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

"" <>
7 Jul 2006 09:49:13 -0700
Doug Harrison [MVP] wrote:

On 7 Jul 2006 07:34:34 -0700, ""
<> wrote:

I have a project which has been running well on VC6 for years now, but
now more and more reasons are telling me it's time to upgrade. The
system consists of a largish number of dynamically loaded DLLs which
are accessed in some kind of COM fashion.

I have now ported a core set of these but run into a big problem: RTTI
does not work correctly (=the way I want it to) between DLLs anymore,
which it did in VC6.

My setup is this:
class IBase
    virtual Foo() = 0;

class IDerivedOne : virtual public IBase
    virtual Bar() = 0;

class IDerivedTwo : virtual public IBase
    virtual Goo() = 0;

class Test : public IDerivedOne, public IDerivedTwo

What I then do is load a module (could be the same executable) where
Test is implemented, get a pointer to a new instance of the object (as
an IBase*) through a class factory and then do a dynamic_cast to
IDerivedOne*. However, the dynamic_cast returns NULL and I can't get it
to understand what I want.

Is this the way it should be or should it give me the pointer I want?
reinterpret_cast does not work either, as a cast to IDerivedOne would
really give me IBase disguised as IDerivedOne, to disastrous results.

Does anyone know how to deal with this? Or do you need more code to see
what's going on?

Certainly, you should be able to do that with dynamic_cast. I've not
encountered the problem you've described, but the seams between modules
(EXE and DLL) are very apparent under the Windows model of dynamic linking,
and this could be a new manifestation of that. Assuming you recompiled
everything from scratch with the new compiler, the next thing to check is
that you're really creating a Test object. After that, see if everything is
compiled with /GR, and in general, that all the relevant compiler options
agree between the modules. What about the class "Test"? Is it implemented
in only one module (DLL or EXE)? If it's all inline, does more than one
module create instances of it? If so, see if it helps to create instances
in only one place, and if it can be said to live in a DLL, try decorating
it with __declspec(dllexport|dllimport), using the well-known macro method.

Doug Harrison
Visual C++ MVP

Thank you for your response.

I have been able to replicate this by keeping all of the code in the
same executable so I have hopefully been able to get away from the
issue of compiler inconsistensies :)

This is the actual code that I am testing with (ITVUnknown is basically
the usual COM IUnknown):

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

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

class CTestImpl : public ITestOne, public ITestTwo

    virtual ~CTestImpl();

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

    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");

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

    if(0 != theTest)
        int i = theTest->GetOne();

    return 0;

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'} *
[0] 0x004577c6 CTestImpl::GetOne(void) *
[1] 0x00459530 CTestImpl::SetOne(double) *
[2] 0x00457d4d tvcomp::CTVComObject<class CTestImpl>::`vector deleting
destructor'(unsigned int) *

but even if I dynamic_cast it to a CTestImpl I still get a NULL
pointer. And Client.exe is the program I am running so it is looking in
its own code.

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