Where to place common function which will be used by class and class member?

Question

For example,

class B {
  int* b1;
  int* b2;
  B(int* x, int* y) {
     b1 = x;
     b2 = y;
  }

};

class A {
  int* a1;
  int* a2;
  B* b1;
  public:
  A() {
    a1 = new int;
    a2 = new int;
    b1 = new B1(a1, b1);
  }
};

I want to access values pointed by a1 and a2 from objects of both class A & B. Where i should write function for the same, so that this function can be called by objects of A and objects of B. ?

Answer 1

In this special context, you are only passing pointers so the objects pointed by a1 and a2 are the objects pointed by b1 and b2. On the other hand, an A object knows what B it contains, but an instance of B has no knowledge about being contained in an A.

Because of that, I think that the function manipulating the pointed objects should be declared in B class. Then if the existence of B is member of the public API of A (with a getter for example) it is enough:

class B {
    ...
    void do_something(...) {
        // do something with objects pointed by b1 and b2
    }
};
...
A a;
a.getB()->do_something();
...

If the B member of A is an implementation detail, you should declare a relay method in A:

class A {
    ...
    void do_something(...) {     // delegate to the `B` member
        b1->do_something(...);
    }
};
...
A a;
a.do_something(...);
...