Re: Deletion of a COM object in VC6.0



"Carl Daniel [VC++ MVP]" wrote:

> lauch2 wrote:
> > I have a project that contains a COM object running in the in-process
> > configuration. The project was developed by VC6.0 with SP 3. The
> > project has been put into production for long period of time and it
> > worked just fine. Right now, I want to migrate the project to VC7.1.
> > During the migration, I find that there is a bug in the COM object:
> > The COM object has wrongly Release() for two times, which result in
> > the reference counter to COM object to 0, triggering the COM object
> > to be deleted in VC7.1 code. This bug is supposed to be the same as
> > in the VC6.0 code. However, I found that, when the reference counter
> > reaches 0(or even a negative value), the VC6.0 code does not delete
> > the COM object and this COM object remains in the COM space. I am
> > wondering:
> > 1. When the reference counter reaches 0, if the VC6.0 code does not
> > delete the COM object immediately, who and when will perform the
> > deletion of the COM object?
> > 2. Why the behavior in the VC7.1 has been changed?
> >
> > Sample code:
> > CoCreateInstance(CLSID_XXX, NULL, CLSCTX_ALL, IID_XXXX,(void**)ppv1);
> > CoCreateInstance(CLSID_XXX, NULL, CLSCTX_ALL, IID_XXXX,(void**)ppv2);
> > ppv2->Release();
> > ppv2->Release();// In VC7.1, the object of IID_XXXX is deleted
> > immediately; // In VC6.0, the object of
> > IID_XXXX would not be
> > deleted
>
> There's got to be something more to the story that you're overlooking. For
> any correctly functioning COM object, a single call to Release() after
> CoCreateInstance will destroy the object. Nothing about this has changed in
> VC7.1 - it's all called out by the COM specification.
>
> -cd
Thanks for the reply.

In fact, I am not familar with the COM development. Anyway, the real
situation is as below:

My COM object is derived from the ATL class. Assume the COM object name is
MyComObject. Since the COM object is generated by the VC6.0's wizard, there
should be CMyComObject and IMyComObject classes generated as follows:
class ATL_NO_VTABLE CMyComObject:
public CComObjectRootEx<CComMultiThreadModel>,
public CComCoClass<IMyComObject, &CLSID_MyComObject>,
public IDispatchImpl<IMyComObject, &IID_IMyComObject, &LIBID_MSQLib>
{
…
};

interface ImsqSendQueue : IDispatch
{
....
};

The sample should be as follows:
IMyComObject* pMyComObject1;
IMyComObject* pMyComObject2;

CoCreateInstance(CLSID_MYCOMOBJECT, NULL, CLSCTX_ALL, IID_
MYCOMOBJECT,(void**) &pMyComObject1);
CoCreateInstance(CLSID_ MYCOMOBJECT, NULL, CLSCTX_ALL, IID_
MYCOMOBJECT,(void**) &pMyComObject2);
pMyComObject2->Release();
pMyComObject2->Release(); // see the source code below

I also trace into the Release() function, here is implementation of
Release() in vc7.1 and vc6.0:
----------------------------------------------------------------------------------
vc7.1:-
template <class Base>
class CComObjectCached : public Base
{
STDMETHOD_(ULONG, Release)() throw()
{
m_csCached.Lock();
InternalRelease();
ULONG l = m_dwRef;
m_csCached.Unlock();
if (l == 0)
delete this;
else if (l == 1)
_pAtlModule->Unlock();
return l;
}
};
----------------------------------------------------------------------------------
vc6.0:-
template <class Base>
class CComObjectGlobal : public Base
{
STDMETHOD_(ULONG, AddRef)() {return _Module.Lock();}
}

class CComModule : public _ATL_MODULE
{
LONG Unlock()
{
return CComGlobalsThreadModel::Decrement(&m_nLockCnt);
}
};
----------------------------------------------------------------------------------

Any idea?

Thanks,
lauch2.

.

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