Singleton MFC extension DLL - dll

Question

I declare a singleton on a MFC extension DLL, like this:
//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;
Code 2:
CMySingleton *a;
a = CMySingleton::Instance();
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:
unresolved external symbol "private: static class CMySingleton* CMySingleton::singleton" (?singleton#CMySingleton##0PAV1#A)
I check correctly all the dependencies, via Project Dependencies.
Any idea?

Answer 1

The problem is in the AFX_EXT_CLASS macro.
#ifdef _AFXEXT
#define AFX_EXT_CLASS __declspec(dllexport)
#else
#define AFX_EXT_CLASS __declspec(dllimport)
#endif
Extension dll defines _AFXEXT and your class is exported, and main app (or a regular dll) doesn't define it so it's imported. But your second extension dll also defines _AFXEXT and your class declaration uses dllimport instead of dllexport and you get a linker error. The solution is to create your own macro for both dlls and use them instead of AFX_EXT_CLASS:
#ifdef EXTENSION_ONE
#define EXT_CLASS_ONE __declspec(dllexport)
#else
#define EXT_CLASS_ONE __declspec(dllimport)
#endif
Create EXTENSION_TWO and EXT_CLASS_TWO for your second dll. Define EXTENSION_ONE only in your first extension dll project, and EXTENSION_TWO only in your second extension dll project.