Virtual Table Explanation

class B1 {
public:
void f0() {}
virtual void f1() {}
int int_in_b1;
};

class B2 {
public:
virtual void f2() {}
int int_in_b2;
};
---------------------------
class D : public B1, public B2 {
public:
void d() {}
void f2() {} // override B2::f2()
int int_in_d;
};
----------------

B2 *b2 = new B2();
D *d = new D();
------------------------------
g++ produce the following 32-bit memory layout:

b2:
+0: pointer to virtual method table of B2
+4: value of int_in_b2

virtual method table of B2:
+0: B2::f2()


d:
+0: pointer to virtual method table of D (for B1)
+4: value of int_in_b1
+8: pointer to virtual method table of D (for B2)
+12: value of int_in_b2
+16: value of int_in_d

Total size: 20 Bytes.

virtual method table of D (for B1):
+0: B1::f1() // B1::f1() is not overridden

virtual method table of D (for B2):
+0: D::f2() // B2::f2() is overridden by D::f2()

Note that for those function not carrying the virtual keyword don't display in the vtable. The D::f2() has overriden B::f2() in the virtual table.