virtual method table for multiple-inheritance

Question

I\'m reading this article \"Virtual method table\"

Example in the above article:

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


        

Answer 1

We cannot pass the D pointer to a virtual function overriding B2::f2(), because all overrides of the same virtual function must accept identical memory layout.

Since B2::f2() function expects B2's memory layout of the object being passed to it as its this pointer, i.e.

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

the overriding function D::f2() must expect the same layout as well. Otherwise, the functions would no longer be interchangeable.

To see why interchangeability matters consider this scenario:

class B2 {
public:
  void test() { f2(); }
  virtual void f2() {}
  int int_in_b2;
};
...
B2 b2;
b2.test(); // Scenario 1
D d;
d.test(); // Scenario 2

B2::test() needs to make a call of f2() in both scenarios. It has no additional information to tell it how this pointer has to be adjusted when making these calls^*. That is why the compiler passes the fixed-up pointer, so test()'s call of f2 would work both with D::f2() and B2::f2().

^* Other implementations may very well pass this information; however, multiple inheritance implementation discussed in the article does not do it.