source: lemon-0.x/src/lemon/concept/graph_component.h @ 1136:8d066154b66a

/* -*- C++ -*-
 * src/lemon/concept/graph_component.h - Part of LEMON, a generic C++ optimization library
 *
 * Copyright (C) 2004 Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
 * (Egervary Combinatorial Optimization Research Group, EGRES).
 *
 * Permission to use, modify and distribute this software is granted
 * provided that this copyright notice appears in all copies. For
 * precise terms see the accompanying LICENSE file.
 *
 * This software is provided "AS IS" with no warranty of any kind,
 * express or implied, and with no claim as to its suitability for any
 * purpose.
 *
 */

///\ingroup graph_concepts
///\file
///\brief The graph components.


#ifndef LEMON_CONCEPT_GRAPH_COMPONENT_H
#define LEMON_CONCEPT_GRAPH_COMPONENT_H

#include <lemon/invalid.h>
#include <lemon/concept/maps.h>

#include <lemon/alteration_notifier.h>

namespace lemon {
  namespace concept {

    /// \addtogroup graph_concepts
    /// @{

    /****************   Graph iterator concepts   ****************/

    /// Skeleton class for graph Node and Edge types

    /// This class describes the interface of Node and Edge (and UndirEdge
    /// in undirected graphs) subtypes of graph types.
    ///
    /// \note This class is a template class so that we can use it to
    /// create graph skeleton classes. The reason for this is than Node
    /// and Edge types should \em not derive from the same base class.
    /// For Node you should instantiate it with character 'n' and for Edge
    /// with 'e'.

#ifndef DOXYGEN
    template <char _selector = '0'>
#endif
    class GraphItem {
    public:
      /// Default constructor.
      
      /// \warning The default constructor is not required to set
      /// the item to some well-defined value. So you should consider it
      /// as uninitialized.
      GraphItem() {}
      /// Copy constructor.
      
      /// Copy constructor.
      ///
      GraphItem(GraphItem const&) {}
      /// Invalid constructor \& conversion.

      /// This constructor initializes the item to be invalid.
      /// \sa Invalid for more details.
      GraphItem(Invalid) {}
      /// Assign operator for nodes.

      /// The nodes are assignable. 
      ///
      GraphItem& operator=(GraphItem const&) { return *this; }
      /// Equality operator.
      
      /// Two iterators are equal if and only if they represents the
      /// same node in the graph or both are invalid.
      bool operator==(GraphItem) const { return false; }
      /// Inequality operator.
        
      /// \sa operator==(const Node& n)
      ///
      bool operator!=(GraphItem) const { return false; }

      /// Artificial ordering operator.

      /// To allow the use of graph descriptors as key type in std::map or
      /// similar associative container we require this.
      ///
      /// \note This operator only have to define some strict ordering of
      /// the items; this order has nothing to do with the iteration
      /// ordering of the items.
      ///
      /// \bug This is a technical requirement. Do we really need this?
      bool operator<(GraphItem) const { return false; }

      template<typename _GraphItem>
      struct Constraints {
        void constraints() {
          _GraphItem i1;
          _GraphItem i2 = i1;
          _GraphItem i3 = INVALID;
          
          i1 = i2 = i3;

          bool b;
          //      b = (ia == ib) && (ia != ib) && (ia < ib);
          b = (ia == ib) && (ia != ib);
          b = (ia == INVALID) && (ib != INVALID);
          //      b = (ia < ib);
        }

        const _GraphItem &ia;
        const _GraphItem &ib;
      };
    };

    /// A type describing the concept of graph node

    /// This is an instantiation of \ref GraphItem which can be used as a
    /// Node subtype in graph skeleton definitions
    typedef GraphItem<'n'> GraphNode;

    /// A type describing the concept of graph edge

    /// This is an instantiation of \ref GraphItem which can be used as a
    /// Edge subtype in graph skeleton definitions
    typedef GraphItem<'e'> GraphEdge;


    /**************** Basic features of graphs ****************/

    /// An empty base graph class.
  
    /// This class provides the minimal set of features needed for a graph
    /// structure. All graph concepts have to be conform to this base
    /// graph.
    ///
    /// \bug This is not true. The minimal graph concept is the
    /// BaseIterableGraphComponent.

    class BaseGraphComponent {
    public:

      typedef BaseGraphComponent Graph;
      
      /// Node class of the graph.

      /// This class represents the Nodes of the graph. 
      ///
      typedef GraphItem<'n'> Node;

      /// Edge class of the graph.

