LRESULT CALLBACK WndProc(HWND hWindow, UINT msg, WPARAM wParam, LPARAM lParam)
{
    // Initialize via wrapper, else route to engine
    switch (msg)
    {
        case WM_CREATE:
            return GEN::Wrapper::GetWrapper()->HandleEvent(hWindow, msg, wParam, lParam);
            break;
        default:
            return GEN::Wrapper::GetEngine()->HandleEvent(hWindow, msg, wParam, lParam) // DefWndProc called from this HandleEvent
    }
}

class WrapperInterface
{
public:
    static std::tr1::shared_ptr<WrapperInterface> // factory function
        create_Wrapper(...); // returns pImpl

    // ... Pure virtuals here
};

std::tr1::shared_ptr<WrapperInterface> WrapperInterface::create_Wrapper(...)
{
    return std::tr1::shared_ptr<WrapperInterface>(new Wrapper(...));
}

std::tr1::shared_ptr<WrapperInterface>
      Wrapper(WrapperInterface::create(...));