Question

Linker error messages “private: static class Worlds::Game * Worlds::Game::instance”
The following are the code segments that are related to the error.

From Game.h

static Worlds::Game* instance;
static Worlds::Game* getInstance();

From Game.cpp

Worlds::Game* instance = 0;

Worlds::Game* Worlds::Game::getInstance()
{
    if (instance)
    {
        return instance;
    }
    else
    {
        return instance = new Worlds::Game();
    }
}

So my question is why am I getting this error as I should have all my bases covered for creating a singleton of my game class?

Edit:

I forgot to add in the Game.cpp I have all my Glut Callbacks which need to call the functions in Game that do something.

void onKeyDownCallback(unsigned char key, int mouseX, int mouseY)
void onKeyUpCallback(unsigned char key, int mouseX, int mouseY)
void timerCallback(int value)
void onWindowReshapeCallback(int w,int h)
void onMouseClickedCallback(int button, int state, int mouseX, int mouseY)
void onMouseMovedCallback(int deltaX, int deltaY)
void displayCallback()

Answer 1

static Worlds::Game* instance; – remove this from header. Because instance is marked static it is copied per each translation unit. And you get as many instances as translated units. But you initialize it only in one translation unit

Also consider implementing a singleton this way:

Worlds::Game & Worlds::Game::getInstance()
{ 
    static Game instance;
    return instance;
}