Re: Derivable singleton implementation

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 28 Dec 2009 10:23:52 -0500

Message-ID:

<#QwTPH9hKHA.2184@TK2MSFTNGP04.phx.gbl>

Faisal wrote:

I have a generic Singleton class.

//Singleton.h

template <class TYPE>
class CSingleTon
{
protected:
CSingleTon()
{}

virtual ~CSingleTon(){}

public:
static TYPE* Instance()
{
if( NULL == m_pInstance )
m_pInstance = new TYPE();

return m_pInstance;
}

static void Destroy()
{
delete m_pInstance;
m_pInstance = NULL;
}

protected:
static TYPE* m_pInstance;
};

template <class TYPE>
TYPE* CSingleTon<TYPE>::m_pInstance = NULL;

#define SET_SINGLETON( classname ) friend class CSingleTon<classname>;

//end of file Singleton.h

And I uses this class to create singletone classes
like

class CGlobalDataStore : public CSingleTon<CGlobalDataStore>
{
SET_SINGLETON(CGlobalDataStore);
};

This is working correctly.

Now I want to derive a class from CGlobalDataStore( which is also
singleton).

Don't derive from CSingleTon in the first place. You can do something =
like this:

class CGlobalDataStore {
  static CGlobalDataStore* Instance() {
    return CSingleTon<CGlobalDataStore>::Instance();
};

class CAnotherStore: public CGlobalDataStore {
  static CAnotherStore* Instance() {
    return CSingleTon<CAnotherStore>::Instance();
};

I don't quite see how this design makes sense though. You now have two =
singletons - one CGlobalDataStore and one CAnotherStore - so there are =
two instances of CGlobalDataStore in the program (one on its own and one =
as a subobject of CAnotherStore), which kind of defeats the point of it =
being a singleton.
--
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not =
necessarily a good idea. It is hard to be sure where they are going to =
land, and it could be dangerous sitting under them as they fly overhead. =
-- RFC 1925