Virtual table/dispatch table

Question

From what I know of CPP, each class has its own vtable.

However this wikipedia link mentions:

An object's dispatch table will contain the addresses of the object's dynamically bound methods. Method calls are performed by fetching the method's address from the object's dispatch table. The dispatch table is the same for all objects belonging to the same class, and is therefore typically shared between them.

Can someone please shed some light.

Thanks !

Answer 1

It's sometimes easier to understand with an example:

class PureVirtual {
   public:
       virtual void methodA() = 0;
       virtual void methodB() = 0;
};

class Base : public PureVirtual {
   public:
        virtual void methodA();
        void methodC();
   private:
        int x;
 };

 class Derived : public Base {
    public:
         virtual void methodB();
    private:
         int y;
 };

So, given an object of type Derived it might look like:

                         ------------
 Known offset for vtable |  0xblah  | -------> [Vtable for type "Derived"]
                         ------------
 Known offset for x      |  3       |
                         ------------
 Known offset for y      |  2       |
                         ------------

With the Vtable for type "Derived" looking something like:

                            ------------
 Known offset for "methodA" | 0xblah1   | ------> methodA from Base
                            -------------
 Known offset for "methodB" | 0xblah2   | ------> methodB from Derived
                            -------------

Note that since "methodC" was not virtual, it is not in the vtable at all. Also note that all instances of class Derived will have a vtable pointer to the same, shared vtable object (since they have the same type).

Though the implementations for C++ and Java are slightly different, the ideas are not incompatible. The key difference, from a conceptual standpoint, is that Java methods are "virtual" unless declared "final". In C++ the keyword "virtual" must be given explicitly for the function to be in the vtable. Anything not in vtable will be dispatched using the compile-time types rather than the runtime type of the object.