Comment by ryao | Hacker News Reader

Comment by ryao | Hacker News Reader

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Object oriented programming implies certain contracts that the compiler enforces that are not enforced with data abstraction. Given that object oriented programming and data abstraction two live side by side in C++, we can spot the differences between member functions that have contracts enforced, and members function pointers that do not. Member functions have an implicit this pointer, and in a derived class, can call the base class version via a shorthand notation to the compiler (BaseClass::func() or super()), unless that base class version is a pure virtual function. Member function pointers have no implicit this pointer unless one is explicitly passed. They have no ability to access a base class variant via some shorthand notation to the compiler because the compiler has no contract saying that OOP is being done and there is a base class version of this function. Finally, classes with unimplemented member functions may not be instantiated as objects, while classes with unimplemented member functions pointers may.

If you think of the differences as being OOP implies contracts with the compiler and data abstraction does not (beyond a simple structure saying where the members are in memory), it becomes easier to see the two as different things.

So you can opt out or in to syntactic sugar, that makes C++ an interesting and useful language, but how you implement OOP, doesn't really affect if it is OOP.

By this logic, C is an objective oriented language. It is widely held to not be. That is why there were two separate approaches to extend it to make it object oriented, C++ and Objective-C.

You can implement OOP in C as you can in any language, the article is an example of this. C is not an OOP language in any way, it doesn't have any syntactic features for it and use the term "object" for something different.

The article mentions file_operations, but ignores that it has what would be a static member function in C++ in the form of ->check_flags(), which is never in a vtable. The article author is describing overlap between object oriented programming and something else, called data abstraction, which is what is really being done inside Linux, and calling it OOP.

You can implement OOP in C if you do vtables for inheritance hierarchies manually, among other things, but that is different than what Linux does.

I honestly don't think how a C++ compiler chooses to implement an object method does matter here.

It's a function belonging to an object, to which is dynamically dispatched with something I would call a vtable. To me that sounds like a classic example of OOP.

Data abstraction is a core of OOP.

This pattern can be used to implement inheritance, when it isn't here that doesn't mean its not OOP.

Data abstraction is a separate invention from OOP since it involves abstract data types. What is being used here is an abstract data type. It is not the pattern used in OOP languages and it is not OOP. It bears similarities and overlap with the vtables used to implement some OOP languages. It is like how thumbs bear similarities and overlap with index fingers, but the two are not the same.

To cite Wikipedia:

> Object-oriented programming (OOP) is a programming paradigm based on the object – a software entity that encapsulates data and function(s). An OOP computer program consists of objects that interact with one another. A programming language that provides OOP features is classified as an OOP language [...]

You don't disagree, that this kernel pattern is about data abstraction. You probably don't disagree, that the kernel uses functions. The kernel uses "objects" (FS implementations) that follow a defined set of functions, sometimes called "class" (vtables/wtables/however you like to call them). Therefore I conclude what the kernel does here is a prime example of OOP.

You can use similar logic to declare English to be an example of Chinese. They both have syllables. They both assemble syllables into words that convey meaning. They both use grammars to form relationships between those words. Thus, they must be the same. It is fallacious logic. Some similarities do not make things the same. Data abstraction is also its own topic that is able to stand independently from OOP. What the kernel does is data abstraction, not OOP. What you are seeing in the kernel are the abstract data types of data abstraction.

Sorry no, thus are very different, but of course that's not what you arguing for.

I know ADT and OOP are different concepts, in another answer I wrote what I think the base differences are. But they are related, and in the definitions I am familiar with, ADTs are a base concept for OOP. And OOP can be an implementation for ADTs.

If you don't think the implementations I provide are enough to apply to the kernel, can maybe provide your own definition according to which we can evaluate this, because I feel like we are beating around the bush. But please not something that says this can only be happening in an OOP language, to these I would plainly disagree with, because OOP is a paradigm and not a property of a language.

