> This sort of begs the question: where does classical OOP, the one taught to all undergrad CS majors in programs that use Java or C++, really fit in nowadays?

I have no idea. The smalltalk ideal of OOP lives on in all sorts of ways, but the deep inheritance chain version that gets taught in classes? I don't think it has a place apart from maintaining existing code.

I like to call that style "modeling a taxonomy of the world". Even real-world carefully studied taxonomies change all the time, good luck adapting your code base when an employee is also a customer. It was a crap style from the very beginning.

Smalltalk style OOP has its own issues, but there's lots of good aspects to it and you really have to experience Smalltalk as a complete system to really get it, it's nothing like Simula/C++/Java-style OOP.

That said, best thing you can learn in your programming career is to get rid of "Customer" classes (or whatever the equivalent is for your particular problem). A customer is a unique entity represented by an ID, a private key if you will, which links data in various systems.

Need to split some part of your codebase that handles authentication into a separate microservice for whatever reason? No problem, the same key can be used to refer to data in that system.

Your program magically becomes more modular, more maintainable, easier to understand, and more performant, all in one go.

Use OOP when you need an abstract interface to something that can be implemented in various different ways, e.g. data structures or plugin systems, stuff like that. Any time I see "PODs" full of getters and setters I cry on the inside (and sometimes on the outside).

Would you mind explaining this part a little more - I do not get the difference between a Customer class and a customer as a unique entity represented by an ID?

"That said, best thing you can learn in your programming career is to get rid of "Customer" classes (or whatever the equivalent is for your particular problem). A customer is a unique entity represented by an ID, a private key if you will, which links data in various systems."

Hi q-base, it all comes down to the single responsibility principle. What is the single responsibility of the Customer class?

What methods should it have? What does a Customer do exactly? Or is it just a POD which simply stores customer data? If so which data? Does it mix authentication information with the user's purchase history? Maybe not that but what about the user's little Avatar in the UI?

The answer is none of the above, the single responsibility of the Customer class is to identify a user, that's it. In the end that's just a number, no need for a class. The purchase history of a user is only relevant to the system that manages the purchase history. The avatar is only relevant to the UI. The authentication information to the authentication service.

A Customer is not in and of itself an "entity" of some sort with associated behavior in the OO sense. But you see this all the time with companies trying to model these taxonomies of their business as class hierarchies.

It's an unhelpful practice that imposes a structure on your code that's not relevant in a any way to the actual functionality of your software.

Thank you very much for for detailed response. Now I understand where you are going with it. It seems very powerful and would illicit endless expandability. Because I have not thought about modelling it on such abstract level before I am left with some questions as to how complex a model this leaves. I do not know if I am too locked in OOP-land but I like that with this thinking, you can have a SSN table for instance that references this ID and limit access to it on that level instead of having to scramble parts of tables for instance. I can also see it being useful for endless additions of tables that can relate to that ID.

But I cannot fully comprehend in which scenarios this would be a clear winner and which it would add too much overhead. There is something to it though, so thanks for opening my eyes.

What is a POD?

"Plain Old Data". An OO term for a simple record/struct with getters and setters and no associated behavior.

GUIs actually map quite well to that OOP model. Different sort of widgets where only the behavior is changed In children.

So does any hardware.

The deepest inheritance chains I saw in practice were the OOP GUI programming toolkits, they'd go six-seven deep and invariable started needing to clone logic to avoid diamond issues (they were always single inheritance it seemed).

Honestly OOP did a lot better in those than the procedural toolkits they replaced in many ways. It was generally "better".

So if we're going to dump OOP for ??? I would say that a really amazing GUI toolkit that is pretty clearly better than the classical OOP inheritance model would really underscore the point. I can see a compositional interface GUI toolkit that is great.

I have since those days not done a native UI, so all of the modern ones, from KDE/Qt, GNOME, whatever the hell replaced MFC on windows, etc I am clueless about.

Plus all UIs are kind of dominated by HTML/CSS/javascript style of code, which is its own special evolutionary tree at this point.

> The smalltalk ideal of OOP lives on in all sorts of ways, but the deep inheritance chain version that gets taught in classes?

What is the deepest chain we find in SmallTalk itself? I can't check it right now, but I'm sure it's not as deep as some classroom examples I've seen. Inheritance makes a lot of sense when you are modelling the world, but most structures we see in the real world aren't that deep.

Just looked in the current Squeak trunk image as an example. The max depth of a Class from Object (rather than from the metaclasses like Behavior, ProtoObject, etc -- these to me are irrelevant to the count) is 8. There are very few 8, 7, or even 6 deep classes. The vast majority are between the 1-3 range.

