dynamic_cast of a COM object to a COM interface do

Question

If I have a C++ class, X, which implements the COM interfaces IY and IZ, and I have a pointer y to the IY interface of an object of type X, and I do this:

IZ *z = dynamic_cast<IZ *> ( y );

That doesn't bump the object's reference count, does it? I don't have to do a Release() to account for it, right?

If it matters, I'm using ATL/COM.

I'm guessing the answer is "no it doesn't bump the reference count, and no you don't have to Release()", but I want to make sure.

Thanks in advance.

Answer 1

In C++Builder, dynamic_cast on a COM interface pointer actually does QueryInterface. and the returned pointer, if the QI succeeds, gets AddRef'd.

Classes which implement COM objects have different vtable layouts to more general C++ classes, so a C++-style dynamic_cast cannot work; so I presume this is why C++Builder does the more sensible thing of doing QueryInterface.

(The original idea of COM was to generalize the C++ object model to be language-agnostic and to a binary standard; they renamed dynamic_cast to QueryInterface).

I guess the top answer is referring to MSVC, if dynamic_cast causes undefined behaviour.

Answer 2

dynamic_cast must not be used on multiple reasons:

• You don't know if destination supports RTTI
• You not sure if OLE doesn't create proxy for you
• ...

Instead use QueryInterface - it would done what you want.

Even if you sure in question above - casting doesn't changes refcounter

Answer 3

Reference counts for COM objects are incremented when someone calls IUnknown::AddRef(). QueryInterface(), according to COM rules since it gives out a new interface pointer, internally calls AddRef().

In your posted code, you're not calling AddRef(), and you're not calling any function that might call AddRef(), so why would you think the reference count would be incremented?

Despite what ATL/MFC does to one's brain, there is no magic involved. When in doubt, you can always view the disassembly in VS and step through it and prove to yourself that AddRef() isn't being called.

Edit: And I want to reiterate what Dewfy said, don't do this. Use QueryInterface(). Or CComQIPtr<> (if you really must).

Further edit: If you use CComPtr<> and CComQIPtr<> then you don't have to call Release() and much of the burden of figuring out the proper ref-counting is alleviated. You should really consider using them.