Re: NSData (again!)

FROM : Joe Osborn
DATE : Sat Dec 14 17:38:13 2002

It's much more flexible to compose the NSData<-->YourObject entity at
runtime.  By using the interface declared in NSData, YourObject can
actually interact with any subclass of NSData, given at runtime, or
even any object that conforms to the NSData interface.  If you subclass
NSData, you end up putting a lot of stuff into the NSData heirarchy
that isn't really NSData's responsibility.  Furthermore, if subclasses
of NSData arise who extend NSData in interesting ways, you won't be
able to take advantage of them as easily.
If you wanted YourData to be mutable, for instance, you'd have to
subclass NSMutableData- if you wanted an immutable version, however,
you'd subclass NSData.  But what if you want to switch between both? 
By making a new class and giving it an NSData instance variable,you can
swap at runtime the mutability of the data.  It wouldn't matter at all
to YourObject.

For more on this, and the difference between compile-time structure and
runtime structure, see www.c2.com/cgi/wiki?WelcomeVisitors and "Design
Patterns: Elements of Reusable Object-Oriented Software"(Gamma, Helm,
Johnson, Vlissides).

--joie

On Saturday, Dec 14, 2002, at 08:32 US/Central, Pascal Goguey wrote:

> Hello!
>
> Summary of the history:
> The other day, I asked a question about how to subclass NSData
> from the interface builder. I got a few replies (thanks again), and
> learned
> about forwardInvocation which is a very nice concept. All the replies
> I got were telling me that "I should not subclass NSData" or "I
> probably
> don't want to subclass NSData".
>
> I guess it is probably true, that there is probably a good technical
> reason
> to prefer wrapping to inheritance, otherwise the people who wrote
> Project Builder and Interface Builder would have enabled the
> possibility
> to subclass NSData.
>
> Now, what I need to know is WHY it is bad practice to subclass.
> There are some subclass (e.g. NSApplocation inherits from NSResponder).
> So why should my class not inherit NSData, but only wrap it?
>
> Thanks,
>
> Pascal
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