Re: Good COM Interface Design

"Alexander Nickolov" <>
Wed, 18 Apr 2007 12:05:03 -0700
For starters, if it doesn't make sense to implement all methods
of a base interface when implementing a derived interface then
you should not derive the interfaces from each other. It's possible
that the original interface needs to be split in the first place and
the other deirved from an interface comprised of only a subset
of the method of the original base interface.

As for maintaining mukltiple interface pointer - check out
CComQIPtr<>. You only need to maintain one pointer - you
QI for any other pointer as needed. In most cases you don't
need to maintain multiple interfaces in the same code location.
You may have to maintain different interface pointers in different
code locations, which is actually good - promotes information

Alexander Nickolov
Microsoft MVP [VC], MCSD

"anand" <> wrote in message

On Apr 17, 9:45 pm, "Alexander Nickolov" <> wrote:

With derivation you lose the capability of extending the base interface.
However, that does not necessarily mean it's a bad option. A properly
designed base interface doesn't ever need to be extended. Case in
point: IPersist. From practical perspective, it more often makes sense
not to derive an interface from another. However, I want to stress
there's no one hard and fast rule for all occasions. E.g. independent
interfaces are _not_ superior to derived interfaces.

Alexander Nickolov
Microsoft MVP [VC], MCSD

"anand" <> wrote in message

On Apr 17, 7:57 pm, ""
<> wrote:

It depends a lot on the problem you are trying to solve.

What are the interfaces trying to achieve? If there an "Is-a"
relationship then inheritance maybe a good solution.

Would it be reasonable to have a client that implemented the methods
of A2 without implementing A? If so then you shouldn't use inheritance
(since any implementation of A2 would be forced to include all the
methods from A).

Yeah that depends on lots of stuff, but what would be good design
approach to make components extensible for future??

I have seen IXMLDocument and IXMDocument2 interfaces , they both are
dereived from IUnknown

The only problem I see with derivation is that we have to implement
all the functions of base interaface in implementing object.

And if we dont use derivation then there is headache of maintaining
all Interface Pointers supporting different functionality.

Interface IA: IUnknown
Interface IA1: IUnknown
Interface IA2: IUnknown

Now in client code we have to maintain pointers to IA,IA1 and IA2
since all these three will have different Functions.

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