Re: Design question: polymorphism after object creation

"Alf P. Steinbach" <>
Mon, 30 Mar 2009 10:23:36 +0200
* James Kanze:

On Mar 29, 1:34 am, Marcel M?ller <>

Even an object, that is not fully complete, has some basic
properties. It may be selected, it may be referenced, it has
a changeable display name and so on. NULL is not sufficient.

In other words, your base class has some behavior. Logically,
this means either the template method pattern or the strategy
pattern---the latter means that you can defer the decision, and
change the implementation at will.

As I wrote else-thread, IMHO it's not a good idea to add complexity just for
ideological reasons. Objects that modify themselves to provide richer
functionality, where that metamorphosis is (1) costly and (2) can fail, well
it's just silly. Instead keep the distinction between URI or directory entry or
whatever, on the one hand, and what's referred to, on the other hand. They might
implement some common interface. But I can't for the life of me see the point in
having e.g. a directory entry metamorphose into a file it represents.

Having said that, I also have an issue with this "pattern talk". Some few
patterns are very often needed in C++ because C++ doesn't provide the relevant
support, e.g. visitor pattern. But I had to look up "strategy pattern" and
darned if I can see what's different from "template pattern"; I guess it's some
detail that Someone thinks is important, like maintaining red Fords are
intrinsicially different from blue Fords, hey even a child can see difference.

Also, I really don't see how it can be helpful for the OP's question to be able
to dynamically replace the implemententaion of a method or set of methods. It's
not like he just wants the behavior of methods to change. He wants dynamic
switching between different but overlapping sets of behaviors, which means
complexity and ample opportunities for bugs to creep in and start families.


- Alf

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