There are times, when you need to have a class which can be instantiated once only. The Singleton Design Pattern provides a solution for such a situation.
There are several possible ways to implement a singleton pattern, but it all pretty much comes down to a class that has a private constructor and a static member function to create and retrieve an instance of the class. My implementation does not differ much from this scenario, with the exception that I created a singleton template class.
So, why a template class?
Well I have searched the Internet for an elegant implementation of a singleton class but I did not really find a solution to my satisfaction. Most classes I found consist of a
Singleton base class that you can use to derive your own singleton class from. The problem with most of these classes is the fact that you still have to override the
GetInstance function to return a pointer to your derived class. A template base class does not have this limitation as I can return any type of pointer.
How it works
To prevent outside source from creating (or copying) an instance of our singleton class, we need to shield the constructor and copy constructor of the singleton class. Further we need to provide a method to create and retrieve a reference to the singleton object:
<PRE lang=c++>static T* Instance()
if (m_instance == NULL) m_instance = new T;
ASSERT(m_instance != NULL);
When this method is called for the first time, it creates an instance of the singleton class, any sequential calls will return a reference to the created class instance. To get a reference to the singleton object, all we have to do is call this method as following:
<PRE lang=c++>CMySingleton* mySingleton = CMySingleton::Instance();
That is almost all that there is to it. Next to shielding the constructors, I also shielded the destructor, so the singleton class cannot be deleted by accident. Just call the
DestroyInstance() method to destroy the singleton object. However, be careful when to call this method, because after you have called this method, all your class data will be destroyed and a sequential to the
Instance() method will create a new instance.
So how do you create a class derived from the singleton template class? Again there is nothing to it. Just include the attached header file and create your object as following:
<PRE lang=c++>class CMySingleton : public CSingleton<CMySingleton>
This implementation of the Singleton Pattern makes creating your own singleton class incredibly easy. But you do have to be careful when to destroy the singleton class instance. If you find this to be a problem you could consider adding (automatic) reference counting.