I can't wrap my head around the differences among these terms.
Are COM and ActiveX synonyms?
Is ActiveX object just a COM object that exposes IDispatch?
Lots of older MSDN pages mention IDispatch without any COM context. Does it have a separate history, and was just introduced under the COM 'umbrella' later in its lifecycle?
Where does OLE fit in? Is its (considerable) presence in MFC naming and MSDN in general - all just legacy?
Wikipedia gives some insight, but not much. I couldn't find a more in depth reference.
Found some quotes from COM Programming by Example:
Another thing you'll quickly find with
COM is that marketing had a field trip
day with the terminology. Rather then
helping to promote understanding,
you'll find in most cases it promotes
mystery. ... OLE, ActiveX, Remotable
Objects - which one means what this
week? ...
COM evolved out of the solution to a
specific problem - how to allow a word
processor to edit the spreadsheet in a
letter without the user having to exit
the word processor to do it. This
solution was called OLE. ... However,
with the second release of OLE (OLE2),
its creators modified OLE32.DLL to
expose more of the basic functionality
of OLE so that its API could be used
to allow any application to acess the
functionality of another. This API,
which hasn't changed much since 1993,
is what is considered to be the COM
API.
...
Once the COM API was exposed,
Microsoft used it to create what were
then called OLE controls to allow
sophisticated controls (e.g., buttons
that spiral, list boxes that play
music) written in Visual C++ to be
accessible to Visual Basic
applications. Eventually this same
technology was used to allow a control
to be downloaded and used by your web
browser. At this point, the name
changed to ActiveX controls or just
plain ActiveX purely for marketing
reasons. Although you can package any
functionality in an ActiveX Control,
its requrirements have been
specialized for supporting a user
interface control.
I'm still curious about the very last sentence - must a COM object satisfy some additional requirements to qualify as an ActiveX control? - but its enough to let it go.
Thanks for your answers!
ActiveX is marketing-ese for COM, although ActiveX is usually used in the context of "ActiveX controls" which are GUI objects.
IDispatch is an interface implemented by COM objects so that client programs can call its methods dynamically at run-time (late-binding), as opposed to at compile time where all the methods and argument types need to be known ahead of time.
OLE is the earlier version of COM.
Regarding OLE: COM has its roots in OLE. But today, OLE may be considered to be built on top of COM and is mostly used for what its name suggests -- document linking and embedding. While COM itself defines only a couple of interfaces itself (IUnknown, IClassFactory, IMalloc etc), OLE adds a whole bunch of additional interfaces like IMoniker, IOleItemComtainer, etc.
To sum up: COM is the foundation, ActiveX and OLE are layers on top of COM.
