Question

I want to implement the Visitor pattern in C++ like this:

class Visitor{
public:
  virtual ~Visitor();
  virtual void visit(C & t)=0;
};

class V : public Visitor{
public:
  void visit(C &c);
};

class C{ 
public:
  void accept(Visitor &v){ v.visit(*this); }
};

But the compiler complains abount 2 syntax errors:
Unknown identifier C and Visitor.
Where is the problem?

Answer 1

At the moment the compiler sees

virtual void visit(C & t)=0;

name C is unknown.

You need to forward-declare class C before Visitor

class C;

class Visitor{
...
}