> This sort of begs the question: where does classical OOP, the one taught to all undergrad CS majors in programs that use Java or C++, really fit in nowadays?

Hot take: nowhere. I mostly kid. But really I don't know of any domain in which strict inheritance and data hiding beats mixins/composition, interfaces, and dataclasses. It's so much easier to reason about the behavior of an interface in a given role, than an "Object" which spans all kinds of scopes.

> This sort of begs the question: where does classical OOP, the one taught to all undergrad CS majors in programs that use Java or C++, really fit in nowadays?

It is not clear that such strawman OOP was ever really taught, let alone ever practiced. Mainly it seems to exist as something to be disparaged.

I have worked on several very large codebases for companies you've heard of in which mutable state, OOP and inheritance are used very heavily over the years. Its not a strawman - it has, despite everything I find wrong with it, generated working systems. Granted I've found them very bug prone and difficult to change compared to others, but there's very large sets of code and coders out there who use it all the time. Its also literally taught as if its a fundamental building block and thats what people will be using all the time.

Oh you sweet summer child :). Have an upvote just because I'm happy someone managed to avoid it. Did you know Facebook's iOS app has around 18000 classes? Yeah.

Edit: It's actually the iOS app, not the Android app, crazy either way. But yeah in university I was taught the whole "a cat is a feline which is a mammal which is an animal" shtick. Completely useless in the real world.

There is nothing wrong with ontological reasoning. A cat is a feline, how you express that relationship, or if it is even worth expressing, is another matter. We also learned “Cat(x) :- Feline(x)”, but never used that either (and no one ever derides the use of predicate logic in programming). I think we spent most of our smalltalk time covering metaobjects (I did my CS program before Java took over).

The problem with the OO style taxonomy is that doesn't just model the taxonomy but it also structures your code, that's usually where the problem is, and it leads to issues like what is the proper superclass: Square or Rectangle.

A square is a kind of rectangle but the "API" of a Square is more restrictive than that of a Rectangle (where both width and height can change independently). I've seen this issue discussed in either this year's or last year's CPP-CON...

> The problem with the OO style taxonomy is that doesn't just model the taxonomy but it also structures your code, that's usually where the problem is, and it leads to issues like what is the proper superclass: Square or Rectangle.

“Rectangle" is a superclass of “Square”, if you are referring to the entities in geometry.

> A square is a kind of rectangle but the "API" of a Square is more restrictive than that of a Rectangle (where both width and height can change independently)

Neither the width nor the height of either can change without changing the identity, just as neither the whole or fractional part of a real number can change without changing what number it is.

If you've got something with mutable side lengths, it's no longer a Square or Rectangle, it's probably some kind of Drawable that might have a mutable state variable for position and a mutable state variable for a shape, but just as numbers, but geometric shapes themselves, like numbers, are immutable values (they can be in a mutable container, but then you are changing which one is in the container, not changing the shape while retaining it's identity.)

The Square vs Rectangle issue is just an example. They shouldn't even be modeled as a hierarchy they should just be simple structs. It's just an example on how OO taxonomies are a silly (and detrimental) exercise.

> The Square vs Rectangle issue is just an example.

Yes, it's an example of a problem that has nothing to do with OOP and everything to do with applying model concepts from one domain to a different domain.

> They shouldn't even be modeled as a hierarchy they should just be simple structs.

Whether they are structs or objects with state and attached methods is an orthogonal concern to whether they form a type heirarchy. It's true that geometric squares and rectangles make sense as value types like structs. They also form part of a natural heirarchy that it is perfectly useful to leverage in code.

> It's just an example on how OO taxonomies are a silly (and detrimental) exercise.

Except that isn't what it is an example of. It's an example of why you can't apply a heirarchy from geometry to things which don't represent the concepts that heirarchy applies to. It doesn't show that OO taxonomies are silly or detrimental.

We will just have to agree to disagree then, for me the Square vs Rectangle inheritance issue is the simplest possible example of the problem, where trying to model an inheritance hierarchy "the right way" directly impacts the structure of your code in a detrimental way, and the solution is to avoid OOP silliness in its entirety.

Even in the immutable case if you have a Square inherit from Rectangle it still stores 2 separate fields for width and height (that it inherited), which are unnecessary. The only winning move is not to play.

You are building a straw man from your misunderstanding of the OOP technique. As 'dragonwriter' tried to explain it to you that mathematical model of a square being a kind of a rectangle has nothing to do with OOP modeling. Liskov substitution principle tells you that a rectangle cannot be a super class of a square.

If modeling taxonomies of the world is not OOP modeling, what is exactly? The very reason Barbara Liskov had to point out the damn principle is that people were doing dumb stuff like this.

