Adding signals to class that inherit from QGraphicsScene - c++

Question

I want to add a signal to a class that inherits from QGraphicsScene.
signals:
void update(std::vector< std::vector<int> > board);
When I do this, Qt Creator warns me that I forgot the Q_OBJECT macro. But somewhere I read that since QGraphicsScene doesn't inherit from QObject, I shouldn't put it in my class definition. But signals need this macro.
How do I add a signal to a class that doesn't inherit from QObject?
boardgui.h
#ifndef BOARDGUI_H
#define BOARDGUI_H
#include <QGraphicsView>
#include <QGraphicsScene>
class BoardGUI : public QGraphicsScene
{
Q_OBJECT
public:
BoardGUI(QGraphicsView*& view, int dimension);
~BoardGUI();
void buildBoard();
signals:
void update(std::vector< std::vector<int> > board);
private:
int dimension;
QGraphicsView* view;
};
#endif // BOARDGUI_H

Answer 1

Reposting as answer, as requested:
Is update() your signal? did you try to implement the signal yourself? If yes, don't do that, signals are defined by moc.

Answer 2

QGraphicsScene does inherit from QObject. You can consult the documentation.
http://doc.qt.io/qt-5/qgraphicsscene.html

Answer 3

Vtables have something to do with virtual functions. When you have an error like Undefined reference to vtable then it cannot find an implementation for a virtual function (I think). Have you implemented all of your functions? Maybe you used the virtual keyword when you weren't supposed to? Or vice versa?
Also, you say that you want to add a signal, but your code chunk shows a slot? Could you show a little more of your class?

Answer 4

If you have not implemented any class with Q_Object before to the project and you add the Q_OBJECT line yourself, you get the vtable error. If you add another c++ class to the project that inherits from QObject, then you get rid of this problem. You can remove this class you created if you dont need it. I am not sure why this happens, but this is easy way to get rid of the problem. Maybe the creator adds some line to the . pro file when you add a class that inherits from qobject.

Answer 5

Have you implemented your destructor? If not, try changing ~BoardGUI(); to ~BoardGUI(){};

Answer 6

Have you added boardgui.h to the MOC preprocessor's file list? Any class that uses QObject and Signals/Slots needs to be passed through the Meta-Object Compiler so it can generate the actual code behind the signals.
I am unsure if Qt Creator handles this automatically.

Answer 7

You simply have to inherit from QObject yourself.
boardgui.h
#ifndef BOARDGUI_H
#define BOARDGUI_H
#include <QGraphicsView>
#include <QGraphicsScene>
class BoardGUI : public QObject, public QGraphicsScene
{
Q_OBJECT
public:
BoardGUI(QGraphicsView*& view, int dimension);
~BoardGUI();
void buildBoard();
signals:
void update(std::vector< std::vector<int> > board);
private:
int dimension;
QGraphicsView* view;
};
#endif // BOARDGUI_H
If you are using Qt Creator, you shouldn't have to worry about moc, everything should be handled by Qt Creator.