Diamond inheritance (C++)

Question

I know that having diamond inheritance is considered bad practice. However, I have 2 cases in which I feel that diamond inheritance could fit very nicely. I want to ask, wou

Answer 1

There's a design-quality difference between implementation-oriented diamond inheritance where implementation is inherited (risky), and subtyping-oriented inheritance where interfaces or marker-interfaces are inherited (often useful).

Generally, if you can avoid the former, you're better off since somewhere down the line the exact invoked method may cause problems, and the importance of virtual bases, states, etc., starts mattering. In fact, Java wouldn't allow you to pull something like that, it supports only the interface hierarchy.

I think that the "cleanest" design you can come up for this is to effectively turn all your classes in the diamond into mock-interfaces (by having no state information, and having pure virtual methods). This reduces the impact of ambiguity. And of course, you can use multiple and even diamond inheritance for this just like you would use implements in Java.

Then, have a set of concrete implementations of these interfaces that can be implemented in different ways (E.g., aggregation, even inheritance).

Encapsulate this framework so that external clients only get the interfaces and never interact directly with the concrete types, and make sure to thoroughly test your implementations.

Of course, this is a lot of work, but if you're writing a central and reusable API, this might be your best bet.