我有一个用 C++ 实现的 COM 对象。我正在使用 VB6 应用程序中的这个对象。
问题是如何实现获取和返回接口(interface)指针的方法。 这是 IDL 的示例:
[...]
interface ICOMCvDC : IUnknown
{
HRESULT GetPen([retval][out] ICOMCvPen** ppPen);
HRESULT SetPen([in] ICOMCvPen* pPen);
};
下面是组件类对象的框架:
class COMCvDC : public ICOMCvDC
{
public:
...
STDMETHODDECL GetPen(
/* [retval][out] */ ICOMCvPen** ppPen);
STDMETHODDECL SetPen(
/* [in] */ ICOMCvPen* pPen);
...
protected:
ICOMCvPen* m_pen;
};
...
STDMETHODIMP COMCvDC::GetPen(
/* [retval][out] */ ICOMCvPen** ppPen)
{
*ppPen = m_pen;
return S_OK;
}
STDMETHODIMP COMCvDC::SetPen(
/* [in] */ ICOMCvPen* pPen)
{
m_pen = pPen;
return S_OK;
}
我是 COM 的初学者,所以我不确定我的做法是否正确。我觉得我需要在某些接口(interface)指针上使用 QueryInterface
方法。了解 VB6 在解释如下代码时在做什么也很有趣:
Dim pen1 As ICOMCvPen
Set pen1 = dc1.GetPen()
是否在GetPen
方法返回的接口(interface)指针上调用了AddRef
方法?
更新1
我已经实现了两个测试对象(COMCvTest 和 COMCvTestFactory),它们只记录所有被调用的方法。然后我执行了以下 VB6 代码:
Dim test1 As ICOMCvTest
Set test1 = New COMCvTest
Debug.Print "Ref: " & test1.GetReferenceCounter
Set test1 = Nothing
下面是这些对象的生命周期日志:
COMCvTestFactory::COMCvTestFactory(); m_cRef = 1
COMCvTestFactory::QueryInterface() --- begin ---
IID is {00000001-0000-0000-C000-000000000046}
IID is IID_IClassFactory
COMCvTestFactory::AddRef(); m_cRef = 2 (was 1)
COMCvTestFactory::QueryInterface() ---- end ----
COMCvTestFactory::Release(); m_cRef = 1 (was 2)
COMCvTestFactory::CreateInstance() --- begin ---
COMCvTest::COMCvTest(); m_cRef = 1
COMCvTest::QueryInterface() --- begin ---
IID is {00000000-0000-0000-C000-000000000046}
IID is IID_IUnknown
COMCvTest::AddRef(); m_cRef = 2 (was 1)
COMCvTest::QueryInterface() ---- end ----
COMCvTest::Release(); m_cRef = 1 (was 2)
COMCvTestFactory::CreateInstance() ---- end ----
COMCvTest::AddRef(); m_cRef = 2 (was 1)
COMCvTest::Release(); m_cRef = 1 (was 2)
COMCvTestFactory::Release(); m_cRef = 0 (was 1); deleting object
COMCvTestFactory::~COMCvTestFactory()
COMCvTest::QueryInterface() --- begin ---
IID is {00000000-0000-0000-C000-000000000046}
IID is IID_IUnknown
COMCvTest::AddRef(); m_cRef = 2 (was 1)
COMCvTest::QueryInterface() ---- end ----
COMCvTest::QueryInterface() --- begin ---
IID is {9F660698-1950-4DE8-BB5F-C8D2D61F7367}
IID is IID_ICOMCvTest
COMCvTest::AddRef(); m_cRef = 3 (was 2)
COMCvTest::QueryInterface() ---- end ----
COMCvTest::QueryInterface() --- begin ---
IID is {7FD52380-4E07-101B-AE2D-08002B2EC713}
IID is IID_IPersistStreamInit
COMCvTest::QueryInterface() --- begin ---
IID is {37D84F60-42CB-11CE-8135-00AA004BB851}
IID is IID_IPersistPropertyBag
COMCvTest::Release(); m_cRef = 2 (was 3)
COMCvTest::Release(); m_cRef = 1 (was 2)
COMCvTest::GetReferenceCounter; m_cRef = 1
COMCvTest::Release(); m_cRef = 0 (was 1); deleting object
COMCvTest::~COMCvTest()
似乎 VB6 正在尝试从 COM 对象查询 IPersistStreamInit
和 IPersistPropertyBag
接口(interface)。为什么?另外我不明白为什么在查询ICOMCvTest
接口(interface)指针之前先查询IUnknown
接口(interface)?
最佳答案
当然,QueryInterface() 使它成为一个干净的单行程序,它利用任何错误处理内置 QI 并处理您必须添加的引用计数:
STDMETHODIMP COMCvDC::GetPen(ICOMCvPen** ppPen)
{
if (m_pen) return m_pen->QueryInterface(__uuidof(ICOMCvPen), (void**)ppPen);
else {
*ppPen = 0;
return E_FAIL;
}
}
STDMETHODIMP COMCvDC::SetPen(ICOMCvPen* pPen)
{
if (m_pen) m_pen->Release();
m_pen = pPen;
return S_OK;
}
不要忘记在构造函数中将 m_pen 初始化为 NULL,并在析构函数中释放它。或者使用智能指针。
关于c++ - 处理指向接口(interface)的指针的方法的正确实现,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/7646852/