pointer to avariable member function outside a class

[wilintec]

Hello, my question is this.
In C++, how do i call a method member of class A from a class B, using a pointer.
By the way Class A and B are of different types.

I read that when a pointer is pointing to member function it can only point member functions within the class. But how can i point to a member function outside the class.?????

for example

class A
{
public:

int add(int x) {

    return x+x;
}
};
int main()

 {

  typedef int (A::*pointer)();
  pointer func = &A::add;
  A objt;
  B objt2;

  obt2.*func(2);// the compiler give me an error of incompatible with object type ‘B’


 return 0;
  }