class KeyboardState
{
public:
        bool operator[] (int value);
        bool operator[] (char value);
        KeyboardState(void);
        ~KeyboardState(void);
private:
        void KeyDown(unsigned char key,int x, int y);
        void SpecKeyDown(int key,int x, int y);
        void KeyUp(unsigned char key,int x, int y);
        void SpecKeyUp(int key,int x, int y);
        int KeysPressed;
        int SpecKeysPressed;
        char KeyNumber[10];
        int SpecKeyNumber[10];
};

...

KeyboardState::KeyboardState(void)
{
        glutIgnoreKeyRepeat(1);

        KeysPressed = 0;
        SpecKeysPressed = 0;

        glutKeyboardFunc(KeyDown); // fails
        glutKeyboardUpFunc(&KeyboardState::KeyUp); // fails because its now the wrong type

        //glutSpecialFunc(SpecKeyDown);
        //glutSpecialUpFunc(SpecKeyUp);
}