Can the class have more than one vtable?

Question

say, we have such classes inheritance

class A
{
    int a;
public:
    A(){}
    virtual void f1(){}
};

class B
{
    int b;
public:
    B(){}
    virtual void f2(){}
    virtual void f3(){}
};

class C: public A, public B
{
    int c;
public:
    C(){}
    virtual void f1(){}
    virtual void f2(){}
    virtual void f3(){}
    virtual void f4(){}
};

And the object: C c;

In object c we have vptr to one vtable, consists of methods of A+C, and another vtable consists of methods for B. For example x64 build:

• +0: vtable A+C
• +8: int a + padding to 8 byte
• +16: vtable B
• +24: int b + padding to 8 byte
• +32: int c + padding to 8 byte

Class C have two instance of vtable:

1. for call own virtual methods and overriden methods of A
2. for call overriden methods of B

I'm confused about statement, that one class have one vtable if it is polymorphic

Answer 1

The C++ standard doesn't say how this should be implemented. It's up to the compiler if it does it by using multiple vtables, combines the vtables [in such a way that they can then be "split" again, because C must be possible to make into a B type object again].

Using multiple vtables is quite a common solution in compilers, so you will see this solution in at least MS's VC++, GNU's g++, LLVM/clang++ and ARM's armcc++. Since I don't know how it works in other compilers, I can't say if other compilers do use this method or not (quite possibly).

To be clear, it is a popular method, but the standard doesn't say how this should be done.

In the future, this may change - someone may come up with a different solution that is better (by some definition of better - e.g. takes up less space, has faster access, etc)