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

You must have the definition of class B before you use the class. How else would the compiler otherwise know that there exists such a function as B::add?

Either define class B before class A, or move the body of A::doSomething to after class B have been defined, like

class B;

class A
{
    B* b;

    void doSomething();
};

class B
{
    A* a;

    void add() {}
};

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