为什么这个dynamic_cast <T>不起作用？

Question

这应该很简单.ID2D1LinearGradientBrush使用有效的vtable从ID2D1Brush派生.我意识到QueryInterface可以在这里工作,但我的问题与dynamic_cast有关.

[definition from d2d1.h]

interface DX_DECLARE_INTERFACE("2cd906ab-12e2-11dc-9fed-001143a055f9") 
ID2D1LinearGradientBrush  : public ID2D1Brush
{
    // ....
}

但是,鉴于过度简化的示例函数......

bool ClampToItem(ID2D1Brush *brush, SizeF itemSize)
{
    // As expected, works when a linear gradient brush is the parameter.
    ID2D1LinearGradientBrush *linearGradientBrush = static_cast<ID2D1LinearGradientBrush *>(brush);
    linearGradientBrush->SetStartPoint(D2D1_POINT_2F{ itemSize.Width, 0.0f });
    linearGradientBrush->SetEndPoint(D2D1_POINT_2F{ itemSize.Width, itemSize.Height });
    return true;
}

bool ClampToItem2(ID2D1Brush *brush, SizeF itemSize)
{
    // this dynamic cast FAILS EVERY TIME with an access violation
    ID2D1LinearGradientBrush *linearGradientBrush = dynamic_cast<ID2D1LinearGradientBrush *>(brush);
    if (!linearGradientBrush)  // <-- never gets here
        return false;

    linearGradientBrush->SetStartPoint(D2D1_POINT_2F{ itemSize.Width, 0.0f });
    linearGradientBrush->SetEndPoint(D2D1_POINT_2F{ itemSize.Width, itemSize.Height });
    return true;
}

由于我不确定将ID2D1LinearGradientBrush作为参数提供,我想使用dynamic_cast.我一定很遗憾.这些COM对象不包含RTTI信息吗？谢谢您的帮助.

// For clarification, this works as expected
bool ClampToItem3(ID2D1Brush *brush, SizeF itemSize)
{
    ID2D1LinearGradientBrush *linearGradientBrush;
    HRESULT hr = brush->QueryInterface(__uuidof(ID2D1LinearGradientBrush), (void **)&linearGradientBrush);
    if (hr == S_OK)
    {
        linearGradientBrush->SetStartPoint(D2D1_POINT_2F{ itemSize.Width, 0.0f });
        linearGradientBrush->SetEndPoint(D2D1_POINT_2F{ itemSize.Width, itemSize.Height });
        linearGradientBrush->Release();
        linearGradientBrush = nullptr;
    }
    return true;
}

编辑:

跟踪动态转换(进入rtti.cpp):

extern "C" PVOID __CLRCALL_OR_CDECL __RTDynamicCast(
    PVOID inptr,            // Pointer to polymorphic object
    LONG VfDelta,           // Offset of vfptr in object
    PVOID SrcType,          // Static type of object pointed to by inptr
    PVOID TargetType,       // Desired result of cast
    BOOL isReference)       // TRUE if input is reference, FALSE if input is ptr
    throw(...)

调用,inptr有效,vfDelta为0,SrcType和TargetType看起来很棒,isRef表示false.进一步跟踪,此处发生内存访问冲突:

// Ptr to CompleteObjectLocator should be stored at vfptr[-1]
_RTTICompleteObjectLocator *pCompleteLocator =
    (_RTTICompleteObjectLocator *) ((*((void***)inptr))[-1]);

char *pCompleteObject = (char *)inptr - COL_OFFSET(*pCompleteLocator);

因为*pCompleteLocator已移至无效位置.

Answer 1

Microsoft的标准COM接口__declspec(novtable)用作其定义的一部分; 如果你看一下你的定义,DX_DECLARE_INTERFACE你会看到这就是这种情况.这意味着基接口类没有vtable,只有具体实现有一个.RTTI没有dynamic_cast正常工作所需的信息.

对于COM接口,您应始终使用QueryInterface动态转换.