Re: about static member

From:

"Giovanni Dicanio" <giovanniDOTdicanio@REMOVEMEgmail.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Wed, 3 Dec 2008 11:04:26 +0100

Message-ID:

<uP0DJ7SVJHA.4928@TK2MSFTNGP05.phx.gbl>

"Tony Johansson" <t.johansson@logica.com> ha scritto nel messaggio
news:%23hg7roSVJHA.4888@TK2MSFTNGP02.phx.gbl...

In this class I have two objects of type CSession and
CDataSource.
I want to be able to access these two from the class itself and not from
the instance
so I tried to declare them as static in the the class Test
But when I do so I get the following link errror.

AlterObj.obj : error LNK2001: unresolved external symbol "public: static
class ATL::CSession Handle_DS::Session"
(?Session@Handle_DS@@2VCSession@ATL@@A)
ReleaseMinDependency/DTHToolObjects.dll : fatal error LNK1120: 1
unresolved externals

Probably CSession is not designed to be defined as 'static' data member?

However, did you try using the classic Singleton pattern?

It builds fine for me on VS2008 (you can just use
CSingleton::GetInstance()->GetSession() to access the CSession "static"
object):

<code>

#include <atlbase.h> // ATL base header
#include <atldbcli.h> // CSession and CDataSource

//
// Use a singleton pattern to get "static" access to CSession and
CDatasource
//
class CSingleton
{
public:

    // Access the only one singleton instance
    static CSingleton* GetInstance();

    ~CSingleton()
    {
        m_isCreated = false;
    }

    CSession * GetSession()
    {
        return &m_session;
    }

    CDataSource * GetDatasource()
    {
        return &m_datasource;
    }

private:
    CSession m_session;
    CDataSource m_datasource;

    //
    // Singleton Mechanics
    //
private:

    // Is the singleton class instance created?
    static bool m_isCreated;

    // Pointer to one and only one singleton instance
    static CSingleton * m_pSingleton;

    // Private constructor, so this class can't be explicitly instantiated
    CSingleton()
    {
    }
};

// Static members initialization
bool CSingleton::m_isCreated = false;
CSingleton * CSingleton::m_pSingleton = NULL;

// Singleton unique accessor
CSingleton* CSingleton::GetInstance()
{
    // Create the singleton if it was not created yet
    if ( ! m_isCreated )
    {
        m_pSingleton = new CSingleton();
        m_isCreated = true;
    }

    // Return the one and only one instance
    return m_pSingleton;
}

int main()
{
    CSession * pSession = CSingleton::GetInstance()->GetSession();
    CDataSource * pDS = CSingleton::GetInstance()->GetDatasource();

    return 0;
}

</code>

HTH,
Giovanni