The Tragic Death of Inheritance
The Tragic Death of Inheritance
This is a story about how I gradually fell in love with composition
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
Vehiclein yourBicycleclass, 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.
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.
What's a good alternative to inheritance in your Animals example?
If you look at Rust for example, then you specify the Traits instead. So, you could define a trait that defines the properties for birth, another to define if the animal have a beak, and another one to define the number and type of legs. The each animal implement these traits, which then properly can define a duck, cat and a platypus.
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.
But even there a TcpStream, a FileStream and a StringStream might have quite differen behaviour, since they all abstract very different things. So, even this simple example may fall apart very fast if you need to care about those. I'm not saying that Inheritance is always bad, but it is quite rigid and might cause problems in a large codebase if you suddenly run in to the corner cases where the assumptions the abstraction is based upon is no longer upheld.
But the fact that TCPStreams isa file-descriptor, Files isa file-descriptor, Pipes isa file-descriptor, and other such "stream-like objects" in the Linux kernel proves that the read/recv and write/send system calls are generic enough to work in a wide variety of circumstances.
Yeah, they're all different. But as far as the Linux API goes, they're all file descriptors under it all and have abstractions that work well for inheritance in practice. In many cases, inheritance doesn't work. But in many cases, it works. And works well, for decades.
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.
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.
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.