visitor.hpp

#ifndef __VISITOR_HPP__
#define __VISITOR_HPP__

template<typename... Types>
struct Visitor;

template<typename... Visitors>
struct Visitable;

template<typename T>
struct Visitor<T>
{
    virtual void visit(T & visitable) = 0;
};

template<typename T, typename... Types>
struct Visitor<T, Types...> : public Visitor<Types...> 
{
    using Visitor<Types...>::visit;

    virtual void visit(T & visitable) = 0;
};

template<typename V>
struct Visitable<V>
{
    virtual void accept(V& visitor) = 0;
};

template<typename Derived, typename V>
struct Visitable<Derived, V> : Visitable<V>
{
    virtual void accept(V& visitor)
    {
        visitor.visit(dynamic_cast<Derived&>(*this));
    }
};

template<typename Derived, typename V, typename... Visitors>
struct Visitable<Derived, V, Visitors...> : public Visitable<Derived, Visitors...>, Visitable<V>
{
    using Visitable<Derived, Visitors...>::accept;

    virtual void accept(V& visitor)
    {
        visitor.visit(static_cast<Derived&>(*this));
    }
};

#endif // end __VISITOR_HPP__