Design Pattern ->Singleton

Layering & Contract Philosophy With additional indirection

class CSingleton
{
     Protect: CSingleton();
     Protect: CSingleton(const CSingleton&);
     Protect: CSingleton& operator=(const CSingleton& rhs);
     Private: ~CSingleton();
     Private: static CSingleton *pInstance;
     Public: static CSingleton * GetInstance() 
    { 
           if ( pInstance != NULL ) pInstance = new CSingleton;  
                  return pInstance;
     }
}
CSingleton::pInstance = NULL;

The Singleton pattern is different from Mono-state pattern which has several static variables shared by all objects of class.

Applicability

Use the Singleton pattern when:

• there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point.
• when the sole instance should be extensible by subclass, and clients should be able to use an extended instance without modifying their code.

Participants

• defines an Instance operation that lets clients access its unique instance. Instance is a class operation (that is, a class method in Smalltalk and a static member function with a return values which also is static member and the constructor must be declared as private for preventing from creating a object with class type  in C++).It may be responsible for creating its own unique instance.

Collaborations

• Clients access a Singleton instance solely through Singleton's Instance operation.

One Reference:http://www.cnblogs.com/Leo_wl/archive/2012/07/19/2599063.html