// -*- mode:C++ -*-

 #ifndef HUGO_INVALID_H

 #define HUGO_INVALID_H

 ///\file

 ///\brief Definition of INVALID.

 namespace hugo {

   /// Dummy type to make it easier to make invalid iterators.

   /// See \ref INVALID, how to use it.

   struct Invalid {

   public:

     bool operator==(Invalid) { return true;  }

     bool operator!=(Invalid) { return false; }

     bool operator< (Invalid) { return false; }

   };

   /// Invalid iterators.

   /// \ref Invalid is a global type that converts to each iterator

   /// in such a way that the value of the target iterator will be invalid.

   // It is also used to convert the \c INVALID constant to the

   // node iterator that makes is possible to write 

   //extern Invalid INVALID;

   //const Invalid &INVALID = *(Invalid *)0;

   const Invalid INVALID = Invalid();

 } //namespace hugo

 #endif