaceB;