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Instantiating _com_ptr_t objects remotely

Sunday Oct 25th 1998 by Keith Rule

Instantiating _com_ptr_t objects remotely


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I was going along, cheerfully using COM with Microsoft's smart pointers when I discovered a glaring omission in the underlying _com_ptr_t code. It turns out that there is no way to use CreateInstance() to instantiate an object remotely. And there is no equivalent member function to CreateInstanceEx().

After a bit of digging around I ended up writing this (partially tested) macro that I believe allows remote or local instantiation of Microsoft's smart COM pointers.

Normally you create an COM instances using Microsoft's Smart Pointers as follows:

m_pMyCOMObj.CreateIntance(CLSID_MyCOMObj);
 

When using this macro, replace the previous code with the following:

HRESULT hr = CreateInstanceEx<IIMyCOMObjPtr>(m_pMyCOMObj, CLSID_MyCOMObj, _T("RemoteHostname"));

This should allow you to connect to your remote host simply by specifying the hostname or IP address in the 3rd argument. Don't forget to check the return value for success.

The code follows:

// This template is used to invoke either local or remote instances of
// C++ builtin smart COM objects.
template <typename ObjPtr>
HRESULT CreateInstanceEx(ObjPtr& output, const CLSID& rclsid, 
	const CString& hostname = _T(""), IUnknown* pOuter = NULL, DWORD dwClsContext = CLSCTX_ALL) 
{
	USES_CONVERSION;
	HRESULT hr;

	if (!hostname.IsEmpty()) {
		// Create Distributed Object

		// Declare some locals
		IUnknown*		pIUnknown;
		COSERVERINFO*	pServerInfo = NULL;
		MULTI_QI		rgmqResults;
		COSERVERINFO	ServerInfo;
		
		// Zero structures
		memset(&rgmqResults, 0, sizeof(rgmqResults));
		memset(&ServerInfo, 0, sizeof(ServerInfo));

		// Initialize data items
		dwClsContext = CLSCTX_REMOTE_SERVER;
		ServerInfo.pwszName = T2W(hostname);
		pServerInfo = &ServerInfo;
		rgmqResults.pIID = &IID_IUnknown;

		// Create the IUnknown object for the remote object object
		hr = CoCreateInstanceEx(rclsid, pOuter, dwClsContext, pServerInfo, 1, &rgmqResults);
		
		// If successful, then get the internal interface we really wanted.
		if (SUCCEEDED(hr)) {
			pIUnknown = rgmqResults.pItf;
			if (SUCCEEDED(hr)) {
				hr = pIUnknown->QueryInterface(ObjPtr::GetIID(), reinterpret_cast<void**>(&output));
			}
			pIUnknown->Release(); // IUnknown has served it's purpose so nuke it.
		}
	} else {
		// It's not remote so use the simpler call.
		hr = CoCreateInstance(rclsid, pOuter, dwClsContext, ObjPtr::GetIID(), reinterpret_cast<void**>(&output));
	}

	return hr;
}

 

Caveats

Don't forget to include _WIN32_DCOM in the preprocessor definitions of your project or you get a syntax error telling you that CoCreateInstanceEx() is not defined. You also need to make sure that OleInit() has been called prior to using the CreateInstanceEx() macro.

Last Modified: October 22, 1998

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