> Object oriented programming implies certain contracts that the compiler enforces

Sorry, but where did you got this definition from? I've always thought OOP as a way of organizing your data and your code, sometimes supported by language-specific constructs, but not necessarily.

Can you organize your data into lists, trees, and hashmaps even if your language does not have those as native types? So you can think in a OO way even if the language has no notion of objects, methods, etc.

> Sorry, but where did you got this definition from?

It is from experience with object oriented languages (mainly C++ and Java). Technically, you can do everything manually, but that involves shoehorning things into the OO paradigm that do not naturally fit, like the article author did when he claimed struct file_operations was a vtable when it has ->check_flags(), which would be equivalent to a static member function in C++. That is never in a vtable.

If Al Viro were trying to restrict himself to object oriented programming, he would need to remove function pointers to what are effectively the equivalent of static member functions in C++ to turn it into a proper vtable, and handle accesses to that function through the “class”, rather than the “object”.

Of course, since he is not doing object oriented programming, placing pointers to what would be virtual member functions and static member functions into the same structure is fine. There will never be a use case where you want to inherit from a filesystem implementation’s struct file_operations, so there is no need for the decoupling that object oriented programming forces.

> I've always thought OOP as a way of organizing your data and your code, sometimes supported by language-specific constructs, but not necessarily.

It certainly can be, but it is not the only way.

> Can you organize your data into lists, trees, and hashmaps even if your language does not have those as native types?

This is an odd question. First, exactly what is a native type? If you mean primitive types, then yes. Even C++ does that. If you mean standard library compound types, again, yes. The C++ STL started as a third party library at SGI before becoming part of the C++ standard. If you mean types that you can define, then probably not without a bunch of pain, as then we are going back to the dark days of manually remembering offsets as people had to do in assembly language, although it is technically possible to do in both C and C++.

What you are asking seems to be exactly what data abstraction is, which involves making an interface that separates use and implementation, allowing different data structures to be used to organize data using the same interface. As per Wikipedia:

> For example, one could define an abstract data type called lookup table which uniquely associates keys with values, and in which values may be retrieved by specifying their corresponding keys. Such a lookup table may be implemented in various ways: as a hash table, a binary search tree, or even a simple linear list of (key:value) pairs. As far as client code is concerned, the abstract properties of the type are the same in each case.

https://en.wikipedia.org/wiki/Abstraction_(computer_science)...

Getting back to doing data structures without object oriented programming, this is often done in C using a structure definition and the CPP (C PreProcessor) via intrusive data structures. Those break encapsulation, but are great for performance since they can coalesce memory allocations and reduce pointer indirections for objects indexed by multiple structures. They also are extremely beneficial for debugging, since you can see all data structures indexing the object. Here are some of the more common examples:

https://github.com/openbsd/src/blob/master/sys/sys/queue.h

https://github.com/openbsd/src/blob/master/sys/sys/tree.h

sys/queue.h is actually part of the POSIX standard, while sys/tree.h never achieved standardization. You will find a number of libraries that implement trees like libuutil on Solaris/Illumos, glib on GNU, sys/tree.h on BSD, and others. The implementations are portable to other platforms, so you can pick the one you want and use it.

As for “hash maps” or hash tables, those tend to be more purpose built in practice to fit the data from what I have seen. However, generic implementations exist:

https://stackoverflow.com/questions/6118539/why-are-there-no...

That said, anyone using hash tables at scale should pay very close attention to how their hash function distributes keys to ensure it is as close to uniformly random as possible, or you are going to have a bad time. Most other applications would be fine using binary search trees. It probably is not a good idea to use hash tables with user controlled keys from a security perspective, since then a guy named Bob can pick keys that cause collisions to slow everything down in a DoS attack. An upgrade from binary search trees that does not risk issues from hash function collisions would be B-trees.

By the way, B-trees are containers and cannot function as intrusive data structures, so you give up some convenience when debugging if you use B-Trees.

