Question

In C++, I have a class A, and a class B.

In class A, there is a object (of class B) , I want to change the class A member data in the object of class B. How can I do that ?

I want to do this:

class A {
    public:
      A() {
          new B(this); 
      }
    private:
      int i;
};

class B {
  public:
     B(A* parent) {
        this->parent = parent;
     }

     change() {
         parent->i = 5;
     }
private:
     A* parent;
};

Answer 1

Rather than setting B as a friend class to A, a better method to preserve encapsulation would be to add a setter method to class A.

Your class A would then look somewhat like this:

class A {
    public:
      A() {
          new B(this); 
      }

      void set_i(int value)
      {
         i = value;
      }
    private:
      int i;
};

Then in your class B implementation, call set_i().

class B {
  public:
     B(A* parent) {
        this->parent = parent;
     }

     change() {
         parent->set_i(5);
     }
private:
     A* parent;
};

This way you’re not exposing and relying on private implementation details of class A in class B.