I don't understand this error:
Qt Code: 
  1. Undefined reference to 'vtable for Spaceship'   Spaceship.cpp  9
  2. Undefined reference to 'vtable for Spaceship'   Spaceship.cpp  9
  3. collect2: ld returned 1 exit status
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spaceship.h:
Qt Code: 
  1. #ifndef SPACESHIP_H
  2. #define SPACESHIP_H
  3.  
  4. #include <QGraphicsItem>
  5. #include <QObject>
  6.  
  7. class Spaceship : public QGraphicsItem
  8. {
  9.  
  10. public:
  11.     Spaceship();
  12.  
  13.     QRectF boundingRect() const;
  14.     QPainterPath shape() const;
  15.     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
  16.  
  17. protected:
  18.     void advance(int step);
  19.  
  20. private:
  21.     qreal angle;
  22.     qreal speed;
  23.     QColor color;
  24. };
  25.  
  26. #endif // SPACESHIP_H
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spaceship.cpp:
Qt Code: 
  1. #include "spaceship.h"
  2. #include <QGraphicsScene>
  3. #include <QPainter>
  4. #include <QStyleOption>
  5. #include <QObject>
  6. #include <math.h>
  7.  
  8. Spaceship::Spaceship()
  9. {
  10.     setRotation(0);
  11. }
  12.  
  13. void Spaceship::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
  14. {
  15.     //Draw the Spaceship
  16.     painter->setBrush(Qt::black);
  17.     painter->drawRect(10, 10, 20, 20);
  18. }
  19.  
  20. void Spaceship::advance(int step)
  21. {
  22.     if (!step)
  23.         return;
  24.  
  25.     angle = 0;
  26.     speed += 1;
  27.     setPos(mapToParent(0, -(3 + speed) *3));
  28. }
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main.cpp:
Qt Code: 
  1. #include "spaceship.h"
  2. #include <QtGui>
  3. #include <math.h>
  4. #include <QObject>
  5.  
  6. int main(int argc, char **argv)
  7. {
  8.     QApplication app(argc, argv);
  10.     scene.setSceneRect(-300, -300, 600, 600);
  11.     scene.setItemIndexMethod(QGraphicsScene::NoIndex);
  12.  
  13.     Spaceship *ship = new Spaceship;
  14.     ship->setPos(0,0);
  15.     scene.addItem(ship);
  16.  
  17.     QGraphicsView view(&scene);
  18.     view.setRenderHint(QPainter::Antialiasing);
  19.  
  20.     view.setWindowTitle(QT_TRANSLATE_NOOP(QGraphicsView, "Spaceship"));
  21.     view.resize(400, 300);
  22.     view.show();
  23.  
  24.     QTimer timer;
  25.     QObject::connect(&timer, SIGNAL(timeout()),&scene, SLOT(advance()));
  26.     timer.start(1000 / 33);
  27.  
  28.     return app.exec();
  29. }
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I was trying to modify the colliding mice example to just show a block move upward.

Thank you very much