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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?

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1 Answer 1

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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.

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