vptr

Let say we have below program: class A { public: virtual fun(){}; }; class B:public A { public: virtual fun(){}; }; int main() { A a1; B b1; } My question is how many vtables and how many vptrs will be created, when we run this program? Its heavily implementation dependent, but generally you'll get one vtable object per class that has any virtual functions (classes with no virtual functions or bases don't need them), and one vptr per object of a class with a vtable (pointing at the class's vtable). Things get more complex if you have multiple inheritance and virtual base classes -- which can

In Java: class Base { public Base() { System.out.println("Base::Base()"); virt(); } void virt() { System.out.println("Base::virt()"); } } class Derived extends Base { public Derived() { System.out.println("Derived::Derived()"); virt(); } void virt() { System.out.println("Derived::virt()"); } } public class Main { public static void main(String[] args) { new Derived(); } } This will output Base::Base() Derived::virt() Derived::Derived() Derived::virt() However, in C++ the result is different: Base::Base() Base::virt() // ← Not Derived::virt() Derived::Derived() Derived::virt() (See http://www

First of all, I want to make myself clear that I do understand that there is no notion of vtables and vptrs in the C++ standard . However I think that virtually all implementations implement the virtual dispatch mechanism in pretty much the same way (correct me if I am wrong, but this isn't the main question). Also, I believe I know how virtual functions work , that is, I can always tell which function will be called, I just need the implementation details. Suppose someone asked me the following: "You have base class B with virtual functions v1, v2, v3 and derived class D:B which overrides