singleton

ensure a class has only one instance and provide a global point of access

applicability

• only one instance of a class
• instance must be extendable by subclassing

implementation

     // .h file 
  
     class Singleton {
     public:
  	static Singleton instance();
     protected: 
 limit access
  	Singleton();
     private:
  	static Singleton* _instance;
     };
  
  
     // .c file
  
     Singleton* Singleton::_instance = 0;
  
     Singleton* Singleton::instance() {
  	if (_instance == 0)
  		_instance = new Singleton();
  	return _instance;
  	}
  

consequences

• coordinate access to a single instance
• reduced namespace
• permit refinement
• permit variable number of instances

etcetera