CComPtrBase 類 為使用基於COM 的記憶體例程的智慧指標類提供了基礎。

/////////////////////////////////////////////////////////////////////////////
// COM Smart pointers

template <class T>
class _NoAddRefReleaseOnCComPtr :
    public T
{
    private:
        STDMETHOD_(ULONG, AddRef)()=0;
        STDMETHOD_(ULONG, Release)()=0;
};


//CComPtrBase provides the basis for all other smart pointers
//The other smartpointers add their own constructors and operators
template <class T>
class CComPtrBase
{
protected:
    CComPtrBase() throw()
    {
        p = NULL;
    }
    CComPtrBase(_Inout_opt_ T* lp) throw()
    {
        p = lp;
        // 普通的引用規則，當介面指標賦值進行賦值操作，應該AddRef
        // 確保了通過這個指標指標進行的介面操作時介面沒有銷燬
        if (p != NULL)
            p->AddRef();
    }
    void Swap(CComPtrBase& other)
    {
        T* pTemp = p;
        p = other.p;
        other.p = pTemp;
    }
public:
    typedef T _PtrClass;
    ~CComPtrBase() throw()
    {
        // 同樣的，當智慧指標銷燬，應該減少引用計數
        // 值觀的一個用途就是，區域性變數的智慧指標，通過構造和解構函式來管理引用計數
        if (p)
            p->Release();
    }
    operator T*() const throw()
    {
        return p;
    }
    T& operator*() const
    {
        ATLENSURE(p!=NULL);
        return *p;
    }
    //The assert on operator& usually indicates a bug.  If this is really
    //what is needed, however, take the address of the p member explicitly.
    T** operator&() throw()
    {
        ATLASSERT(p==NULL);
        return &p;
    }
    _NoAddRefReleaseOnCComPtr<T>* operator->() const throw()
    {
        ATLASSERT(p!=NULL);
        return (_NoAddRefReleaseOnCComPtr<T>*)p;
    }
    bool operator!() const throw()
    {
        return (p == NULL);
    }
    bool operator<(_In_opt_ T* pT) const throw()
    {
        return p < pT;
    }
    bool operator!=(_In_opt_ T* pT) const
    {
        return !operator==(pT);
    }
    bool operator==(_In_opt_ T* pT) const throw()
    {
        return p == pT;
    }

    // Release the interface and set to NULL
    // 顯式的釋放這個指標
    void Release() throw()
    {
        T* pTemp = p;
        if (pTemp)
        {
            p = NULL;
            pTemp->Release();
        }
    }
    // Compare two objects for equivalence
    inline bool IsEqualObject(_Inout_opt_ IUnknown* pOther) throw();

    // Attach to an existing interface (does not AddRef)
    // 這裡需要注意，Attach 到一個已經存在的介面，不需要呼叫AddRef
    // 但是，會將之前繫結的介面，進行Release 操作
    // 這就要求我們，當重複、連續Attach 到同一個介面，將導致，該介面的引用計數異常
    // 這個問題本身就是一個，引用計數的問題，因為，Attach 和 Detach 要對應才保證程式的正常執行
    void Attach(_In_opt_ T* p2) throw()
    {
        if (p)
        {
            ULONG ref = p->Release();
            (ref);
            // Attaching to the same object only works if duplicate references are being coalesced.  Otherwise
            // re-attaching will cause the pointer to be released and may cause a crash on a subsequent dereference.
            ATLASSERT(ref != 0 || p2 != p);
        }
        p = p2;
    }
    // Detach the interface (does not Release)
    // 和上面的Attach 對應
    T* Detach() throw()
    {
        T* pt = p;
        p = NULL;
        return pt;
    }
    // 將當前的介面copy 到另外的指標，理所應當的會需要我們增加當前介面指標的引用計數
    _Check_return_ HRESULT CopyTo(_COM_Outptr_result_maybenull_ T** ppT) throw()
    {
        ATLASSERT(ppT != NULL);
        if (ppT == NULL)
            return E_POINTER;
        *ppT = p;
        if (p)
            p->AddRef();
        return S_OK;
    }
    _Check_return_ HRESULT SetSite(_Inout_opt_ IUnknown* punkParent) throw()
    {
        return AtlSetChildSite(p, punkParent);
    }
    _Check_return_ HRESULT Advise(
        _Inout_ IUnknown* pUnk,
        _In_ const IID& iid,
        _Out_ LPDWORD pdw) throw()
    {
        return AtlAdvise(p, pUnk, iid, pdw);
    }
    _Check_return_ HRESULT CoCreateInstance(
        _In_ REFCLSID rclsid,
        _Inout_opt_ LPUNKNOWN pUnkOuter = NULL,
        _In_ DWORD dwClsContext = CLSCTX_ALL) throw()
    {
        ATLASSERT(p == NULL);
        return ::CoCreateInstance(rclsid, pUnkOuter, dwClsContext, __uuidof(T), (void**)&p);
    }
#ifdef _ATL_USE_WINAPI_FAMILY_DESKTOP_APP
    _Check_return_ HRESULT CoCreateInstance(
        _In_z_ LPCOLESTR szProgID,
        _Inout_opt_ LPUNKNOWN pUnkOuter = NULL,
        _In_ DWORD dwClsContext = CLSCTX_ALL) throw()
    {
        CLSID clsid;
        HRESULT hr = CLSIDFromProgID(szProgID, &clsid);
        ATLASSERT(p == NULL);
        if (SUCCEEDED(hr))
            hr = ::CoCreateInstance(clsid, pUnkOuter, dwClsContext, __uuidof(T), (void**)&p);
        return hr;
    }
#endif // _ATL_USE_WINAPI_FAMILY_DESKTOP_APP
    template <class Q>
    _Check_return_ HRESULT QueryInterface(_Outptr_ Q** pp) const throw()
    {
        ATLASSERT(pp != NULL);
        return p->QueryInterface(__uuidof(Q), (void**)pp);
    }
    T* p;
};
 

需要注意的一點：智慧指標已經幫助我們進行了引用計數的操作，此時，我們不應該顯式的呼叫Addref 和 Release 函式。