Why won't this compile (link) with the Q_OBJECT macro in place?

Question

Hi,

I made a prototype of a project with PyQt and made it work there, now I'm trying to convert it to C++ and am having some problems.

If I don't put the Q_OBJECT macro in, it compiles and works, but if I comment it out, I get the following errors:

Undefined symbols:
  "vtable for MapView", referenced from:
      MapView::~MapView()in mapview.o
      MapView::~MapView()in mapview.o
      MapView::MapView(QObject*)in mapview.o
      MapView::MapView()in mapview.o
  "MapView::staticMetaObject", referenced from:
      MapView::MapView(QObject*)in mapview.o
      MapView::MapView()in mapview.o

Here's the header:

#ifndef MAPVIEW_H
#define MAPVIEW_H

#include <QtGui>
#include <QObject>

class MapView : public QGraphicsScene
{
    //Q_OBJECT

public:
    MapView();
    explicit MapView(QObject *parent = 0);
    QGraphicsPixmapItem *mappixmap;
    ~MapView();

private:
    bool dragging;
    float offsetX, offsetY, downoffsetX, downoffsetY;

protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);

};

#endif // MAPVIEW_H

Secondary question is will Bad Things happen if I just omit the Q_OBJECT macro here?

And yes, I'm aware of that it's stupid to call a QGraphicsScene a "view".

Answer 1

This kind of errors usually happen when you add the Q_OBJECT macro and forget to rerun moc. If you use qmake, just run make qmake after you added the macro.

As for your second question: you won't be able to use signals/slots (among other things) without the Q_OBJECT macro. See the docs for more information about this.