'undefined reference to vtable' when adding Q_OBJECT

kitfox

I'm having a compiler issue I can't understand. I have two classes which inherit from a common base class which in turn inherits from QObject. I'm doing this because I'm storing the objects in a list and want to be able to distinguish them later:

for (int i: _elements.keys())
{
    DrawableElement* de = _elements[i];

    if (qobject_cast<DrawableElementContour*>(de))
    {
        DrawableElementContour* ctr = qobject_cast<DrawableElementContour*>(de);
        ...
    }
    else if (qobject_cast<DrawableElementStroke*>(de))
    {
        DrawableElementStroke* ctr = qobject_cast<DrawableElementStroke*>(de);
        ...
    }

However, when I add the Q_OBJECT macro to my DrawableElementStroke class, it causes an error when I compile:

C:/Qt/5.12.3/mingw73_32/include/QtCore/qobject.h:504: undefined reference to `Gf::DrawableElementStroke::staticMetaObject'
debug/DrawableElementStroke.o: In function `ZN2Gf21DrawableElementStrokeC2EiP7QObject':
D:\dev\starling2\game\2019\build-GoldfinchQt-Desktop_Qt_5_12_3_MinGW_32_bit2-Debug/../GoldfinchQt/core/DrawableElementStroke.cpp:9: undefined reference to `vtable for Gf::DrawableElementStroke'
debug/DrawableElementStroke.o: In function `ZN2Gf21DrawableElementStrokeD2Ev':
D:\dev\starling2\game\2019\build-GoldfinchQt-Desktop_Qt_5_12_3_MinGW_32_bit2-Debug/../GoldfinchQt/core/DrawableElementStroke.cpp:13: undefined reference to `vtable for Gf::DrawableElementStroke'
collect2.exe: error: ld returned 1 exit status

This is strange because my DrawableElementContour class has almost the same structure. If I omit the Q_OBJECT from DrawableElementStroke I can compile, but then I am unable to use the qobject_cast<>.

These are my headers:

DrawableElement.h

class ScenePanel;

class DrawableElement : public QObject
{
    Q_OBJECT

    int _id;

public:
    DrawableElement(int id, QObject* parent = nullptr);

    int id() { return _id; }
    virtual QRectF bounds() = 0;
    virtual void paint(QPainter&) {}

    virtual void toXml(QDomElement root) {}
    virtual void initFromXml(QDomElement root) {}

};

DrawableElementStroke.h

class DrawableElementStroke : public DrawableElement
{
    Q_OBJECT

    StrokeShape _stroke;

public:
    DrawableElementStroke(int id, QObject* parent = nullptr);
    ~DrawableElementStroke() override;

    StrokeShape stroke() { return _stroke; }
    void setStroke(const StrokeShape& stroke) { _stroke = stroke; }

    QRectF bounds() override { return _stroke.bounds(); }
    void paint(QPainter& p) override;

    void toXml(QDomElement root) override;
    void initFromXml(QDomElement root) override;
};

DrawableElementCountour.h

class DrawableElementContour : public DrawableElement
{
    Q_OBJECT

    Shape2D _shape;

public:
    DrawableElementContour(int id, QObject* parent = nullptr);
    ~DrawableElementContour() override;

    Shape2D shape() { return _shape; }
    void setShape(const Shape2D& shape) { _shape = shape; }

    QRectF bounds() override { return _shape.bounds(); }
    void paint(QPainter& p) override;


    void toXml(QDomElement root) override;
    void initFromXml(QDomElement root) override;
};

SGaist

Hi,

This question hasn't been asked in while but the answer is still the same: re-run qmake after you add/remove the Q_OBJECT macro.