Circular dependencies of declarations

Question

I am trying to implement example of visitor pattern, but I have trouble with circular dependecies of declarations of classes. When I do forward declaration of class Visitor,

Answer 1

Alexy gave one part of the answer already.

However, if you're not going to implement accept for Land then you need:

class Land {
  public:
    virtual void accept(const Visitor *v)= NULL;
};