Singleton MFC Extension DLL

Hi.

I declare a singleton on a MFC extension DLL, like this:

<pre>
//header file: SingleTon.h
class AFX_EXT_CLASS CMySingleton
{
public:
  static CMySingleton* Instance()
  {
    if(!singleton)
        singleton = new CMySingleton();
    return singleton;
  }

  int a;

// Other non-static member functions
private:
  CMySingleton() {}; // Private
constructor
  CMySingleton(const CMySingleton&); // Prevent copy-
construction
  CMySingleton& operator=(const CMySingleton&); // Prevent
assignment
  virtual ~CMySingleton() {};

  static CMySingleton* singleton;
};

And in a cpp file I code the following line:

CMySingleton* CMySingleton::singleton = NULL;
</pre>

Code 2:
<pre>
CMySingleton *a;

a = CMySingleton::Instance();
</pre>

The problem is when I code "code 2" in a Regular Dll, all works fine,
but when I code "code 2" in another MFC extension DLL gives an error:

<pre>
unresolved external symbol "private: static class CMySingleton*
CMySingleton::singleton" (?singleton@CMySingleton@@0PAV1@A)
</pre>

Any idea?