Question

I want to have multiple instances of a DLLInterface class but as each object will have pointers to the same DLL functions I want these to be static.

The below code doesn’t compile. (I do have a reason for needing multiple instances containing the same pointers which the stripped down code below doesn’t illustrate.)

//Header file:

typedef void (*DLLFunction)();
class DLLInterface
{
private:
    static HINSTANCE hinstDLL;
public:
    static DLLFunction Do;

    DLLInterface()
    {
        if(!hinstDLL || !Do)
        {
            hinstDLL = LoadLibrary("DoubleThink.dll");
            Do = (DLLFunction)GetProcAddress(hinstDLL, "Do");
        }
    }
};

I need to have all code contained within this header file also. (I know I can make it compile by adding statements like the following into a cpp file on the EXE but I need to avoid doing this.

HINSTANCE DLLInterface::hinstDLL = 0;

Thanks!

Answer 1

One trick I can think of is to use templates. The idea is you make the class the includes the static data a template class and then derive your actual class from it with some specific type:

template <class T>
class DLLInterfaceImpl;
{
private:
    static HINSTANCE hinstDLL;
public:
    static DLLFunction Do;
};

template <class T>
HINSTANCE DllInterfaceImpl<T>::hInstDLL;

template <class T>
DLLFunction DllInterfaceImpl<T>::Do;

class DllInterface : public DllInterfaceImpl<int>
{
};

Because DllInterfaceImpl is a template, you can put your static definitions in the header file and the compiler will do some “tricks” so that multiple definitions do not cause the link to fail.