What does "<vtable for A + 16>" mean?

Question

This is my code. I just want to take a look at the virtual inheritance memory layout.

#include<iostream>
using namespace std;

class A{
    private:
        int a;
    public:
        virtual void print() const{
            cout << a << endl;
        }

};

class B:public virtual A{
    private:
        int b;
    public:
        void print() const{
            cout << b << endl;
        }
};



int main(){
    A a;
    B b;
    return 0;
}

Then in gdb I used

p a
p b

Output signal

(gdb) p a
$1 = {
  _vptr.A = 0x400b40 <vtable for A+16>, 
  a = 0
}
(gdb) p b
$2 = {
  <A> = {
    _vptr.A = 0x400b18 <vtable for B+56>, 
    a = 4196384
  }, 
  members of B: 
  _vptr.B = 0x400af8 <vtable for B+24>, 
  b = 0
}
(gdb) 

I know the meanings of _vptr.A and _vptr.B, but I don’t understand what vtable means for B + 24 or + 16.

Thanks for the answer!

Answer 1

The list asays that it is a class with two fields: a vptr, which points to a byte with an offset of 16 inside the vtable for A and a data item acontaining zero.

The second - along the same lines, but more complex due to the virtual base class. He says that b consists of three fields: an instance of A with fields similar to those indicated above, and then two elements of B, again as above. This time vptr is directed to offset 24.

vptr , ? gcc, , . , RTTI ( ), . .

NB. , 2 , vtable: , RTTI. 64 ( , ), 16 . B 24 , .