      /// This class represents the Edges of the graph. 
      ///
      typedef GraphItem<'e'> Edge;

      ///Gives back the target node of an edge.

      ///Gives back the target node of an edge.
      ///
      Node target(const Edge&) const { return INVALID;}

      ///Gives back the source node of an edge.

      ///Gives back the source node of an edge.
      ///
      Node source(const Edge&) const { return INVALID;}


      template <typename _Graph>
      struct Constraints {
        typedef typename _Graph::Node Node;
        typedef typename _Graph::Edge Edge;
      
        void constraints() {
          checkConcept<GraphItem<'n'>, Node>();
          checkConcept<GraphItem<'e'>, Edge>();
          {
            Node n;
            Edge e;
            n = graph.source(e);
            n = graph.target(e);
          }      
        }
      
        const _Graph& graph;
      };
    };

    /// An empty iterable base graph class.
  
    /// This class provides beside the core graph features
    /// core iterable interface for the graph structure.
    /// Most of the base graphs should be conform to this concept.

    class BaseIterableGraphComponent : virtual public BaseGraphComponent {
    public:

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// Gives back the first Node in the iterating order.
      
      /// Gives back the first Node in the iterating order.
      ///     
      void first(Node&) const {}

      /// Gives back the next Node in the iterating order.
      
      /// Gives back the next Node in the iterating order.
      ///     
      void next(Node&) const {}

      /// Gives back the first Edge in the iterating order.
      
      /// Gives back the first Edge in the iterating order.
      ///     
      void first(Edge&) const {}
      /// Gives back the next Edge in the iterating order.
      
      /// Gives back the next Edge in the iterating order.
      ///     
      void next(Edge&) const {}


      /// Gives back the first of the Edges point to the given Node.
      
      /// Gives back the first of the Edges point to the given Node.
      ///     
      void firstIn(Edge&, const Node&) const {}

      /// Gives back the next of the Edges points to the given Node.


      /// Gives back the next of the Edges points to the given Node.
      ///
      void nextIn(Edge&) const {}

      /// Gives back the first of the Edges start from the given Node.
      
      /// Gives back the first of the Edges start from the given Node.
      ///     
      void firstOut(Edge&, const Node&) const {}

      /// Gives back the next of the Edges start from the given Node.
      
      /// Gives back the next of the Edges start from the given Node.
      ///     
      void nextOut(Edge&) const {}


      template <typename _Graph>
      struct Constraints {
      
        void constraints() {
          checkConcept< BaseGraphComponent, _Graph >();
          typename _Graph::Node node;      
          typename _Graph::Edge edge;
          {
            graph.first(node);
            graph.next(node);
          }
          {
            graph.first(edge);
            graph.next(edge);
          }
          {
            graph.firstIn(edge, node);
            graph.nextIn(edge);
          }
          {
            graph.firstOut(edge, node);
            graph.nextOut(edge);
          }
        }

        const _Graph& graph;
      };
    };

    /// An empty idable base graph class.
  
    /// This class provides beside the core graph features
    /// core id functions for the graph structure.
    /// The most of the base graphs should be conform to this concept.
    /// The id's are unique and immutable.
    class IDableGraphComponent : virtual public BaseGraphComponent {
    public:

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// Gives back an unique integer id for the Node. 

      /// Gives back an unique integer id for the Node. 
      ///
      int id(const Node&) const { return -1;}

      /// \brief Gives back the node by the unique id.
      ///
      /// Gives back the node by the unique id.
      /// If the graph does not contain node with the given id
      /// then the result of the function is undetermined. 
      Node fromId(int id, Node) const { return INVALID;}

      /// \brief Gives back an unique integer id for the Edge. 
      ///
      /// Gives back an unique integer id for the Edge. 
      ///
      int id(const Edge&) const { return -1;}

      /// \brief Gives back the edge by the unique id.
      ///
      /// Gives back the edge by the unique id.
      /// If the graph does not contain edge with the given id
      /// then the result of the function is undetermined. 
      Edge fromId(int id, Edge) const { return INVALID;}

      template <typename _Graph>
      struct Constraints {

        void constraints() {
          checkConcept< BaseGraphComponent, _Graph >();
          typename _Graph::Node node;
          int nid = graph.id(node);
          nid = graph.id(node);
          node = graph.fromId(nid, Node());
          typename _Graph::Edge edge;
          int eid = graph.id(edge);
          eid = graph.id(edge);
          edge = graph.fromId(eid, Edge());
        }

        const _Graph& graph;
      };
    };


    /// An empty max-idable base graph class.
  
