[C++] Tworzenie prywatnego konstruktora - błąd podczas kompilacji

class TimeKeeper
{
private:
   
    static sf::Clock Clock;
    static TimeKeeper * instance;
    TimeKeeper();
public:
    static TimeKeeper * getInstance();
    static double getTime();
   
   
};


TimeKeeper * TimeKeeper::instance = NULL;
sf::Clock TimeKeeper::Clock;

double TimeKeeper::getTime()
{
    return Clock.restart().asSeconds();
}

TimeKeeper * TimeKeeper::getInstance()
{
    if( instance == NULL )
         instance = new TimeKeeper;
   
    return instance;
}