deba@1418: namespace lemon {
deba@1418: /*!
deba@1418: 
deba@1418: \page adaptor-references Smart Reference Handling
deba@1418: 
deba@1418: The adaptor classes are very useful tools in the lemon library. It makes
deba@1418: possible to create various data view of the current data structures without
deba@1418: of the copy of them. This makes the lemon programming efficient and 
deba@1418: consederably fast.
deba@1418: 
deba@1418: \section problem The problem
deba@1418: 
deba@1418: The adaptors usually use references or pointers to reference to an 
alpar@1591: existing data structure. We may use an algorithm in the following way:
deba@1418: \code
deba@1418: function_algorithm(adaptor(structure));
deba@1418: \endcode 
deba@1418: 
deba@1418: But what about the class algorithms:
deba@1418: \code
deba@1418: class_algorithm alg;
deba@1418: alg.add(adaptor(structure));
deba@1418: alg.run();
deba@1418: \endcode
deba@1418: The algorithm store a reference to the given structure. It is a created
deba@1418: as temporarly and when the expression of the \c add() function is evaluated it
deba@1418: will be destructed. It is very dangerous handling of the adaptors.
deba@1418: 
deba@1418: \section solution The solution
deba@1418: 
deba@1418: We made reference to a temporarly constructed adaptor but we should make
deba@1418: a copy of the adaptor when it is given as parameter to a class.
deba@1418: It is not impossible with a little tranformation of the code.
deba@1418: 
deba@1418: Let's first create some helper class:
deba@1418: \code
deba@1418: template <typename _Type, typename Enable = void>
deba@1418: struct SmartConstReference {
deba@1418:   typedef const _Type& Type;
deba@1418: };
deba@1418:   
deba@1418: template <typename _Type>
deba@1418: struct SmartConstReference<
deba@1418:   _Type, 
deba@1418:   typename enable_if<typename _Type::NeedCopy, void>::type
deba@1418: > {
deba@1418:   typedef const _Type Type;
deba@1418: };
deba@1418: \endcode
deba@1418: Then we can place NeedCopy tags in the adaptor classes:
deba@1418: \code
deba@1418: template<class M1,class M2> 
deba@1418: class AddMap {
deba@1418:   typename SmartConstReference<M1>::Type m1;
deba@1418:   typename SmartConstReference<M2>::Type m2;
deba@1418: 
deba@1418: public:
deba@1418: 
deba@1418:   typedef True NeedCopy;
deba@1418: 
deba@1418:   typedef typename M1::Key Key;
deba@1418:   typedef typename M1::Value Value;
deba@1418: 
deba@1418:   AddMap(const M1 &_m1,const M2 &_m2) : m1(_m1), m2(_m2) {};
deba@1418:   Value operator[](Key k) const {return m1[k]+m2[k];}
deba@1418: };
deba@1418: \endcode
deba@1418: Then we can transform all of the template map and graph references to
deba@1418: \c SmartConstReference<Map>::Type or \c SmartConstReference<Graph>::Type.
deba@1418: This way we copy all of maps and graphs what we should copy but the
deba@1418: greater data structures are not copied.
deba@1418: 
deba@1418: \section not-const If the adaptor is not const
deba@1418: The solution is very similar but it gives an additional problem.
deba@1418: We make the same \c SmartReferences:
deba@1418: \code
deba@1418: template <typename _Type, typename Enable = void>
deba@1418: struct SmartReference {
deba@1418:   typedef _Type& Type;
deba@1418: };
deba@1418: 
deba@1418: template <typename _Type>
deba@1418: struct SmartReference<
deba@1418:   _Type, 
deba@1418:   typename enable_if<typename _Type::NeedCopy, void>::type
deba@1418: > {
deba@1418:   typedef _Type Type;
deba@1418: };
deba@1418: \endcode
deba@1418: Let's create a class that use it:
deba@1418: \code
deba@1418: template <typename Map>
deba@1418: class Algorithm {
deba@1418: public:
deba@1418:   SmartReference<Map>::Type map;
deba@1418:   Algorithm(Map& _map) : map(_map) {}
deba@1418:   ...
deba@1418: };
deba@1418: \endcode
deba@1418: But if we want to give an adaptor function as parameter
deba@1418: it will be a compile error. The adaptor will be created as temporarly
deba@1418: so it cannot give as reference just as const reference.
deba@1418: 
deba@1418: Make more helper class:
deba@1418: \code
deba@1418: template <typename _Type, typename Enable = void>
deba@1418: struct SmartParameter {
deba@1418:   typedef _Type& Type;
deba@1418: };
deba@1418: 
deba@1418: template <typename _Type>
deba@1418: struct SmartParameter<
deba@1418:   _Type, 
deba@1418:   typename enable_if<typename _Type::NeedCopy, void>::type
deba@1418: > {
deba@1418:   typedef const _Type& Type;
deba@1418: };
deba@1418: \endcode
deba@1418: And the repaired code:
deba@1418: \code
deba@1418: template <typename Map>
deba@1418: class Algorithm {
deba@1418: public:
deba@1418:   SmartReference<Map>::Type map;
deba@1418:   Algorithm(SmartParameter<Map>::Type _map) : map(_map) {}
deba@1418:   ...
deba@1418: };
deba@1418: \endcode
deba@1418: But some times it does not help:
deba@1418: \code
deba@1418: class Algorithm {
deba@1418: public:
deba@1418:   ...
deba@1418: 
deba@1418:   template <typename Map>
deba@1418:   void addMap(SmartParameter<Map>::Type _map) {
deba@1418:     ...
deba@1418:   }
deba@1418:   ...
deba@1418: };
deba@1418: \endcode
deba@1418: Actually, it is a good code but the template parameter should
deba@1418: be written because it cannot be found out from the parameter type.
deba@1418: This can be solved with a bigger transformation:
deba@1418: \code
deba@1418: class Algorithm {
deba@1418: public:
deba@1418:   ...
deba@1418: 
deba@1418:   template <typename Map>
deba@1418:   void addMap(const Map& _map) {
deba@1418:     _addMap<SmartParameter<Map>::Type, Map>(_map);
deba@1418:   }
deba@1418:   template <typename Map>
deba@1418:   void addMap(const Map& _map) {
deba@1418:     _addMap<SmartParameter<Map>::Type, Map>(_map);
deba@1418:   }
deba@1418: 
deba@1418: private:
deba@1418:   template <typename MapParameter, typename Map>
deba@1418:   void _addMap(MapParameter _map) {
deba@1418:     ...
deba@1418:   }
deba@1418:   ...
deba@1418: };
deba@1418: \endcode
deba@1418: This way we solved the smart referencing problem.
deba@1418: \author Balazs Dezso
deba@1418: */
deba@1418: }