> The only one that springs to mind is a View hierarchy in UI libraries.

I'd like to generalize that a little bit and say: graph structures in general. A view hierarchy is essentially a tree, where each node has a bunch of common bits (tree logic) and a bunch of custom bits (the actual view). There are tons of "graph structures" that fit that general pattern: for instance, if you have some sort of data pipeline DAG where data comes in on the left, goes out on the right, and in the middle has to pass through a bunch of transformations that are linked in some kind of DAG. Inheritance is great for this: you just have your nodes inherit from some kind of abstract "Node" class that handles the connection and data flow, and you can implement your complex custom behaviors however you want and makes it very easy to make new ones.

I'm very much in agreement that OOP inheritance has been horrendously overused in the 90s and 00s (especially in enterprise), but for some stuff, the model works really well. And works much better than e.g. sum types or composition or whatever for these kinds of things. Use the right tool for the right job, that's the central point. Nothing is one-size-fits-all.

turns out that using composition and polymorphism is usually simpler than inheritance in such cases.

> But even then, I notice React (& friends) have all moved away from this approach. Modern web development usually makes components be functions.

But what do those functions return? Oh look, it's DOM nodes, which are described by and implemented with inheritance.

I would agree that view hierarchies in UI libraries are one of the primary use-cases for inheritance. But it's a pretty big one.

> But what do those functions return? Oh look, it's DOM nodes, which are described by and implemented with inheritance.

Well of course. React builds on what the browser provides. And the DOM has been defined as a class hierarchy since forever. But react components don’t inherit from one another. If the react devs could reinvent the DOM, I think it would look very different than it looks today.

The problem is of course that there is no useful default behavior you can define when the trait is so isolated and generic.

It doesn't have to be "so isolated". The trait can still have required methods that don't have a default implementation. Eg:

    trait Robot {
        fn send_command(&mut self, command: Command);

        fn stop(&mut self) {
            self.send_command(Command.STOP);
        }
    }

    struct BenderRobot;
    
    impl Robot for BenderRobot {
        // Required.
        fn send_command(&mut self, command: Command) { todo!(); } 
    }

This is starting to look a lot like C++ class inheritance. Especially because traits can also inherit from one another. However, there are two important differences: First, traits don't define any fields. And second, BenderRobot is free to implement lots of other traits if it wants, too.

If you want a real world example of this, take a look at std::io::Write[1]. The write trait requires implementors to define 2 methods (write(data) and flush()). It then has default implementations of a bunch more methods, using write and flush. For example, write_all(). Implementers can use the default implementations, or override them as needed.

Docs: https://doc.rust-lang.org/std/io/trait.Write.html

Source: https://doc.rust-lang.org/src/std/io/mod.rs.html#1596-1935

> First, traits don't define any fields.

How does one handle cases where fields are useful? For example, imagine you have a functionality to go fetch a value and then cache it so that future calls to get that functionality are not required (resource heavy, etc).

    // in Java because it's easier for me
    public interface hasMetadata {
        Metadata getMetadata() {
            // this doesn't work because interfaces don't have fields
            if (this.cachedMetadata == null) {
                this.cachedMetadata = generateMetadata();
            }
            return this.cachedMetadata;
        }
        // relies on implementing class to provide
        Metadata fetchMetadata(); 

    }

Getters and setters that get specified by the implementing type.

But then you have the getters, setters, and field on every class that implements the functionality. It works, sure, it just feels off to me. This is code that will be the same everywhere, and you're pulling it out of the common class and implementing it everywhere.

Yep. Or ... don't put that in the interface at all. It looks like an implementation concern to me.

But if there's a lot of classes that implement the same thing, then not duplicating code makes sense. And saying "it's an implementation detail" leads to having the same code in a bunch of different classes. It feels very similar to the idea of default implementations to me; when the implementation will be the same everywhere, it makes sense to have it in one place.

So to be clear about your example: You have a whole lot of different - totally distinct - types of things, which all need to have the same logic to cache HTTP requests? Can you give some examples of these different types you're creating? Why do you have lots of distinct types that need exactly the same caching logic?

