What are oaidl.idl and ocidl.idl for?

A default ATL Simple Object has the following at the top of its IDL file:

import "oaid开发者_开发百科l.idl";
import "ocidl.idl";

What are these files for, and how do I know when I need to import them? Is there any documentation which describes what is defined in these two files? I have no idea what "oa" and "oc" stand for, and there are no helpful comments in the files themselves.

oaidl.idl and ocidl.idl are shipped with Platform SDK/Windows SDK. They are used to generate oaidl.h and ocidl.h

ocidl is for OLE container (e.g. ActiveX) while oaidl is for OLE automation. search oaidl.h and ocidl.h in MSDN or Windows SDK documentation for information on interfaces defined in these two files.

You may not find enough comments in the files, but the interfaces that are defined there are documented in MSDN.