C++ – Returning an object as a property in ATL

atlc

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?

Best Answer

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.

Related Solutions

C++ – object slicing

"Slicing" is where you assign an object of a derived class to an instance of a base class, thereby losing part of the information - some of it is "sliced" away.

For example,

class A {
   int foo;
};

class B : public A {
   int bar;
};

So an object of type B has two data members, foo and bar.

Then if you were to write this:

B b;

A a = b;

Then the information in b about member bar is lost in a.

C++ – Unable to get ATL Connection Points working

Try using the method outlined at this location:

Adding an Event (ATL) @ MSDN

With the example names you have given above, in your MyObject.idl file, you should see a declaration for your outbound even under the section for dispinterface _IMyObjectEvents. Since you right clicked on CProxy_IMyObjectEvents<T> and not on _IMyObjectEvents under the library, your IDL is probably missing the definition. The auto-generated file _IMyObjectEvents_CP.h file should be present in your project header files and should contain the Fire_someEvent() method as a result of adding the method correctly. If not, you may find that it just created a method named someEvent() instead.

Here is an outline you can try in a test project.

Create a new ATL DLL Project. I've called it ATLEventTest1.
Build Project.
Class View: Right clicked on ATLEventTest1, Add a new ATL Simple Object class. Call it MyObject.
Create the definition with a) Apartment Threading b) Dual Interface c) Connection points and whatever else you need.
Rebuild Project.
Class View: Locate the ATLEventTest1Lib library, right click on _MyObjectEvents, and "Add Method".
Method is of void return type, named someEvent, and takes an int testParam with parameter attribute [in].
You should see "[in] int testParam" as a result of adding the parameter.
On the IDL Attributes tab, change the id if necessary. Click Finish.
Class View: Right click on CMyObject, go to Add... , go to Implement Connection Point.
Select _IMyObjectEvents and click ">" to move it to the implemented connection points list. Click Finish.
Rebuild.
Now, in your CMyObject class, you should be able to call Fire_someEvent(). You will see a new method in your ATLEventTest1.idl file under the dispinterface _IMyObjectEvents; the header file _IMyObjectEvents_CP.h file will be created and will create the broadcast code for Fire_someEvent().

Best Answer

Related Solutions

C++ – object slicing

C++ – Unable to get ATL Connection Points working

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