Re: dynamic_cast does not work across modules with multiple inheritance (VC2005)

From:

Ulrich Eckhardt <eckhardt@satorlaser.com>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 10 Jul 2006 09:06:23 +0200

Message-ID:

<fsa9o3-lll.ln1@satorlaser.homedns.org>

kaislavirta@gmail.com wrote:

class ITestOne : virtual public ITVUnknown
{
public:
virtual int GetOne() const = 0;
virtual void SetOne(double inOne) = 0;
};

class ITestTwo : virtual public ITVUnknown
{
public:
virtual int GetTwo() const = 0;
virtual void SetTwo(double inTwo) = 0;
};

class CTestImpl : public ITestOne, public ITestTwo
{
public:
BEGIN_TVINTERFACE_MAP
TVINTERFACE_MAP_ENTRY(TVIID_ITestOne, ITestOne)
TVINTERFACE_MAP_ENTRY(TVIID_ITestTwo, ITestTwo)
END_TVINTERFACE_MAP

public:
CTestImpl();
virtual ~CTestImpl();

public:
virtual int GetOne() const { return 0; }
virtual void SetOne(double inOne) { ; }

public:
virtual int GetTwo() const { return 1; }
virtual void SetTwo(double inTwo) { ; }
};

The Macro definitions in the beginning of the class does not seem to be
the problem here, as I will show below.

int main(int argc, char* argv[])
{
HMODULE theModule = LoadLibrary(L"Client.exe");

const LPFNTVGETCLASSOBJECT fnGCO =
reinterpret_cast<LPFNTVGETCLASSOBJECT>(GetProcAddress(theModule,
"TVGetClassObject"));

Where is the declaration of this function? Could it be that it doesn't
return an ITVUnknown? The reason I ask is that you say:

ITVClassFactoryPtr ptrCF = fnGCO();
ITVUnknown* theUnk = ptrCF->CreateInstance(TVCLSID_TestTwo,
TVIID_ITestTwo);
ITestOne* theTest = dynamic_cast<ITestOne*>(theUnk);

[...]

The important bit I have discovered is that when I run this in the
debugger and hover over theUnk when it has been initialised, I see:
- __vfptr 0x0050c4f0 const tvcomp::CTVComObject<class
CTestImpl>::`vftable'{for `ITestOne'} *

I.e. the debugger thinks it is an ITestOne object, and not a ITVUnknown. It
should however either be an ITVUnknown (because the pointer really points
to such an object, even if it is a subobject of another) or to a CTestImpl,
because that is what it really is.

Just in case you were wondering, otherwise your code seems fine.

Uli