3 ms·

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 lo

by ryao 1y ago

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.

1718627440 1y ago
It can still depend on the type, the answer just doesn't need information from the instance. What speed limits can this road possibly have is a question I want to ask about this specific road. Yet this can be answered by referring to the country, which is already known when you create the road. But the user that asks this question can still ask this about roads in different countries, so this question still is valid. Different objects can have different methods. When the method to be used is known at the time of object creation it can be chosen by assigning the appropriate function pointer in the vtable. The method itself might not necessarily need a instance pointer though.
- ryao 1y ago
  Let’s say I have ClassA::Foobar() and ClassB::Foobar(), and ClassB inherits from ClassA. Now let’s say I want to use ClassA::Foobar(), and I access it from the object because this pointer is in the vtable and the object is of type ClassB. Now ClassB::Foobar() executed, which is wrong. This is why static functions are not put into vtables. What Linux is doing is not a vtable, even if it shares similarities. Perhaps you would understand this if I said all thumbs are fingers, but not all fingers are thumbs.
  - 1718627440 1y ago
    That's a very good example. When I invoke object->Foobar() I want to invoke the appropriate method for this object, from whatever class that might be. This is exactly what's happening in the kernel here. When I actually intend to call the method from ClassA, I would either call something like object->base->Foobar() or ClassA->Foobar(object). Note how this is the very example that you are replying to: https://news.ycombinator.com/user?id=1718627440 https://news.ycombinator.com/user?id=1718627440
    - ryao 1y ago
      You don’t implement calls to static member functions by putting them into a vtable, which is one of many reasons why this is not a vtable. The proper way to implement static functions is through static dispatch.
      - 1718627440 1y ago
        > You don’t implement calls to static member functions by putting them into a vtable Yes, you claimed it is like a static member function, I don't think it can be. > The proper way to implement static functions is through static dispatch Yes, but we are talking about dynamic dispatch here. To quote your earlier comment: > which is most unlike a vtable since static member functions are never in vtables You conclude it isn't a vtable, I conclude, it's not a static member function, because it uses dynamic dispatch. I don't think we actually disagree on how and when to use static and dynamic dispatch.
        ryao 1y ago
        If you leave out the this pointer, it is the equivalent of putting a function pointer to a global function (or a static member function) into the structure. This is not how OOP works. Omitting the this pointer also breaks inheritance, since hypothetical child classes would not be able to override the definition while using the this pointer. Having to edit the parent class to be able to do that is not how OOP works. This is not OOP nor is it intended to be.
        1718627440 1y ago
        > If you leave out the this pointer, it is the equivalent of putting a function pointer to a global function (or a static member function) into the structure. No it is not. Unless you can show me a virtual overridden static member function. > since hypothetical child classes would not be able to override the definition while using the this pointer Yes! That's the entire idea here. The non-hypothetical, but indeed existing, child classes are not able to access the object. This is to prescribe potential behaviour for child implementations.

by ryao 1y ago

1718627440 1y ago

It can still depend on the type, the answer just doesn't need information from the instance. What speed limits can this road possibly have is a question I want to ask about this specific road. Yet this can be answered by referring to the country, which is already known when you create the road. But the user that asks this question can still ask this about roads in different countries, so this question still is valid. Different objects can have different methods. When the method to be used is known at the time of object creation it can be chosen by assigning the appropriate function pointer in the vtable. The method itself might not necessarily need a instance pointer though.

ryao 1y ago
Let’s say I have ClassA::Foobar() and ClassB::Foobar(), and ClassB inherits from ClassA. Now let’s say I want to use ClassA::Foobar(), and I access it from the object because this pointer is in the vtable and the object is of type ClassB. Now ClassB::Foobar() executed, which is wrong. This is why static functions are not put into vtables. What Linux is doing is not a vtable, even if it shares similarities. Perhaps you would understand this if I said all thumbs are fingers, but not all fingers are thumbs.
- 1718627440 1y ago
  That's a very good example. When I invoke object->Foobar() I want to invoke the appropriate method for this object, from whatever class that might be. This is exactly what's happening in the kernel here. When I actually intend to call the method from ClassA, I would either call something like object->base->Foobar() or ClassA->Foobar(object). Note how this is the very example that you are replying to: https://news.ycombinator.com/user?id=1718627440 https://news.ycombinator.com/user?id=1718627440
  - ryao 1y ago
    You don’t implement calls to static member functions by putting them into a vtable, which is one of many reasons why this is not a vtable. The proper way to implement static functions is through static dispatch.
    - 1718627440 1y ago
      > You don’t implement calls to static member functions by putting them into a vtable Yes, you claimed it is like a static member function, I don't think it can be. > The proper way to implement static functions is through static dispatch Yes, but we are talking about dynamic dispatch here. To quote your earlier comment: > which is most unlike a vtable since static member functions are never in vtables You conclude it isn't a vtable, I conclude, it's not a static member function, because it uses dynamic dispatch. I don't think we actually disagree on how and when to use static and dynamic dispatch.
      - ryao 1y ago
        If you leave out the this pointer, it is the equivalent of putting a function pointer to a global function (or a static member function) into the structure. This is not how OOP works. Omitting the this pointer also breaks inheritance, since hypothetical child classes would not be able to override the definition while using the this pointer. Having to edit the parent class to be able to do that is not how OOP works. This is not OOP nor is it intended to be.
        1718627440 1y ago
        > If you leave out the this pointer, it is the equivalent of putting a function pointer to a global function (or a static member function) into the structure. No it is not. Unless you can show me a virtual overridden static member function. > since hypothetical child classes would not be able to override the definition while using the this pointer Yes! That's the entire idea here. The non-hypothetical, but indeed existing, child classes are not able to access the object. This is to prescribe potential behaviour for child implementations.