Re: Instantiating ComObjects - best practice

From:
"Igor Tandetnik" <itandetnik@mvps.org>
Newsgroups:
microsoft.public.vc.atl
Date:
Sat, 26 Jul 2008 10:59:37 -0400
Message-ID:
<uRom8Az7IHA.2260@TK2MSFTNGP03.phx.gbl>
"Alexander Lamaison" <newsgroups@lammy.co.uk> wrote in message
news:u6Yt5Re7IHA.1952@TK2MSFTNGP02.phx.gbl

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
{
   pObj->AddRef();
   hr = pObj->Initialize( some stuff specific to implementation );
   if (SUCCEEDED(hr))
   {
       hr = pObj->QueryInterface( __uuidof(IMyInterface), &pFace );
   }
   pObj->Release();
}


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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