doc_external_value_traits.cpp

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/////////////////////////////////////////////////////////////////////////////
//
// (C) Copyright Ion Gaztanaga  2007
//
// Distributed under the Boost Software License, Version 1.0.
//    (See accompanying file LICENSE_1_0.txt or copy at
//          http://www.boost.org/LICENSE_1_0.txt)
//
// See http://www.boost.org/libs/intrusive for documentation.
//
/////////////////////////////////////////////////////////////////////////////
//[doc_external_value_traits
#include <boost/intrusive/list.hpp>
#include <vector>

using namespace boost::intrusive;

//This type is not modifiable so we can't store hooks or custom nodes
typedef int identifier_t;

//This value traits will associate elements from an array of identifiers with
//elements of an array of nodes. The element i of the value array will use the
//node i of the node array:
class external_traits
{
   //Non-copyable
   external_traits(const external_traits &);
   external_traits& operator=(const external_traits &);

   public:
   typedef list_node_traits<void*>           node_traits;
   typedef node_traits::node                 node;
   typedef node *                            node_ptr;
   typedef const node *                      const_node_ptr;
   typedef identifier_t                      value_type;
   typedef identifier_t *                    pointer; 
   typedef const identifier_t *              const_pointer; 
   static const link_mode_type link_mode =   normal_link;

   external_traits(pointer ids, std::size_t NumElements)
      :  ids_(ids),  nodes_(NumElements)
   {}

   ///Note: non static functions!
   node_ptr to_node_ptr (value_type &value)
      {  return &this->nodes_[0] + (&value - this->ids_); }
   const_node_ptr to_node_ptr (const value_type &value) const
      {  return &this->nodes_[0] + (&value - this->ids_); }
   pointer to_value_ptr(node_ptr n)
      {  return this->ids_ + (n - &this->nodes_[0]); }
   const_pointer to_value_ptr(const_node_ptr n) const
      {  return this->ids_ + (n - &this->nodes_[0]); }

   private:
   pointer  ids_;
   //This is an array of nodes that is necessary to form the linked list
   std::vector<list_node_traits<void*>::node> nodes_;
};

//This is the value traits class that will be stored in the container
//and that will lead to the external traits using the address
//of the container.
struct internal_traits
{
   static const bool external_value_traits = true;
   typedef external_traits value_traits; 

   template<class Container>
   value_traits &get_value_traits(Container &cont);

   template<class Container>
   const value_traits &get_value_traits(const Container &cont) const;
};

//The intrusive list that will use external value traits
typedef list<identifier_t, value_traits<internal_traits> > List;

class data_holder
   :  public List
{
   public:
   data_holder(identifier_t *ids, std::size_t NumElements)
      :  List()
      ,  external_traits_(ids, NumElements)
   {}
   external_traits external_traits_;
};

template<class Container>
internal_traits::value_traits &internal_traits::get_value_traits(Container &cont)
{  return static_cast<data_holder&>(cont).external_traits_; }

template<class Container>
const internal_traits::value_traits &internal_traits::get_value_traits(const Container &cont) const
{  return static_cast<const data_holder&>(cont).external_traits_; }

int main()
{
   const int NumElements = 100;

   //This is an array of ids that we want to "store"
   identifier_t ids [NumElements];

   //Initialize id objects, each one with a different number
   for(int i = 0; i != NumElements; ++i)   ids[i] = i;

   //The data holding the list and the external traits
   data_holder data(ids, NumElements);

   //This list will store ids without modifying identifier_t instances
   //Stateful value traits must be explicitly passed in the constructor.
   List &my_list = data;

   //Insert ids in reverse order in the list
   for(identifier_t * it(&ids[0]), *itend(&ids[NumElements]); it != itend; ++it)
      my_list.push_front(*it);

   //Now test lists
   List::const_iterator   list_it (my_list.cbegin());
   identifier_t *it_val(&ids[NumElements]-1), *it_rbeg_val(&ids[0] -1);

   //Test the objects inserted in the base hook list
   for(; it_val != it_rbeg_val; --it_val, ++list_it){
      if(&*list_it  != &*it_val)   return 1;
   }

   return 0;
}
//]