Design Patterns

1. Design Patterns • Difficult to describe abstractly • Elements: • Pattern Name • Problem to Solve • Solution • Consequences: Results and Trade-Offs • Higher level abstraction than classes

2. Example: Singleton • Intent: Define a class pattern that is guaranteed to have only one instance, and provide a global point of access to it. • Uses • A good global variable/class • Simulator has one radar, one physical display, etc.

3. Singleton Implementation Class Singleton { public: static Singleton* Instance(); protected: Singleton(); private: static Singleton* _instance; }; Singleton* Singleton::Instance = 0; Singleton* Singleton::Instance() { if (_instance == 0) { _instance = new Singleton; } return _instance; }

4. Example: Abstract Factory • Provide and interface for creating families of related or dependent objects without specifying their concrete class. • A system can be created that is independent of how its elements(products) are created, composed, and represented • Participants: • Abstract Factory (WidgetFactory): interface to the client • Concrete Factory(MotifWidgetFactory, MFCWidgetFactory): implements the abstract interface • Abstract Product(Window, Scrollbar): what the client sees • Concrete Product(MotifWindow, MotifScrollBar): defines the actual object • Client: who uses the products via the abstract interfaces

5. Iterator • Goal: Provide a way to sequentially access the elements of an aggregate object without exposing its underlying representation. • Using an iterator, it would be possible to replace an array with a linked list or binary tree • This construct is heavily used in STL

6. Iterator Implementation Template <class Item> class Iterator { public: virtual void First() = 0; virtual void Next()= 0; virtual bool IsDone() const = 0; virtual Item CurrentItem const = 0; protected: Iterator(); };

7. List Iterator Template <class Item> class ListIterator : public Iterator<Item> { public: ListIterator(const List<Item>* aList); virtual void First(); … private: const List<Item>* _list; long _current; };

8. STL • Standard Template Libaray: Part of C++ • Objects • vector • list • map & multimap • set & multiset • deque (double ended queues) • stack, queue, priority_queue

9. STL cont’d • All objects have iterators • Generic Algorithms can be done on these objects • for_each, find, count, equal, search, sort, fill, merge, etc.