Using a Variable of an Instance within the Method of another Class in C++

I was wondering how I would go about using a variable of a specific instance of a class within a function of another class.

To provide an example of what I'm trying to do, say I've 3 classes a,b and c. Class c inherits from class b, and a single instance of b and c are called within a method in class a and b respectively. How would I go about using the variable of int pos (see below) within a specific instance of class a in class c?

class a
{
    private:
    void B(); //Calls an instance of class c
    int pos; //Variable that I want to use in c
};

class b : public c
{
    private:
    void C(); //Calls an instance of class b
};

class c
{
    private:
    void calculate(int _pos); //Method which requires the value of pos from class a 
};

Help would be greatly appreciated, thank you!