Create COM DLL from Unmanaged C++ LIB

Question

I have followed the steps here to create a COM DLL in Visual Studio 2008. My objective is to wrap an existing unmanaged C++ .lib.

Is there an easy way to implement the COM interface for the lib. Or do I just have to keep adding ATL simple objects which essentially wrap the objects in my library?

For example, I have added the simple ATL object, CMyObject to my COM library, am I to create wrapper code including function members etc in CMyObject that essentially wrap the behavior of MyObject contained in the unmanaged C++ library?

Answer 1

I think adding wrappers the way you describe is the best way to go. Given that parameter types for OLE will be different to the C++ parameters in many cases, e.g. BSTR rather than string or char*, some wrapping is required for COM. The only alternative is to have a non-COM DLL.

Answer 2

In order to expose your functionality to COM you'll need to perform two major steps:

• introduce COM interfaces
• implement those interfaces using functionality of code you already have

So yes, the scenario you described is the typical way to solve this problem.

Using ATL will simplify things a lot. However you have to take care of exceptions as well. Since your code is in C++ it can throw exceptions. COM doesn't allow propagating exceptions outside COM methods - if the client is not in C++ it will crash once exception is thrown from your code. So your wrapper layer needs to have try-catch for every method directly called from COM.