Re: static_cast interface pointer to implementation class

"Igor Tandetnik" <>
Sun, 3 Aug 2008 09:19:40 -0400
"Alexander Lamaison" <> wrote in message

I've been having some problems caused by static-casting an interface
pointer, e.g.:

IShellFolderViewCB *pExplorerCB;
hr = CoCreateInstance(
  __uuidof(CExplorerCallback), NULL, CLSCTX_ALL,


Inside the Initialise call, the member variables are all pointing at
the wrong memory (they appear to be slightly higher in memory than
they should be). What am I doing wrong?

The code that does the casting and the code that implements
CExplorerCallback see slightly different definitions of
CExplorerCallback class during compilation. Figure out why.

To avoide this issue I replaced this code with a call to
CComObject<>::CreateInstance, initialise and then use QI to get the
interface pointer (the recommended way, I believe)

Very much so.

but there are some
situations where this isn't possible, for instance if handed the
interface pointer as a callback.

If you are handed an interface pointer as a callback, how do you even
know the pointer is implemented by a specific class? What's to stop the
client from implementing the interface on their own class, and handing
you a pointer to their implementation? Perfectly legal in COM. If your
design relies on that not happening, you should reconsider it.

If you insist, there's a trick that goes like this:

class CExplorerCallback {
        // ... usual interface entries go here
        COM_INTERFACE_ENTRY_IID(IID_Internal, CExplorerCallback)

Invent a new IID for IID_Internal, don't publish it anywhere (some use
the class' CLSID for this). This way, you can query an incoming
interface pointer for IID_Internal and get back a raw CExplorerCallback
pointer (assuming the interface is in fact implemented by
CExplorerCallback). Don't forget to Release when you are done.
With best wishes,
    Igor Tandetnik

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