我有一个用C ++实现的COM对象。我在VB6应用程序中使用此对象。
问题是如何实现获取和返回指向接口的指针的方法。 以下是IDL的示例:
[...]
interface ICOMCvDC : IUnknown
{
HRESULT GetPen([retval][out] ICOMCvPen** ppPen);
HRESULT SetPen([in] ICOMCvPen* pPen);
};
以下是组件类对象的骨架:
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,所以我不确定我是否以正确的方式做到了。我觉得我需要在某些接口指针上使用QueryInterface
方法。另外有趣的是,在解释代码时,VB6正在做什么:
Dim pen1 As ICOMCvPen
Set pen1 = dc1.GetPen()
是否在AddRef
方法返回的接口指针上调用GetPen
方法?
我已经实现了两个测试对象(COMCvTest和COMCvTestFactory),它们只记录被调用的所有方法。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
以下是这些对象生命周期的日志:
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。为什么?另外我不明白为什么在查询IUnknown
接口指针之前有ICOMCvTest
接口的查询?
答案 0 :(得分:1)
接口指针管理没有什么特别之处,除了一件事:你必须处理AddRef / Release调用。这就是为什么强烈建议使用自动管理的智能指针包装器(C ++伪代码)的原因:
CComPtr<ICOMCvPen> m_pen;
HRESULT Get(IPen** ppPen) { ... *ppPen = CComPtr<ICOMCvPen>(m_pen).Detach(); ... }
HRESULT Set(IPen* pPen) { ... m_pen = pPen; ... }
就是这样!如果你不使用CComPtr,你必须一直做“if(!x)x-&gt; Release()”的东西,这很容易犯错误。
IUnknown
是VB6的CoCreateInstance中的参数,则可以先查询IID_IUnknown
;那么它QueryInterface
是感兴趣的界面。IPersistStreamInit
和IPersistPropertyBag
的查询可能是VB6运行时标准对象初始化的一部分;如果该对象应该加载表单,它可以从持久数据初始化。 另请注意,通常在初始化期间从对象中预先查询某些接口,以便在以后需要时准备就绪,尤其是如果要重复使用它们。
答案 1 :(得分:1)
当然,QueryInterface()使它成为一个干净的单线程,利用任何错误处理内置QI并处理你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。或使用智能指针。