Object-oriented design patterns in C and kernel development

Post by frutiger » Thu, Aug 28, 2025, 11:01 AM UTC

1718627440 wrote: post #50

moregrist wrote: post #36

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> In Smalltalk and Objective-C, you just check at runtime whether an object instance responds to a message. This is the original OOP way. This introduces performance issues larger than the typical ones associated with vtable lookups. Not all domains can afford this today and even fewer in the 80s/90s when these languages were first designed. > It's sad that OOP was corrupted by the excessively class-centric C++ and J…

When you fear about the branch miss, then you can also just call the method and catch the SIGSEGV. I think if you allow for the possibility of there being no implementation, then you can't really not have some decision there. This would also apply for say a C++ virtual method.

Post by pjmlp » Thu, Aug 28, 2025, 12:25 PM UTC

f1shy wrote: post #18

ryao wrote: post #11

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My point is that this pattern is not object oriented programming. As for a default behavior with it, you usually would do that by either always adding the default pointer when creating the structure or calling the default whenever the pointer is NULL. In the Linux VFS for example, there are optimized functions for reading and writing, but if those are not implemented, a fallback to unoptimized functions is done at th…

To be fair, OOP is not 100% absolutely perfectly defined. Strustrup swears C++ is OOP, Alan Key, at least at some point laughed at C++, and people using CLOS have yet another definition

Post by 1718627440 » Thu, Aug 28, 2025, 4:05 PM UTC

ryao wrote: post #152

1718627440 wrote: post #142

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As it isn't the compiler that creates the vtable, you can also have the equivalent of this as the last parameter or where you want it to be. > It could also contain variables that are non-pointers. The convention of it being a pure vtable is that it just doesn't. > Neither is allowed in a vtable Who is the vtable membership authority? :-)

You can take the API for a linked list and implement a balanced binary search tree behind it, but continuing to call it a linked list after doing that is wrong. Similarly, you can do pointer indirections the same way a vtable would do them, but if the things you get are not the equivalent of member function pointers, it is not a vtable.

Post by 1718627440 » Thu, Aug 28, 2025, 4:16 PM UTC

ryao wrote: post #114

1718627440 wrote: post #85

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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…

Post by 1718627440 » Thu, Aug 28, 2025, 4:33 PM UTC

ryao wrote: post #154

zozbot234 wrote: post #134

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The term "vtable" is not exclusive to C++. Trait function dictionaries in Rust are called vtables and behave as described here, there's not necessarily any 'this' or 'self' object.

The point of the vtable is to allow dynamic dispatch based on the actual type of an object. When you have a function that does not need a this pointer, it no longer depends on the type of the object and putting it in there anyway could cause you to execute a variant depending on the type of the object, which seems like a buggy undesirable behavior.