Code organizing

Question

I have this visitor pattern implementation:

class Visitor {
  public:
    Visitor() {}
    void visit(A *a){ a->doSomething(); }
    void visit(B *b){ b->doSomething(); }
};

class Base {
  public:
    Base() {}
    virtual void accept(Visitor *v) = 0;
};

class A: public Base {
  public:
    A():Base() {}
    void accept(Visitor *v) override { v->visit(this) };
    .....
};

class B: public Base {
  public:
    B():Base() {}
    void accept(Visitor *v) override { v->visit(this) };
};

The problem is that A and B have incomplete types in the Visitor implementation. But I don't want to put implementation in a separate source file.

Is there a way to put it in a single header file?

Answer 1

Do forward declaration A and B. Move your visit(A*a); and visit(B*b); outside of Visitor's class declaration and declare it after A and B classes. To avoid duplicate symbols put this to header file, for example Header.h:

//forward declarations
class A;
class B;

class Visitor {
public:
    Visitor() {}
    void visit(A *a);
    void visit(B *b);
};

class Base {
public:
    Base() {}
    virtual void accept(Visitor *v) = 0;
};

class A: public Base {
public:
    A():Base() {}
    void accept(Visitor *v) override { v->visit(this); };
    void doSomething(){};
};

class B: public Base {
public:
    B():Base() {}
    void accept(Visitor *v) override { v->visit(this); };
    void doSomething(){};
};

This declaration should be putted in cpp file, to avoid duplicate symbols, for example add it to main.cpp:

#include "Header.h"

//outside declarations of methods
void Visitor::visit(A *a){ a->doSomething(); }
void Visitor::visit(B *b){ b->doSomething(); }