What's the difference between COM_INTERFACE_ENTRY_IID and COM_INTERFACE_ENTRY2_IID?

Seems like both COM_INTERFACE_ENTRY_IID and COM_INTERFACE_ENTRY2_IID are for the case when the class is derived from two or more classes each derived from a common interface. Like this:

class CMyClass : public IPersistFile, public IPersistStream { 

};

(both IPersistStream and IPersistFile derive from IPersist).

Looks like I'm free to include either

COM_INTERFACE_ENTRY_IID( __uuidof( IPersist ), IPersistFile )

or

COM_INTERFACE_ENTRY2_IID( __uuidof( IPersist ), IPersist, IPersis开发者_如何学PythontFile )

into the COM map of my class and it will work allright.

Is there any difference between the two?

According to ATL Internals (a book you should definitely get, based on your questions here -- most of them are covered thoroughly!), the two are mostly equivalent.

• COM_INTERFACE_ENTRY only mentions the interface name, and maps its IID to a vtable offset
• COM_INTERFACE_ENTRY2 mentions the interface name and which branch in the inheritance tree to use for vtable offsetting
• COM_INTERFACE_ENTRY_IID maps IID to interface, and thereby allows you to choose inheritance tree branch
• COM_INTERFACE_ENTRY2_IID does it all; maps IID to interface, and is explicit about inheritance tree branch

The author of ATL Internals says:

Since COM_INTERFACE_ENTRY2[_IID] provides no extra functionality beyond that provided by COM_INTERFACE_ENTRY[_IID], I tend to always use the latter and forget the former.

I think they meant to say that if you use COM_INTERFACE_ENTRY_IID, you already choose the branch, so the COM_INTERFACE_ENTRY2[_IID] family doesn't add anything. But COM_INTERFACE_ENTRY2 is somewhat simpler to use that COM_INTERFACE_ENTRY_IID.