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One can separate data from behaviour and still use object oriented means and polymorphism. …

Does this violate contemporary design principles? No. Is there really a problem with interface members having parameters that are used by some implementations, but ignored by others? Not by …

Your confusion comes from the fact Eric did not say much about the actual execution model (or evaluation model) for his proposed system. He leaves this to some degree "as an exercise to the reader". I …

It seems to be a common cargo cult today to ask if something is "a good practice". Usually, this is the wrong question, since in programming there is almost nothing "good" or "bad" per se - it always …

, then it is usually a good idea to implement this using neither conditionals nor polymorphism. … Moreover, you wrote that your original example is "bad", implying that "replacing conditional by polymorphism" would be "good". …

Somewhere in your code you have to store the specifics of each product. In your example, the natural place for this seems to be the Specs class, since I guess it is something like the "configuration" …

JohnWu's approach of using templates is fine. Alternatively, instead of avoiding casts at any price, one could encapsulate them at a single place inside the BookSorter class, implemented in terms of a …

However, that only makes sense if I create a different class for each different element, which doesn't seem right To me, this feels perfectly right, since, it is just an application of the classi …

it can have a private constructor and expose only static methods Yes, it can, but there is no technical need for. So (besides some other good suggestions here) you should also consider the most s …

I think the easiest way to understand this is by looking at a simple example in C: the qsort function of the standard C library. As a user of that lib, you can provide an arbitrary comparison function …

One can use MVP (Model View Presenter) here. This decouples the view from the view's logic and is typically used to make views exchangable, but there is no reason why it cannot also be used to exchang …

I think I understand what you are after, but first note class FontDialogDirector is essentially a container for a specific list of Widgets, it is actually the coded definition of which Widgets your di …

You are confused because you think of inheritance and polymorphism as separate concepts. … Another example is "static polymorphism" in C++ template meta programming, or polymorphic functions in Python, like shown here. …