    /// This class provides beside the core graph features
    /// core max id functions for the graph structure.
    /// The most of the base graphs should be conform to this concept.
    /// The id's are unique and immutable.
    class MaxIDableGraphComponent : virtual public BaseGraphComponent {
    public:

      /// Gives back an integer greater or equal to the maximum Node id. 

      /// Gives back an integer greater or equal to the maximum Node id. 
      ///
      int maxId(Node = INVALID) const { return -1;}

      /// Gives back an integer greater or equal to the maximum Edge id. 

      /// Gives back an integer greater or equal to the maximum Edge id. 
      ///
      int maxId(Edge = INVALID) const { return -1;}

      template <typename _Graph>
      struct Constraints {

        void constraints() {
          checkConcept<BaseGraphComponent, _Graph>();
          int nid = graph.maxId(typename _Graph::Node());
          ignore_unused_variable_warning(nid);
          int eid = graph.maxId(typename _Graph::Edge());
          ignore_unused_variable_warning(eid);
        }
      
        const _Graph& graph;
      };
    };

    /// An empty extendable base graph class.
  
    /// This class provides beside the core graph features
    /// core graph extend interface for the graph structure.
    /// The most of the base graphs should be conform to this concept.
    class BaseExtendableGraphComponent : virtual public BaseGraphComponent {
    public:

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// Adds a new Node to the graph.

      /// Adds a new Node to the graph.
      ///
      Node addNode() {
        return INVALID;
      }
    
      /// Adds a new Edge connects the two Nodes to the graph.

      /// Adds a new Edge connects the two Nodes to the graph.
      ///
      Edge addEdge(const Node& from, const Node& to) {
        return INVALID;
      }

      template <typename _Graph>
      struct Constraints {
        void constraints() {
          checkConcept<BaseGraphComponent, _Graph >();
          typename _Graph::Node node_a, node_b;
          node_a = graph.addNode();
          typename _Graph::Edge edge;
          edge = graph.addEdge(node_a, node_b);
        }

        _Graph& graph;
      };
    };

    /// An empty erasable base graph class.
  
    /// This class provides beside the core graph features
    /// core erase functions for the graph structure.
    /// The most of the base graphs should be conform to this concept.
    class BaseErasableGraphComponent : virtual public BaseGraphComponent {
    public:

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// Erase a Node from the graph.
      
      /// Erase a Node from the graph. This function should not
      /// erase edges connecting to the Node.
      void erase(const Node&) {}    

      /// Erase an Edge from the graph.

      /// Erase an Edge from the graph.
      ///
      void erase(const Edge&) {}

      template <typename _Graph>
      struct Constraints {
        void constraints() {
          checkConcept<BaseGraphComponent, _Graph>();
          typename _Graph::Node node;
          graph.erase(node);
          typename _Graph::Edge edge;
          graph.erase(edge);
        }

        _Graph& graph;
      };
    };

    /// An empty clearable base graph class.
  
    /// This class provides beside the core graph features
    /// core clear functions for the graph structure.
    /// The most of the base graphs should be conform to this concept.
    class ClearableGraphComponent : virtual public BaseGraphComponent {
    public:

      /// Erase all the Nodes and Edges from the graph.

      /// Erase all the Nodes and Edges from the graph.
      ///
      void clear() {}    

      template <typename _Graph>
      struct Constraints {
        void constraints() {
          checkConcept<BaseGraphComponent, _Graph>();
          graph.clear();
        }

        _Graph graph;
      };
    };


    /// Skeleton class for graph NodeIt and EdgeIt

    /// Skeleton class for graph NodeIt and EdgeIt.
    ///
    template <typename _Graph, typename _Item>
    class GraphIterator : public _Item {
    public:
      /// \todo Don't we need the Item type as typedef?

      /// Default constructor.
      
      /// @warning The default constructor sets the iterator
      /// to an undefined value.
      GraphIterator() {}
      /// Copy constructor.
      
      /// Copy constructor.
      ///
      GraphIterator(GraphIterator const&) {}
      /// Sets the iterator to the first item.
      
      /// Sets the iterator to the first item of \c the graph.
      ///
      explicit GraphIterator(const _Graph&) {}
      /// Invalid constructor \& conversion.

      /// This constructor initializes the item to be invalid.
      /// \sa Invalid for more details.
      GraphIterator(Invalid) {}
      /// Assign operator for items.

