Re: Instantiating ComObjects - best practice

"Igor Tandetnik" <>
Sat, 26 Jul 2008 10:59:37 -0400
"Alexander Lamaison" <> wrote in message

My question is about the recommended way to initialising ATL interface
implementations (i.e. ComObjects) with values.

At the moment my programs are littered with code like the following:

IMyInterface pFace;
CComObject<CMyObject> *pObj;
hr = CComObject<CMyObject>::CreateInstance( &pObj );
if(SUCCEEDED(hr)) // Perform implementation-specific initialisation
   hr = pObj->Initialize( some stuff specific to implementation );
   if (SUCCEEDED(hr))
       hr = pObj->QueryInterface( __uuidof(IMyInterface), &pFace );

Yes, that's about right. That's how you have to do it.

Of course, you don't have to "litter" your code with this. Write a
static CMyObject::MakeInstance function that does all this and returns a
properly AddRef'ed interface pointer.

I've also seen people do this which looks a little cleaner:

CComPtr<IMyInterface> spFace;
hr = spFace.CoCreateInstance( __uuidof(CMyObject) );
if (SUCCEEDED(hr))
   static_cast<CMyObject *>(spFace.p)->Initialize( some imp-specific
); }

While it would work, I personally prefer the first variant.

Is the second way in any way inferior to the first

The second way only works on creatable objects - those with a CLSID
registered in the registry. More often than not, you use
CComObject::CreateInstance to create an instance of non-coclass object -
one that is not externally creatable.

Also, if somebody modifies your CLSID registration (e.g. with TreatAs
clause), this code will fail spectacularly.
With best wishes,
    Igor Tandetnik

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