Putting a singleton in a DLL

[tlann]

I'm getting errors about a singleton when I put it into a dll. The dll compiles without any problems. However, when I try to link it to another dll I get the following error.

libFoo-Bar error LNK2001: unresolved external symbol "private: static class MSGTMS:ObjectManager * MSGTMS::ObjectPostManager::m_instance" (?m_instance@ObjectManager@MSGTMS@@0PAV12@A)


The Singleton class looks like this

#define LIBMSGTM_Managers_EXPORT __declspec (dllexport)
 
namespace MSGTMS
{
 
class LIBMSGTM_Managers_EXPORT ObjectManager
{
public:
    static ObjectManager* instance()
    {
        if (m_instance==0)
            m_instance=new ObjectManager;
        return m_instance;
    }
 
    void destroy()
    {
        if (m_instance!=0)
            delete m_instance;
    }
 
    bool updateFPStatus(const unsigned int FPindex, FencePostStatus& fpStatus);
    bool updateFP(const unsigned int FPindex, FencePost& fp);
 
    bool isFPActive(const unsigned int FP);
    bool getFP(const unsigned int FPindex, FencePost& fp);
    bool getUpdatedStatus(bool* arrayEightFencePosts);
 
    bool getUpdatedPosition(bool* arrayEightFencePosts);
 
    bool deleteFP(const unsigned int FPindex);
 
protected:
    ObjectManager();
    ~ObjectManager();
    Object m_Object[8];
 
 
private:
    static ObjectManager* m_instance;
    CRITICAL_SECTION cs;  //For multithreading protection
 
};
}//namespace MSGTMS
 
Implementation
 
namespace MSGTMS
{
ObjectManager* ObjectManager::m_instance=0;
 
//Constructor Destructor and other things implemented
 
}

Should I be doing something else to a static variable in a dll?

[weaknessforcats]

private:
static ObjectManager* m_instance;

You are not exporting the m_instance.

The error is that m_instance should not be a member of this singleton class.

Instead the m_instance should be in an anonymous namespace accessible onbly by the Instance() function. You export the Instance function().

There is an article in the C++ HowTos forum on singleton classes that shows the correct code setup.

BTW: m_ does not mean a member variable. It just identifies that the name of the variable starts with m_. Avoid naming conventions that lead to assumptions.