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> while OP use void pointers OP doesn't use void pointers, he uses void. He writes about functions having no arguments and returning nothing for the same reas

> while OP use void pointers OP doesn't use void pointers, he uses void. He writes about functions having no arguments and returning nothing for the same reason other blog posts name functions foo and bar. > OP uses this vtable as a form of indirection to implement runtime method swapping and polymorphism The kernel uses vtables to implement polymorphism, it doesn't store the vtable in the object to save space. If there is no polymorphism, you don't use a vtable at all, that's saving even more space.

wosined 1y ago
> If there is no polymorphism, you don't use a vtable at all, that's saving even more space. Not true. You use a vtable even if there is no polymorphism, in cases where you want to have objects store pointers to their methods (OO interface), but don't want each instance to have pointers to all methods. I was referring to this article, which the OP links in his post https://lwn.net/Articles/444910/ https://lwn.net/Articles/444910/
- 1718627440 1y ago
  When do objects need to have function pointers? If the function never changes per object, then you would just directly call the function. When do you use function pointers? When the caller only knows the type of struct foo_ops, but not the actual called function. That's polymorphism.
  - wosined 1y ago
    > When do objects need to have function pointers? You can redefine the methods at runtime. If for example I have a logger that logs on the local drive and another one that logs on a networked drive I can swap the method pointer. This is not polymorphism since the object does have to change. I can also load the function from a shared library that is unknown during compilation and use that as part of the objects methods.
    - 1718627440 1y ago
      I would call that polymorphism, since the object type the caller sees, still stays the same, i.e. the vtable itself or some containing struct/class. So as far as the caller is concerned, the object he has access to behaves differently.

wosined 1y ago

> If there is no polymorphism, you don't use a vtable at all, that's saving even more space. Not true. You use a vtable even if there is no polymorphism, in cases where you want to have objects store pointers to their methods (OO interface), but don't want each instance to have pointers to all methods. I was referring to this article, which the OP links in his post https://lwn.net/Articles/444910/ https://lwn.net/Articles/444910/

1718627440 1y ago
When do objects need to have function pointers? If the function never changes per object, then you would just directly call the function. When do you use function pointers? When the caller only knows the type of struct foo_ops, but not the actual called function. That's polymorphism.
- wosined 1y ago
  > When do objects need to have function pointers? You can redefine the methods at runtime. If for example I have a logger that logs on the local drive and another one that logs on a networked drive I can swap the method pointer. This is not polymorphism since the object does have to change. I can also load the function from a shared library that is unknown during compilation and use that as part of the objects methods.
  - 1718627440 1y ago
    I would call that polymorphism, since the object type the caller sees, still stays the same, i.e. the vtable itself or some containing struct/class. So as far as the caller is concerned, the object he has access to behaves differently.