Can we have one vtable shared my multiple classes

Question

Is it possible in C++ to have one vtable shared by multiple classes? As per my understanding if a class is having a virtual function then it will generate a vtable.So every class should have its own vtable.

Answer 1

Polymorphism could be implemented several ways. And vtables could also be implemented several ways. But usually in following case

class A {
    virtual foo(){}
}

class B : public A {
    virtual foo(){}
}

class C : public B {
    void fooNonVirtual(){};
}

classes B and C should have the same vtable.

Example:

A *a = new A;
B *b = new B;
C *c = new C;
a->foo(); // A::foo is called
((A*)b)->foo(); // B::foo is called
((A*)c)->foo(); // B::foo is called

foo for a and b calls different methods because A and B has different vtables. For b and c the same method is called. So compiler could make some optimization and create only one vtable for B and C classes.

fooNonVirtual is not virtual and do not require vtables at all.