Returning an object as a property in ATL

Question

I am creating a COM object using Visual Studio 2008 and ATL. Adding simple properties and methods is easy enough but now I want to do something more complicated. I want to give access to a C++ object via a property of my COM object, so I can do something like:

// Pseudo-code
var obj = CreateObject("progid");
obj.aProperty.anotherProperty = someValue;

So, aProperty returns a reference to another object which exposes a property called anotherProperty.

I'm guessing that I need to add another simple ATL object to my project for the second object (call it IClass2), and have something like the following in the IDL:

[propget, id(1)] HRESULT aProperty([out, retval] IClass2** ppValue);

Am I on the right track here? Does anyone know of a good tutorial for this sort of thing?

Answer 1

I just came across this article:

HOWTO: Implement static object hierarchies in ATL

This looks very similar to what I am trying to achieve.

Answer 2

If you're going to call it from an automation language, you'll need the interface returned to be derived from IDispatch, and you'll likely need to return it at least as an IDispatch**. For retval I think that's good enough; for simple [out] parameters you need to pass it as a VARIANT* (with the variant type set to VT_LPDISPATCH) so that the automation language can understand it.

I'm not sure if there's a good tutorial; it's been a while since I looked for a comprehensive reference. The best advice I could give would be to make sure everything you're passing is automation compatible (eg: is a type which you can put into a VARIANT), and that should take care of 80% of your problems. It's very doable, though; just read up on MSDN and you should be fine.