Most of us have bad memories of over-complex hierarchies we regret seeing, but this is probably due to the dominance of OOP in recent decades.

This sentence here is why inheritance gets a bad reputation, rightly or wrongly. Inheritance sounds intuitive when you're inheriting Vehicle in your Bicycle class, but it falls apart when dealing with more abstract ideas. Thus, it's not immediately clear when and why you should use inheritance, and it soon becomes a tangled mess.

Thus, OO programs can easily fall into a trap of organizing code into false hierarchies. And those hierarchies may not make sense from developer to developer who is reading the code.

I'm not a fan of OO programming, but I do think it can occasionally be a useful tool.

Inheritance is useful.

However, "Dog isa Animal" is worse than useless, it actively hampers your code and makes your life worse.

Useful inheritance patterns are all over the place in GUI / Model View Controller code however. "Button isa Window", and "FileStream isa Stream", and "StringStream isa Stream" in C++. If you stick with SOLID principles, inheritance helps your code significantly.

The core problem I see with Inheritance is that the abstractions tend to fall apart and no longer be true. Lets use the Animal example. It is easy, when you have Animal -> Cat and Animal -> Dog. But as soon as we become more specific like Animal -> Mammal -> Cat, Animal -> Fish -> Hammerhead Shark, Animal -> Bird -> Bald eagle, we risk of getting in trouble. Because now for all purposes we assume things about the Fish, Birds and Mammals, like fish is in the sea and mammals are live on land. We know that this is not strictly true, but for everything we need it works. Later we need to handle a dolphin.... should that be a fish, or do we need to restructure the whole program. If we treat it like a fish, then we might be even deeper in trouble later if we would need to handle birth. And even if we restructure our program to be correct to handle birth, we might stil forget that some mammals lay eggs like the Platypus, so then things break again if we need to handle that. We tend to see Inheritance as a rigid fact based structure, but the core problem is that it is just a bunch of assumptions we dictate in a very rigid way that is hard to change.

Composition have no problem with specifying the platypus as a mammal that lays eggs and have a beak.

Inheritance is a fine abstraction. Easy to understand, but can't bring you very far. It's like a necessary evolutionary niche. It has its places, but it's most important as a gateway to get us to better abstractions.

I remember the old times where every class in my codebase extended Database and I thought how clever that was and how cool inheritance is.

I find that inheritance isn't that useful when it comes to helping me reason about programs, which is more important to me than avoiding code duplication. The ability to create subclasses can be convenient for adding new code but the same extensibility also means that I have to consider all potential future uses of a class when I'm using one. It's easy to make assumptions when writing code that uses a class than can then be broken by future subclasses. You can try to mitigate this by writing clear invariants in advance and updating them when you find that you need to make more assumptions, but then this means that you have to give up the flexibility of inheritance.

Good riddance.

I also used to love inheritance. That is, right up until I started dabbling with game development and started hitting the limitations. Then I discovered ECS systems and everything changed.

Conpsition is better in every case. I know some people will say inheritance is still good for things like GUIs, but I'm not buying in. I'm now firmly in the Rust camp, and there's nothing someone could do with inheritance that I couldn't do with traits and trait bounds.

Composition ftw.

6First off, Inheritance is not "dead". We all just learned to favour interface inheritance over class inheritance.

Secondly, class inheritance is not bad or useless, it's just poorly taught and wildly overused. The fact that the article still uses the bullshit Animal kingdom example is indicative of that. There's no value in trying to model cats and dogs in OOP.

Inheritance is pretty useful in niche scenarios, mostly involving polymorphism and probably in the context of Library or Framework code. Trying to re-use code between classes with inheritance is always the incorrect approach. Two classes that are unrelated but have similar properties don't actually need a common base class, they can each have their own version of the data.

The big, big, big problem outside the education system comes back to the top down, design-first approach. There's still this strange practice of trying to model class hierarchies and abstract classes in neat little diagrams before you've actually started writing code. No UML type document has ever survived contact with the real world. If you need any form of inheritance, it will become obvious as you build out your system.

There are many reasons one might use inheritance. The one which is often first introduced by OOP proponents is creating a “is-a” relationships in some domain model where the goal is to somehow model “real life” with classes and objects.

This always ends badly in my experience. Not worth doing. Just consider the “rectangle square controversy” https://henrietteharmse.com/2015/04/18/the-rectanglesquare-controversy/. Inheritance as a domain modelling tool doesn’t really mean anything outside of textbooks talking about animals and vehicles.

One should understand how computers and programming work and model data in a way which makes sense for the concrete scenario. Look in any algorithms and data structures book. Nothing is ever explained and presented through object models. No UML diagrams needed.

But inheritance also bring actual concrete functionality. In particular dynamic single dispatch. If you need dynamic single dispatch, inheritance is good. That being said, I think that parametric polymorphism (“generics” in OOP terminology) is a better solution than subtype polymorphism in almost all cases. Although languages build around OOP and inheritance sometimes make it too inconvenient to use.

In Python, everything is a Dictionary.

Great article. I think it's been a long time coming that OOP gets a serious second look, especially regarding the blind adoption of design patterns that add complexity for the sake of code isolation and segmentation.

Great read!

I think a bonus point in favour of composition here is the power of static typing. Introducing advanced features like protocols can bring back some of that safety that this article describes as being exclusive to inheritance.

Overall, I think composition will continue to be the future going forward, and we'll find more ways to create that kind of compilation-time safety without binding ourselves into too restrictive or complicated models.

Take Rust as an example, composition implemented in modern languages is really a mix of both paradigms.

In the Advantages of Inheritance section, we can emulate the inheritance way by adding a IndefinitelyDoStuff type class (or interface). The type's capability is extent by type class hierarchy and code duplication is mitigated by default implementation.

Inheritance provides a safer experience due to compile time guarantees [...] the code simply doesn’t compile until you’ve updated all child classes

Similarly, adding that to the type class declaration will enforce implementation for each instance to be added at compile time.

C++ dev with minimal experience of composition here, but this is a great article and it's something I've been trying to play with more and more. Out of curiosity, does anyone know if there are any known big pitfalls to avoid with composition (thinking of things like the diamond problem)?

Well written article that mirrors exactly what I’ve discovered over the years as well.

Typo in one part if the author is reading: compilated