Question

Situation:
I have a Dialog class in Qt on which I draw a raster of squares. The squares are implemented in the MySquare class (MySquare: QGraphicsItem).

Question:
I want to signal the Dialog slot setTarget() that a square was clicked (and obviously I want to give it some information about that square like for example it’s x and y coordinates with it later). However I get an ‘undefined reference to ‘MySquare::targetChanged()‘ error. I have looked for a solution but have not found any; anybody an idea?

edit: I have added a Q_OBJECT macro inside the MySquare however the error does not dissapear and I get an additional “‘undefined reference to ‘vtable for MySquare()‘ error

dialog.h

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);

  ~Dialog();


public slots:
    void setTarget();

private:
    Ui::Dialog *ui;
    QGraphicsScene *scene;
};

dialog.cpp

Dialog::Dialog(QWidget *parent) : QDialog(parent), ui(new Ui::Dialog)
{
    ui->setupUi(this);

    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);

    MySquare *item;

    item = new MySquare(30,30,30,30);
    QObject::connect(item,SIGNAL(targetChanged()),this,SLOT(setTarget()));
}

void Dialog::setTarget(){
    qDebug() << "signal recieved" << endl;
}

mysquare.h

#include <QGraphicsItem>
#include <QPainter>
#include <QDebug>
#include <QKeyEvent>
#include <QObject>

class MySquare : public QGraphicsItem, public QObject
{

Q_OBJECT

public:
    MySquare(int x,int y,int h, int w);
    QRectF boundingRect() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
    int x,y,h,w;

signals:
    void targetChanged();

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

mysquare.cpp

#include "mysquare.h"
#include <QGraphicsSceneDragDropEvent>
#include <QWidget>

MySquare::MySquare(int _x,int _y, int _w, int _h)
{

    setAcceptDrops(true);
    color=Qt::red;
    color_pressed = Qt::green;
    x = _x;
    y = _y;
    w = _w;
    h = _h;
}


QRectF MySquare::boundingRect() const
{
    return QRectF(x,y,w,h); 
}


void MySquare::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    QRectF rec = boundingRect();
    QBrush brush(color);

        if (Pressed){
            brush.setColor(color);
        } else {
            brush.setColor(color_pressed);
        }

    painter->fillRect(rec,brush);
    painter->drawRect(rec);
}


void MySquare::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed=true;
    update();
    QGraphicsItem::mousePressEvent(event);

    emit targetChanged();
}



void MySquare::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed=false;
    update();
    QGraphicsItem::mouseReleaseEvent(event);
    qDebug() << "mouse Released";
}

void MySquare::mouseMoveEvent(QGraphicsSceneMouseEvent *event){
    qDebug() << "mouse Moved";
    QDrag *drag = new QDrag(event->widget());
    QMimeData *mime = new QMimeData;
    drag->setMimeData(mime);
    drag->exec();
}


void MySquare::keyPressEvent(QKeyEvent *event){
    //out of bounds check?
    int x = pos().x();
    int y = pos().y();

    QGraphicsItem::keyPressEvent(event);

}

Answer 1

Edit: If you have an undefined reference to a vtable, then it’s probably because you did not implement certain virtual functions. Have you implemented the mouse event handlers of the MySquare class somewhere? Have you also implemented the boundingRect() and paint() functions of the MySquare class?

Old answer:
You need to write the Q_OBJECT macro after the opening ‘{‘ in the MySquare class. Also Qt will complain about multiple inheritance at some point. Therefore, instead of inheriting from QGraphicsItem and QObject, inherit from QGraphicsObject instead.

The actual reason, the linker complains about a missing function definition is the following: When you put a Q_OBJECT macro into the right place, then the Qt Meta Object Compiler (MOC) will generate a new cpp file for the project which contains the definition of the signal functions of the respective class. So Qt implements a function for every signal for you. If you don’t insert the Q_OBJECT macro, then MOC won’t do anything for you and the function definition for the signal will be missing to the linker.