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vv2.h

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template <typename T>
class Visitor
{
   protected:
   //Defines the type that is being operated on
   typedef T ArgBase;

   virtual void do_visit(ArgBase &obj)=0;

   virtual void try_visit(ArgBase & obj)
   {
      do_visit(obj);
   }

   public:
   virtual void visit(ArgBase &obj)
   {
      do_visit(obj);
   }
};

namespace visitor 
{
   template <typename S, typename P>
   class SV: public P
   {
      protected:
      typedef P Parent;

      typedef S ArgSub;

      typedef typename Parent::ArgBase ArgBase;

      void try_visit(ArgBase &ob)
      {
    // We dynamically cast pointers because this is probably faster than
    // casting references
    ArgSub *s=dynamic_cast<ArgSub *>(&ob);
    if (s==NULL) Parent::try_visit(ob);
    else do_visit(*s);
      }

      virtual void do_visit(ArgSub &ob)=0;

      public:
      void visit(ArgBase &ob)
      {
    try_visit(ob);
      }
   };

   template <typename Mytype, typename ListRemain>
   class TlVisitor_helper
   {
   public:
      typedef SV<Mytype, typename TlVisitor_helper<typename ListRemain::Head, 
    typename ListRemain::Tail>::Ret> Ret;
   };


   template <typename Mytype>
   class TlVisitor_helper<Mytype, meta::Nil>
   {
   public:
      typedef Visitor<Mytype> Ret;
   };


   template <typename List>
   class TlVisitor
   {
   public:
      typedef typename TlVisitor_helper<typename List::Head, typename List::Tail>::Ret Ret;
   };
}

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