It sounds like you could solve that problem in a lot of different ways. For example, you could make an HTTP client wrapper which internally cached responses. Or make a LazyResource struct which does the caching - and use that in all those different types you're making. Or make a generic struct which has the caching logic. The type parameter names the special individual behaviour. Or something else - I don't have enough information to know how I'd approach your problem.

Can you describe a more detailed example of the problem you're imagining? As it is, your requirements sound random and kind of arbitrary.

From a very modified version of something I was working on recently, but with the stuff I couldn't do actually done here (and non-functionality code because of that, but is shows the idea)

    public interface MetadataSource {
        Metadata metadata = null;

        default Metadata getMetadata() {
            if (metadata == null) {
                metadata = fetchMetadata();
            }
            return metadata;
        }
        
        // This can be relatively costly
        Metadata fetchMetadata();
    }

    public class Image implements MetadataSource {
        public Metadata fetchMetadata() {
            // goes to externally hosted image to fetch metadata
        }
    }

    public class Video implements MetadataSource {
        public Metadata fetchMetadata() {
            // goes to video hosting service to get metadata
        }
    }

    public class Document implements MetadataSource {
        public Metadata fetchMetadata() {
            // goes to database to fetch metadata
        }
    }

Each of the above have completely different ways to fetch their metadata (ex, Title and Creator), and of them has different characteristics related to the cost of getting that data. So, by default, we want the interface to cache the result so that the

1. The thing that _has_ the metadata only needs to know how to fetch it when it's asked for (implementation of fetchMetadata), and it doesn't need to worry about the cost of doing so (within limits of course)

2. The things that _use_ the metadata only need to know how to ask for it (getMetadata) and can assume it has minimal cost.

3. Neither one of those needs to know anything about it being cached.

I had a case recently where I needed to check "does this have metadata available" separate from "what is the metadata". And fetching it twice would add load.

Here's my take on implementing this in rust. I made a trait for fetching metadata, that can be implemented by Image, Video, Document, etc:

    trait MetadataSource {
        fn fetch_metadata(&self) -> Metadata;
    }
    impl MetadataSource for Image { ... } 
    impl MetadataSource for Video { ... } 
    impl MetadataSource for Document { ... }

And a separate object which stores an image / video / document alongside its cached metadata:

    struct ThingWithMetadata<T> {
        obj: T, // Assuming you need to store this too?
        metadata: Option<Metadata>
    }

    impl<T: MetadataSource> ThingWithMetadata {

        fn get_metadata(&self) -> &Metadata {
            if self.metadata.is_none() {
                self.metadata = Some(self.obj.fetch_metadata());
            }
            self.metadata.as_ref().unwrap()
        }
    }

Its not the most beautiful thing in the world, but it works. And it'd be easy enough to add more methods, behaviour and state to those metadata sources if you want. (Eg if you want Image to actually load / store an image or something.)

In this case, it might be even simpler if you made Image / Video / Document into an enum. Then fetch_metadata could be a regular function with a match expression (switch statement).

If you want to be tricky, you could even make struct ThingWithMetadata also implement MetadataSource. If you do that, you can mix and match cached and uncached metadata sources without the consumer needing to know the difference.

https://play.rust-lang.org/?version=stable&mode=debug&editio...

Isn't this essentially the generic typestate pattern in Rust? In my view there is a pretty obvious connection between that particular pattern and how other languages implement OO inheritance, though in all fairness I don't think that connection is generally acknowledged.

(For one thing, it's quite obvious to see that the pattern itself is rather anti-modular, and the ways generic typestate is used are also quite divergent from the usual style of inheritance-heavy OO design.)

When you call myImageInstance.fetchMetadata, what does it do? I don't know rust, so it's not clear to me how the value gets cached.

In this example, ThingWithMetadata does the caching. image.fetch_metadata fetches the image and returns it. It’s up to the caller (in ThingWithMetadata) to cache the returned value.