      /// The items are assignable. 
      ///
      GraphIterator& operator=(GraphIterator const&) { return *this; }      
      /// Next item.

      /// Assign the iterator to the next item.
      ///
      GraphIterator& operator++() { return *this; }
      //        Node operator*() const { return INVALID; }
      /// Equality operator

      /// Two iterators are equal if and only if they point to the
      /// same object or both are invalid.
      bool operator==(const GraphIterator&) const { return true;}
      /// Inequality operator
        
      /// \sa operator==(Node n)
      ///
      bool operator!=(const GraphIterator&) const { return true;}
      
      template<typename _GraphIterator>
      struct Constraints {
        void constraints() {
          //      checkConcept< Item, _GraphIterator >();
          _GraphIterator it1(g);
        
          /// \todo Do we need NodeIt(Node) kind of constructor?
          //    _GraphIterator it2(bj);
          _GraphIterator it2;

          it2 = ++it1;
          ++it2 = it1;
          ++(++it1);
          /// \bug This should be: is_base_and_derived<BaseItem, _GraphIterator>
          _Item bi = it1;
          bi = it2;
        }
        _Graph& g;
      };
    };

    /// Skeleton class for graph InEdgeIt and OutEdgeIt

    /// \note Because InEdgeIt and OutEdgeIt may not inherit from the same
    /// base class, the _selector is a additional template parameter. For 
    /// InEdgeIt you should instantiate it with character 'i' and for 
    /// OutEdgeIt with 'o'.
    /// \todo Is this a good name for this concept?
    template <typename Graph,
              typename Edge = typename Graph::Edge,
              char _selector = '0'>
    class GraphIncIterator : public Edge {
    public:
      /// Default constructor.
      
      /// @warning The default constructor sets the iterator
      /// to an undefined value.
      GraphIncIterator() {}
      /// Copy constructor.
      
      /// Copy constructor.
      ///
      GraphIncIterator(GraphIncIterator const&) {}
      /// Sets the iterator to the first edge incoming into or outgoing 
      /// from the node.
      
      /// Sets the iterator to the first edge incoming into or outgoing 
      /// from the node.
      ///
      explicit GraphIncIterator(const Graph&, const typename Graph::Node&) {}
      /// Invalid constructor \& conversion.

      /// This constructor initializes the item to be invalid.
      /// \sa Invalid for more details.
      GraphIncIterator(Invalid) {}
      /// Assign operator for nodes.

      /// The nodes are assignable. 
      ///
      GraphIncIterator& operator=(GraphIncIterator const&) { return *this; }      
      /// Next edge.

      /// Assign the iterator to the next node.
      ///
      GraphIncIterator& operator++() { return *this; }

      //        Node operator*() const { return INVALID; }

      /// Equality operator

      /// Two iterators are equal if and only if they point to the
      /// same object or both are invalid.
      bool operator==(const GraphIncIterator&) const { return true;}

      /// Inequality operator
        
      /// \sa operator==(Node n)
      ///
      bool operator!=(const GraphIncIterator&) const { return true;}

      template <typename _GraphIncIterator>
      struct Constraints {
        typedef typename Graph::Node Node;
        void constraints() {
          checkConcept<GraphItem<'e'>, _GraphIncIterator>();
          _GraphIncIterator it1(graph, node);
          /// \todo Do we need OutEdgeIt(Edge) kind of constructor?
          //    _GraphIncIterator it2(edge);
          _GraphIncIterator it2;

          it2 = ++it1;
          ++it2 = it1;
          ++(++it1);
          Edge e = it1;
          e = it2;
        }

        Edge& edge;
        Node& node;
        Graph& graph;
      };
    };


    /// An empty iterable base graph class.
  
    /// This class provides beside the core graph features
    /// iterator based iterable interface for the graph structure.
    /// This concept is part of the StaticGraphConcept.
    class IterableGraphComponent : virtual public BaseGraphComponent {

    public:
    
      typedef IterableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// This iterator goes through each node.

      /// This iterator goes through each node.
      ///
      typedef GraphIterator<Graph, Node> NodeIt;
      /// This iterator goes through each node.

      /// This iterator goes through each node.
      ///
      typedef GraphIterator<Graph, Edge> EdgeIt;
      /// This iterator goes trough the incoming edges of a node.

      /// This iterator goes trough the \e inccoming edges of a certain node
      /// of a graph.
      typedef GraphIncIterator<Graph, Edge, 'i'> InEdgeIt;
      /// This iterator goes trough the outgoing edges of a node.