They still do btw. Universities still teach that Mammals have a walk() method that you implement for Dog and Cat, except then you need to add a Whale and now you're screwed. Silly example, but real world cases are much more subtle. I have seen plenty of NotImplementedExceptions strung across various codebases.

If you haven't done Domain-Driven Design, with cute UML diagrams and everything, then we aren't talking about the same thing (and I have no idea what you are talking about).

Reflecting the business domain into class hierarchies and structuring your codebase around that structure has ruined more codebases than NULL ever did. Code structure should not be dictated by business taxonomy concerns, only by concrete business data.

> If modeling taxonomies of the world is not OOP modeling, what is exactly?

It is, but modelling the taxonomy of a different domain than the one you are operating in is not. “An mutable object whose current state will always correspond to some square and can be mutated to correspond to any square” and “A mutable object whose current state will always correspond to some rectangle and can be mutated to represent any rectangle” (which are the entities in the domain being discussed) are not the same things as “a square” and “a rectangle” (entities in the domain of geometry), and taking the is-a relationship that holds between the latter and trying to apply it to the former is bad modelling at a level prior to how it reduces to implementation in any particular programming paradigm.

> Universities still teach that Mammals have a walk() method that you implement for Dog and Cat, except then you need to add a Whale and now you're screwed.

To the extent this is true, it's a pedagogical problem not an paradigmatic one.

> If you haven't done Domain-Driven Design, with cute UML diagrams and everything

DDD is at least 3 decades more recent than OOP, and is not equivalent to it.

> It is, but modelling the taxonomy of a different domain than the one you are operating in is not.

What I'm trying to get across is that this happens all the time. All the freaking time. I will concede that attributing this to a flaw in OOP might be unfair to OOP, but this comment thread started from a comment on how this style of OOP modelling is a strawman. It's not a strawman because I've seen this happen all the time. Universities still teach it really poorly. It is still discussed in conferences.

Now, you can do proper OOP modelling that is actually useful. My approach is to use inheritance exclusively for the purpose of code reuse and even then only when it is trivially correct (if you need to think about it, that's enough of a sign to not use it). Interfaces are for is-a relationships so you can have a DAG instead of a tree (which is too limiting in practice). The other 3 pillars of OOP are very useful, meaning abstraction, subtype polymorphism and encapsulation.

But I only use OOP to model software entities that have actual behavior + data, not business concerns (I will rant about how "Customer" classes with associated methods are a bad idea till the cows come home, they break the single responsibility principle by default).

But, and this may be more of DDD problem than a OOP problem, I've seen people invest tons of effort into modeling UML diagrams where every class corresponds to some business concept, and then they think of all the little methods these classes should have, and then this gets turned into code.

The design is *always* wrong because the behavior is associated to the wrong data, and then you end up with the single responsibility principle broken, horrifically, everytime. The performance is abysmal because state is spread across RAM like my cats spread sand all over the house.

Maintenance is also a pain because you feel like you need to maintain this utterly suboptimal class hierarchy to fit the "business taxonomy" even if it doesn't help in any way with transforming data A into data B, so you add all these additional classes and abstractions and dependency injection to make it somewhat usable in practice.

Things get a lot easier when you focus on using paradigms as tools rather than ideals.

>If modeling taxonomies of the world is not OOP modeling, what is exactly?

You are still focused on a mathematical property of a square being a special case of a rectangle. Inheritance is not the tool to model such a relationship. Again it has nothing to do with taxonomy.

>Universities still teach,...

The same people teach functional programming with silly recursion examples and linked lists being the most important data structures.

OOP code can be convoluted, but a large imperative mess is worse. I haven't seen any large collaborative project written in a purely functional language, so I can't comment about that.

I'm not suggesting getting rid of OOP or switching to functional code. I've seen far worse horrors on "functional" codebases. Rather I'm merely complaining of something I've seen happen in practice, from companies that are "proudly OOP". The moment you turn a tool into an ideal, you end up with a mess. Doesn't matter what it is.

You're too hung up on the square vs rectangle example. It's just an example. Replace it with a big tree-shaped UML model encoding business concepts, and it's the same problem. Business concept relationships are a DAG or even an undirected graph and no edge in that graph should be given "preferential treatment" (as is the case for inheritance relationships, since they form a tree). The moment you do, you screwed up, and people do all the time.

> Liskov substitution principle tells you that a rectangle cannot be a super class of a square.

It does not. The Liskov substitution principle states, as it applies here, that a property that holds for all rectangles has to hold for all squares. This is true, as every square is a rectangle.

A constant rectangle can be a superclass of a constant square; a mutable rectangle cannot be a superclass of a mutable square (without some kind of type changing mutation?).

