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A Singleton Template Class

, 19 Mar 2003
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A Singleton Template Class

Introduction

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:

static T* Instance()
{
if (m_instance == NULL) m_instance = new T;
            ASSERT(m_instance != NULL);
return m_instance;
};

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:

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:

class CMySingleton : public CSingleton<CMySingleton>
{
    friend CSingleton<CMySingleton>;   
 
       private:      

           CMySingleton();
           ~CMySingleton();
 
    ...
}

Conclusion

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.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

About the Author

Brian van der Beek
Web Developer
Netherlands Netherlands
Brian van der Beek has been working as a software engineer at Philips TASS since 1999. He worked on a Digital Video Broadcasting system based on COM components. After that he has worked on test and factory equipment for DVD+RW drives. And since the last 2 year he is working on firmware for the Philips/BenQ Blu-ray drive.

Comments and Discussions

 
GeneralAnother good link PinmemberPeace ON20-Jul-13 8:41 
QuestionAnother possibility PinmemberJames_7225-Jul-11 19:03 
GeneralSingleton flawed PinmemberMark Ginnane24-Jul-10 15:03 
GeneralI want to subclass CMySingleton but I'm having problems... PinmemberDemented-TED24-Mar-09 19:06 
Questionwont compile on vs2005. Pinmembermiche_mannen26-Sep-07 9:39 
Generalimplemetation PinmemberPaul the Great18-Sep-07 1:33 
GeneralSimilar threadsafe (+older) article: PinmemberPaul Evans9-Nov-03 21:51 
Hey there,
 
http://www.codeproject.com/cpp/pdesingletontemplatenomfc.asp
is my take on the subject, may help you out if this didn't switch on any light-bulbs! Smile | :)
 
Cheers,
 
Paul

 
/**********************************
Paul Evans, Dorset, UK.
Personal Homepage "EnjoySoftware" @
http://www.enjoysoftware.co.uk/
**********************************/

GeneralYour sample makes you still have to destroy the Singleton. PinmemberWREY6-Jul-03 12:01 
GeneralRe: You sample makes you still have to destroy the Singleton. PinmemberBrian van der Beek6-Jul-03 20:34 
GeneralRe: Your sample makes you still have to destroy the Singleton. PinmemberWREY7-Jul-03 0:34 
GeneralArticle updated: PinmemberBrian van der Beek20-Mar-03 20:58 
GeneralDoes not fully implement the singleton pattern I'm afraid PinmemberJörgen Sigvardsson20-Mar-03 13:05 
GeneralRe: Does not fully implement the singleton pattern I'm afraid PinmemberJim A. Johnson20-Mar-03 13:53 
GeneralRe: Does not fully implement the singleton pattern I'm afraid PinmemberJörgen Sigvardsson20-Mar-03 22:24 
GeneralRe: Does not fully implement the singleton pattern I'm afraid PinmemberBrian van der Beek20-Mar-03 20:31 
QuestionUhhhhh? Singleton Template? PinsussAnonymous20-Mar-03 12:37 
AnswerRe: Uhhhhh? Singleton Template? PinmemberBrian van der Beek20-Mar-03 20:14 
GeneralRe: Uhhhhh? Singleton Template? PinsussAnonymous21-Mar-03 8:16 
GeneralIf templates are your thing PineditorBarry Lapthorn20-Mar-03 11:58 
GeneralRe: If templates are your thing PinmemberJoão Paulo Figueira20-Mar-03 13:02 
GeneralRe: If templates are your thing PinmemberJörgen Sigvardsson20-Mar-03 13:11 
GeneralRe: If templates are your thing PinmemberPeter Hancock20-Mar-03 13:35 
GeneralRe: If templates are your thing PinmemberBrian van der Beek20-Mar-03 20:11 

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