error: member access into incomplete type : forward declaration of

Question

I have two classes in the same .cpp file:

// forward
class B;

class A {       
   void doSomething(B * b) {
      b->add();
   }
};

class B {
   void a         


        

Answer 1

Move doSomething definition outside of its class declaration and after B and also make add accessible to A by public-ing it or friend-ing it.

class B;

class A
{
    void doSomething(B * b);
};

class B
{
public:
    void add() {}
};

void A::doSomething(B * b)
{
    b->add();
}