I think this generalizes to any "this is a more constrained that".

If the square and rectangles are not mutable, you can indeed treat a square as a rectangle (however you want to express that type). If they are mutable, then a square is of course not a rectangle.

In OOP systems that support predicate-style dynamic inheritance, rectangles pick up the square trait only when their width is equal to their height. We don't talk about such systems very much today (JavaScript does not have a notion of type, and updating an object's type is a mutable operation rather than one driven by a rule), but there were experimental systems from the 90s that looked at this.

Pretty cool to hear about those, didn't know there was such a dynamic approach to the problem.

Either way, the problem in my view is that the question is silly to begin with, Squares and Rectangles should be PODs (or, even better, simple structures), and not have any associated behavior. The decision of whether they are mutable or not then depends solely on the needs of the software, and not on irrelevant modelling constraints unrelated to the problem.

There's no need for them to have a taxonomy relation unless the problem being solved involves taxonomies of shapes.

What have you solved by having squares and rectangles as PODs? Now you won't be able to treat them as same in some way (e.g. IShape, IDrawable, IPrintable, IArea,...) and you will need twice as many functions for doing same things.

If you make a square and a rectangle into IShapes (with no inheritance hierarchy) you still need 2 separate implementations for GetWidth().

Either way, it's simply not true that you can't treat them the same way because you've made them PODs, there are more kinds of polymorphism beyond subtype polymorphism. Even for the latter modern languages split the data (structs) from the behavior (traits/protocols), see Rust or Swift for example.

If you explicitly need to treat squares and rectangles as a single entity to draw them (alongside other shapes), rather than thinking of each of them having a "draw()" function (likely breaking the single responsibility principle), you should instead have a function square_to_drawable, rectangle_to_drawable etc, where a drawable is also a POD but one that has the relevant drawing data (textures, triangles, whatever). Then a drawing system is responsible for actually rendering these drawables.

That's how it's done on modern game engines. It takes a while to wrap your head around but it works really well. This style is used a lot by modern game engines because performance is way better with this style.

Everything you are describing is still OOP. Again, inheritance in not necessarily the best tool to model all relationships.

The most basic principle of OOP is the bundling of behavior and data. If you don't have that, you don't have OOP.

PODs + independent functions is not OOP. ECS-style designs are not OOP because ECS-style designs are PODs + independent functions (it's a relational model). OOP and the relational model are not equivalent, if they were the object-relational impedance mismatch wouldn't exist.

Polymorphism exists independently of OOP. OOP is not the only way to model relationships. OOP is not the only way to encapsulate data. OOP is not the only way to abstract things. Sometimes it is by far the best way to do all three. That's when you use it, not because of some silly attempt at purity of style.

People have a really distorted view of procedural-style programming where it's all global variables and copy pasted code. Couldn't be further from the truth. Even within OO codebases you have tons of procedural-style code that just happens to be stored in a class with the little "static" keyword in front. Many "procedural" codebases (i.e. stuff written in C) have lots of OO-style code with structs full of function pointers.

But it's wrong to say that it's all just OOP in disguise. If behavior and data are not bundled it's not OOP, and most code doesn't need that, not even when you need abstraction + polymorphism. Encapsulation is the main one where OOP is often the best approach.

> This sort of begs the question: where does classical OOP, the one taught to all undergrad CS majors in programs that use Java or C++, really fit in nowadays?

As everyone is telling you, it’s a poor paradigm that makes the developer’s job harder compared to both non-OOP approaches and better approaches (mixins / multiple inheritance).

That said, one should be cognizant of the reason it was created in the first place - single inheritance allows C++ to implement fast polymorphism via virtual function tables.

> single inheritance allows C++ to implement fast polymorphism via virtual function

Multiple inheritance can be implemented to be as fast as single inheritance regarding virtual function calls, and typically is in C++ by keeping multiple virtual table pointers per object. The problem this brings is that we can have pointers/references to the different parts of the same same object. A Child* may point to the start of the object, but Parent* may point to somewhere in the middle (because that's where the parent's vtable pointer happens to be). This also means that casting a pointer can change it. This can introduce some "interesting" bugs, as you can imagine.

OTOH, there are languages like C# that don't do that, but require 2 "hops" when calling an interface method, causing a slight performance penalty (a class can inherit from at most one other class, but can implement multiple interfaces). But a reference to an object always points to the object's start, which is very important for garbage collector.

C++ supports multiple inheritance though. And it still does it with (multiple) vtables. Did you mean single dispatch?

No, I just had an incorrect memory / mental model of how C++ vtables work.

Yes, but multiple dispatch is much slower than single dispatch.