I'm working a project, where the natural approach is to implement a main object with sub-components based on different classes, e.g. a PC consisting of CPU, GPU, ...

I've started with a composition structure, where the components have attributes and functions inherent to their sub-system and whenever higher level attributes are needed, they are given as arguments.

Now, as I'm adding more functionality, it would make sense to have different types of the main object, e.g. a notebook, which would extend the PC class, but still have a CPU, etc. At the moment, I'm using a separate script, which contains all the functions related to the type.

Would it be considered bad practice to combine inheritance and composition, by using child classes for different types of the main object?

CodePudding user response：

In short

Preferring composition over inheritance does not exclude inheritance, and does not automatically make the combination of both a bad practice. It's about making an informed decision.

More details

The recommendation to prefer composition over inheritance is a rule of thumb. It was first coined by GoF. If you'll read their full argumentation, you'll see that it's not about composition being good and inheritance bad; it's that composition is more flexible and therefore more suitable in many cases.

But you'll need to decide case by case. And indeed, if you consider some variant of the composite pattern, specialization of the leaf and composite classes can be perfectly justified in some situations:

• polymorphism could avoid a lot of if and cases,
• composition could in some circumstances require additional call-forwarding overhead that might not be necessary when it's really about type specialization.
• combination of composition and inheritance could be used to get the best of both worlds (caution: if applied carelessly, it could also give the worst of both worlds)

Note: If you'd provide a short overview of the context with an UML diagram, more arguments could be provided in your particular context. Meanwhile, this question on SE, could also be of interest