      /// This iterator goes trough the \e outgoing edges of a certain node
      /// of a graph.
      typedef GraphIncIterator<Graph, Edge, 'o'> OutEdgeIt;
    };
    
    template <typename _Graph> 
    struct Constraints {
      void constraints() {
        checkConcept< BaseGraphComponent, _Graph>();

        checkConcept<GraphIterator<_Graph, typename _Graph::Edge>,
          typename _Graph::EdgeIt >();
        checkConcept<GraphIterator<_Graph, typename _Graph::Node>,
          typename _Graph::NodeIt >();
        checkConcept<GraphIncIterator<_Graph>, typename _Graph::InEdgeIt >();
        checkConcept<GraphIncIterator<_Graph>, typename _Graph::OutEdgeIt >();
      }
    };

    /// An empty alteration notifier base graph class.
  
    /// This class provides beside the core graph features
    /// alteration notifier interface for the graph structure.
    /// This is an observer-notifier pattern. More Obsevers can
    /// be registered into the notifier and whenever an alteration
    /// occured in the graph all the observers will notified about it.
    class AlterableGraphComponent : virtual public BaseGraphComponent {
    public:

      /// The edge observer registry.
      typedef AlterationNotifier<Edge> EdgeNotifier;
      /// The node observer registry.
      typedef AlterationNotifier<Node> NodeNotifier;
      
      /// \brief Gives back the edge alteration notifier.
      ///
      /// Gives back the edge alteration notifier.
      EdgeNotifier getNotifier(Edge) const {
        return EdgeNotifier();
      }
      
      /// \brief Gives back the node alteration notifier.
      ///
      /// Gives back the node alteration notifier.
      NodeNotifier getNotifier(Node) const {
        return NodeNotifier();
      }
      
    };


    /// Class describing the concept of graph maps

    /// This class describes the common interface of the graph maps
    /// (NodeMap, EdgeMap), that is \ref maps-pages "maps" which can be used to
    /// associate data to graph descriptors (nodes or edges).
    template <typename Graph, typename Item, typename _Value>
    class GraphMap : public ReadWriteMap<Item, _Value> {
    protected:      
      GraphMap() {}
    public:
      /// \brief Construct a new map.
      ///
      /// Construct a new map for the graph.
      explicit GraphMap(const Graph&) {}
      /// \brief Construct a new map with default value.
      ///
      /// Construct a new map for the graph and initalise the values.
      GraphMap(const Graph&, const _Value&) {}
      /// \brief Copy constructor.
      ///
      /// Copy Constructor.
      GraphMap(const GraphMap&) {}
      
      /// \brief Assign operator.
      ///
      /// Assign operator.
      GraphMap& operator=(const GraphMap&) { return *this;}

      template<typename _Map>
      struct Constraints {
        void constraints() {
          checkConcept<ReadWriteMap<Item, _Value>, _Map >();
          // Construction with a graph parameter
          _Map a(g);
          // Constructor with a graph and a default value parameter
          _Map a2(g,t);
          // Copy constructor. Do we need it?
          _Map b=c;
          // Copy operator. Do we need it?
          a=b;

          ignore_unused_variable_warning(a2);
        }

        const _Map &c;
        const Graph &g;
        const typename GraphMap::Value &t;
      };

    };

    /// An empty mappable base graph class.
  
    /// This class provides beside the core graph features
    /// map interface for the graph structure.
    /// This concept is part of the StaticGraphConcept.
    class MappableGraphComponent : virtual public BaseGraphComponent {
    public:

      typedef MappableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// ReadWrite map of the nodes.
    
      /// ReadWrite map of the nodes.
      ///
      template <typename _Value>
      class NodeMap : public GraphMap<Graph, Node, _Value> {
      private:
        NodeMap();
      public:
        /// \brief Construct a new map.
        ///
        /// Construct a new map for the graph.
        /// \todo call the right parent class constructor
        explicit NodeMap(const Graph&) {}
        /// \brief Construct a new map with default value.
        ///
        /// Construct a new map for the graph and initalise the values.
        NodeMap(const Graph&, const _Value&) {}
        /// \brief Copy constructor.
        ///
        /// Copy Constructor.
        NodeMap(const NodeMap&) {}

        /// \brief Assign operator.
        ///
        /// Assign operator.
        NodeMap& operator=(const NodeMap&) { return *this;}

      };

      /// ReadWrite map of the edges.
    
