处理接口指针的方法的正确实现

A correct implementation of the methods that deal with a pointer to an interface

本文关键字：实现 方法 指针 处理 接口      更新时间：2023-10-16

我有一个用c++实现的COM对象。我是从一个VB6应用程序中使用这个对象的。

问题是如何实现获取和返回指向接口的指针的方法。下面是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;
}

我是一个初学者在COM，所以我不确定我是否做正确的方式。我觉得我需要在一些接口指针上使用QueryInterface方法。同样有趣的是，当VB6解释如下代码时，它正在做什么:

Dim pen1 As ICOMCvPen
Set pen1 = dc1.GetPen()

是否在GetPen方法返回的接口指针上调用AddRef方法?

更新1

我已经实现了两个测试对象(COMCvTest和COMCvTestFactory)，只是记录所有的方法被调用。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正试图从COM对象查询IPersistStreamInit和IPersistPropertyBag接口。为什么?我也不明白为什么在查询ICOMCvTest接口指针之前有IUnknown接口的查询?

当然，QueryInterface()使它成为一个干净的单行程序，它利用了内置QI的任何错误处理，并负责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

接口指针管理没有什么特别的，除了一件事:你必须照顾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):

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->Release()"之类的事情，这很容易出错。

Re: Update 1

    如果IID_IUnknown是VB6的CoCreateInstance的参数，
  • IUnknown可能首先被查询;然后QueryInterface 'd感兴趣的接口。
    • IPersistStreamInit和IPersistPropertyBag的查询可能是VB6运行时标准对象初始化的一部分;如果这个对象加载了表单，它可以从持久数据初始化。

还请注意，通常在初始化期间预先查询对象的某些接口，以便在以后需要时准备好，特别是在要重用它们时。

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