> handle accesses to that function through the “class”, rather than the “object”

You don't need classes for OOP. C++ not putting methods that logically operate on an object, but don't need a pointer to it, into the automatically created vtable, is an optimization and an implementation detail. I don't know why you think that putting this function into a vtable precludes OOP.

Wait, how does inheritance work when the method is not in the vtable?

The calling convention for C++ non-static member functions always includes a this pointer, even if the function does not use it. Removing it on member functions that do not use it would pose a problem if another class inherited from this class and overrode the function definition with one that did use it. Maybe in very special cases whole program optimization could safely remove the this pointer, but it is questionable whether any compiler author would go through the trouble given that the exception handling would need to know about the change. Outside of whole program optimization, it is unlikely removing this from member functions that do not use it would ever happen because it would break ABI stability.

As for how inheritance works when the member function is not in the vtable, that depends on what kind of member function it is. All C++ functions are given a mangled name that is stuffed into C’s infrastructure for linking symbols. For static member functions, inheritance is irrelevant since they are tied to the class. Calls to static member functions go directly to the mangled function with no indirections, just as if it had been a global function. For non-static virtual member functions, you use the vtable pointer to find it. For non-virtual member functions, the call goes straight to the function as if a global function had been called (and the this pointer is still passed, even if the function does not use it), since the compiler knows the type and thus can tell the linker to have calls there go to the function through the appropriately mangled name. It is just like calling a global function.

> The calling convention for C++ non-static member functions always includes a this pointer, even if the function does not use it.

Yes. Since we are not in C++ we can choose to get rid of this useless pointer.

> Removing it on member functions that do not use it would pose a problem if another class inherited from this class and overrode the function definition with one that did use it.

That problem has nothing to do with the this pointer specifically. When you change the method signature of an inherited method you always have this problem. This simply means, that the superclass prescribes limits to subclasses, which is why it's possible to use a subclass inplace of a superclass.

> Maybe in very special cases whole program optimization could safely remove the this pointer, but it is questionable whether any compiler author

Yes, that's why its not done in C++, but we can do it, if we handroll it.

> it would break ABI stability

It does not if it has always been like this.

> For static member functions, inheritance is irrelevant since they are tied to the class. Calls to static member functions go directly to the mangled function with no indirections

In other words, ->check_flags() can't be implemented as a static member functions in C++. It would simply have a this pointer, that it just wouldn't use, since C++ has no way to express non-static member functions, that just don't take a this pointer.

> thus can tell the linker to have calls there go to the function

In our case the linker can only resolve the call to the appropriate vtable, since the type isn't known until runtime.

> Yes. Since we are not in C++ we can choose to get rid of this useless pointer.

If you were trying to implement OOP in the kernel in C and implemented a vtable, you cannot get rid of the this pointer in vtable entries since a child class might want to use it in the overrode definition. It is one of the same reasons why you cannot remove it in C++. The entire point of a vtable is to enable inheritance. If OOP really were being done, an out of tree module could make a class that inherits from this one without needing any code changes and use the this pointer, but you cannot do that if you drop the this pointer. I already explained this.

This is one interpretation. The other is that the interface of check_flags() specifies, that any implementation of it is only allowed to differ on the type of the object and not any other property.

You already prescribe with the chosen arguments in the superclass on which things the child implementation can depend. Why not also do this with the first argument?

You would typically put the this pointer into the first argument when doing OOP in C. You can put the this pointer in the last argument to have it work too. However, you cannot omit it entirely. That is something that is not OOP. It is an ADT.

So suppose you have it, but never use it. Then why have it you can just remove the first parameter. You can have object methods in C++ too, that don't use the this pointer.

Also why do you care exactly about the order of arguments? The nature of the function doesn't change, it's entirely arbitrary and orthogonal to the paradigm the function implements. Another example is the implementation of the equality operator between objects. In languages with syntactic sugar you typically have (self, other), but if its the true equality operator then the order doesn't matter.

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