      /// ReadWrite map of the edges.
      ///
      template <typename _Value>
      class EdgeMap : public GraphMap<Graph, Edge, _Value> {
      private:
        EdgeMap();
      public:
        /// \brief Construct a new map.
        ///
        /// Construct a new map for the graph.
        /// \todo call the right parent class constructor
        explicit EdgeMap(const Graph&) {}
        /// \brief Construct a new map with default value.
        ///
        /// Construct a new map for the graph and initalise the values.
        EdgeMap(const Graph&, const _Value&) {}
        /// \brief Copy constructor.
        ///
        /// Copy Constructor.
        EdgeMap(const EdgeMap&) {}

        /// \brief Assign operator.
        ///
        /// Assign operator.
        EdgeMap& operator=(const EdgeMap&) { return *this;}

      };

      template <typename _Graph>
      struct Constraints {

        struct Type {
          int value;
          Type() : value(0) {}
          Type(int _v) : value(_v) {}
        };

        void constraints() {
          checkConcept<BaseGraphComponent, _Graph>();
          { // int map test
            typedef typename _Graph::template NodeMap<int> IntNodeMap;
            checkConcept<GraphMap<_Graph, typename _Graph::Node, int>, 
              IntNodeMap >();
          } { // bool map test
            typedef typename _Graph::template NodeMap<bool> BoolNodeMap;
            checkConcept<GraphMap<_Graph, typename _Graph::Node, bool>,
              BoolNodeMap >();
          } { // Type map test
            typedef typename _Graph::template NodeMap<Type> TypeNodeMap;
            checkConcept<GraphMap<_Graph, typename _Graph::Node, Type>,
              TypeNodeMap >();
          } 

          { // int map test
            typedef typename _Graph::template EdgeMap<int> IntEdgeMap;
            checkConcept<GraphMap<_Graph, typename _Graph::Edge, int>,
              IntEdgeMap >();
          } { // bool map test
            typedef typename _Graph::template EdgeMap<bool> BoolEdgeMap;
            checkConcept<GraphMap<_Graph, typename _Graph::Edge, bool>,
              BoolEdgeMap >();
          } { // Type map test
            typedef typename _Graph::template EdgeMap<Type> TypeEdgeMap;
            checkConcept<GraphMap<_Graph, typename _Graph::Edge, Type>, 
              TypeEdgeMap >();
          } 
        }

        _Graph& graph;
      };
    };

    /// \brief An empty extendable extended graph class.
    ///
    /// This class provides beside the core graph features
    /// item addition interface for the graph structure.
    /// The difference between this class and the 
    /// \c BaseExtendableGraphComponent is that it should
    /// notify the item alteration observers.
    class ExtendableGraphComponent : virtual public BaseGraphComponent {
    public:

      typedef ExtendableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// \brief Add a node to the graph.
      ///
      /// Add a node to the graph and notify the observers.
      Node addNode() {
        return INVALID;
      }
    
      /// \brief Add an edge to the graph.
      ///
      /// Add an edge to the graph and notify the observers.
      Edge addEdge(const Node& from, const Node& to) {
        return INVALID;
      }

      template <typename _Graph>
      struct Constraints {
        void constraints() {
          checkConcept<BaseGraphComponent, _Graph >();
          typename _Graph::Node node_a, node_b;
          node_a = graph.addNode();
          typename _Graph::Edge edge;
          edge = graph.addEdge(node_a, node_b);      
        }
        _Graph& graph;
      };
    };

    /// \brief An empty erasable extended graph class.
    ///
    /// This class provides beside the core graph features
    /// item erase interface for the graph structure.
    /// The difference between this class and the 
    /// \c BaseErasableGraphComponent is that it should
    /// notify the item alteration observers.
    class ErasableGraphComponent : virtual public BaseGraphComponent {
    public:

      typedef ErasableGraphComponent Graph;

      typedef BaseGraphComponent::Node Node;
      typedef BaseGraphComponent::Edge Edge;

      /// \brief Erase the Node and notify the node alteration observers.
      ///
      ///  Erase the Node and notify the node alteration observers.
      void erase(const Node&) {}    

      /// \brief Erase the Edge and notify the edge alteration observers.
      ///
      ///  Erase the Edge and notify the edge alteration observers.
      void erase(const Edge&) {}

      template <typename _Graph>
      struct Constraints {
        void constraints() {
          checkConcept<BaseGraphComponent, _Graph >();
          typename _Graph::Node node;
          graph.erase(node);
          typename _Graph::Edge edge;
          graph.erase(edge);      
        }

        _Graph& graph;
      };
    };

    /// @}

  }

}

#endif