But part of the goal is to not need the caller to cache it. Nor have the class that knows how to fetch it need to know how to cache it either. The responsibility of knowing how to cache the value is (desired to be) in the MetadataSource interface.

The rule is that you can't cache a value in an interface, because interfaces don't store data. You need to cache a value in a struct somewhere. This implementation wraps items (like images) in another struct which stores the image, and also caches the metadata. Thats the point of ThingWithMetadata. Maybe it should instead be called WithCachedMetadata. Eg, WithCachedMetadata<Image>.

You can pass WithCachedMetadata around, and consumers don't need to understand any of the implementation details. They just ask for the metadata and it'll fetch it lazily. But it is definitely more awkward than inheritance, because the image struct is wrapped.

As I said, there's other ways to approach it - but I suspect in this case, using inheritance as a stand-in for a class extension / mixin is probably going to always be your most favorite option. A better approach might be for each item to simply know the URL to their metadata. And then get your net code to handle caching on behalf of the whole program.

It sounds like you really want to use mixins for this - and you're proposing inheritance as a way to do it. The part of me which knows ruby, obj-c and swift agrees with you. I like this weird hacky use of inheritance to actually do class mixins / extensions.

The javascript / typescript programmer in me would do it using closures instead:

    function lazyResource(url) {
      let cached = null
      return async () => {
        if (cached == null) cached = await fetch(url)
        return cached
      }
    }

    // ...
    const image = {
      metadata: lazyResource(url)
    }

Of all the answers, I think this is actually my favorite solution. Its probably the most clear, simple and expressive way to solve the problem.

> The rule is that you can't cache a value in an interface, because interfaces don't store data.

Right, but the start of where I jumped into this thread was about the fact that there are places where fields would make things better (specifically in relation to traits, but interfaces, too). And then proceeding to discuss a specific use case for that.

> A better approach might be for each item to simply know the URL to their metadata.

Not everything is a coming from a url and, even when it is, it's not always a GET/REST fetch.

> but I suspect in this case, using inheritance as a stand-in for a class extension / mixin is probably going to always be your most favorite option

Honestly, I'd like to see Java implement something like a mixin that allows adding functionality to a class, so the class can say "I am a type of HasAuthor" and everything else just happens automatically.

One way you could fix this with composition is:

    class CachedMetadataSource implements MetadataSource {
      CachedMetadataSource(MetadataSource uncachedSource) {}
      Metadata getMetadata() {
        if (metadata == null) {
          metadata = uncachedSource.getMetadata();
        }
        return metadata;
      }
    }

I don't see how that solves the problem. It seems like Video will need to keep it's own copy of CachedMetadaSource, which points back to itself, and go through that access it's metadata in the getMetadata implementation it makes available to it's users. At that point, it might as well just cache the value itself without the extra hoops. The difficult part isn't caching the value, it's preventing every class that implements MetadataSource from having to do so.

It would be the other way around. You wouldn't pass around the underlying suppliers directly, you'd wrap them. But if you must have state _and_ behavior, then `abstract class` is your friend in Java (while in Scala traits can have fields and constructors, so there is no problem).

Don't mix implementation and interface inheritance.

The commenter used inheritance and thought it was fine. Probably not necessary to re-write in Rust just to be able to say that it doesn't use inheritance while being functionally the same thing.

> And yes, javascript supports many kinds of inheritance

Funny you mention it, since JavaScript has absolutely no concept of contracts, which is one of the most important side-effects of inheritance. Especially not at compile time, but even at runtime you can compose objects willy-nilly, pass them anywhere, and the only way to test if they adhere to some kind of trait is calling a method and hoping for the best.

At least that had been the case till ES6 came around, but good luck finding anyone actually using classes in JavaScript. Mainly because it adds near-zero benefits, basically just the ability to overwrite method behavior without too much trickery.

Yes. JavaScript is an incredibly dynamic language. If you don’t like that, don’t use it.

How does that work in a language without inheritance?

(yes, I guess it's the fake vtable of structure full of pointers)

Structure composition is a form of inheritance.