share variable between classes [SOLVED]



  • I created an application that exploits three different classes of objects. Basically I have the first class that "all the interface and the main code" the second one that is a simple rect object and the last one that is the edge between different rect (more or less as in the elasticnode example ). Basically every time the user moves the rect the edge is modified and I need to seed the "length" to the main class.

    here is the code of the movement:

      void Edge::adjust()
      {
    
      QLineF line(mapFromItem(source, 0, 0), mapFromItem(dest, 0, 0));
    
     prepareGeometryChange();
    
     QPointF edgeOffset(5, 5);
     sourcePoint = line.p1() + edgeOffset;
     destPoint = line.p2() + edgeOffset;
    
     length_reff = sqrt((source->x()-dest->x())*(source->x()-dest->x())+(source->y()-dest->y())*(source->y()-dest->y()));
    
     emit length_COMPUTED(length_reff);
    //Here I have to send the lenght_ref variable to the MainWindow class
    }
    

    I tried to implement SIGNAL/SLOT in this way:

    Edge.h:

      public:
      Edge(MyItem *sourceNode, MyItem *destNode);
    
     void adjust();
    
      signals:
      void length_COMPUTED(qreal &length_reff);
    

    MainWindow.h:

      class MainWindow: public QMainWindow
     {
      Q_OBJECT
    
      public:
      explicit MainWindow(QWidget *parent = 0);
      ~MainWindow();
      ...
      public slots:
      void official_length_computation();
     ...
    

    in Mainwindow.cpp:

       this->connect(this, SIGNAL(length_COMPUTED(&length_reff)), this,SLOT(official_length_computation()));
    

    I guess I'm completely wrong with the connect function.

    Any help?

    Thanks


  • Moderators

    Hi and welcome to devnet

    You are not completely wrong with the connect. However, you are missing the parameter in your slot method.

    MainWindow.h:

          class MainWindow: public QMainWindow
         {
          Q_OBJECT
    
          public:
          explicit MainWindow(QWidget *parent = 0);
          ~MainWindow();
          ...
          public slots:
          void official_length_computation(qreal &length_reff);
    
            this->connect(this, SIGNAL(length_COMPUTED( qreal ), this,SLOT(official_length_computation(qreal )));
    

    Just consider the signal-slot connection as a fancy function call. Basically the parameters shall match (forget the even more fancy stuff at the moment). The connect says in your case, there is a function call with a qreal parameter and it will call the function in the other class with a qreal parameter.



  • Thank you for you answer and for the clarification about the SIGNAL/SLOT concept. I did as you suggested me, but I still get the error:

    LKN2019 unresolved external symbol "public: void __cdecl Edge::length_COMPUTED(double &)" (?length_COMPUTED@Edge@@QEAAXAEAN@Z) not referenced in function "public: void __cdecl Edge::adjust(void)" (?adjust@Edge@@QEAAXXZ)

    am I wrong in the declaration of the length_COMPUTED signal?


  • Moderators

    @tmoranduzzo
    Sorry, partly forgot to change your code.

    Should better say:

          public slots:
          void official_length_computation(qreal length_reff);
    

    In the simplest case the signal-slot is exactly as a function call. Therefore, the parameters have to match. Passing a reference is possible, but you should use a "const &" instead of a "&". The connect has basically to know the signal and its parameter definition (it is a simple qreal or double) and it has to match with the same definition for the slot.

    A slot function is basically a completely normal function as you typically implement. It is only sitting in the declaration under "public slots". Consider "Q_OBJECT" and "public slots:" as a marker for the moc process which runs in advance of the compiler.

    When a slot function is as a simple function there has to be an implementation of course. I suspect that you have no implementation in implementation file.

    In this case it should be similar to:

    void MainWindow::official_length_computation(qreal length_reff)
    {
         ...
    //     doing something with length_reff e.g.
        double myLength = length_reff * 2.;
         ...
    }
    


  • I did the implementation of the function.

    I remove all the "&" but is still having the same error. do you think could be a problem how I declared length_reff? I declared it in edge.h.

    Or is it a problem of the object Edge itself?


  • Moderators

    @tmoranduzzo
    Is Edge derived from QObject and does it also contain Q_OBJECT?
    Otherwise please post complete Edge.h declaration.


  • Moderators

    In case of doubt the reference symbol "&" has to be removed from the signal definition as well.



  • @koahnig

    I defined Edge as:

     class Edge : public QGraphicsItem  {
    
     public:
     Edge(MyItem *sourceNode, MyItem *destNode);
     MyItem *sourceNode() const;
     MyItem *destNode() const;
    
     void adjust();
    
    qreal length_reff;
    
    signals:
     void length_COMPUTED(double cazzo);
    
     protected:
    QRectF boundingRect() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
    
     private:
     QPointF sourcePoint;
     QPointF destPoint;
     qreal arrowSize;
     MyItem *source, *dest;
    

    };

    #endif // EDGE_H

    do I need to transform it in Q_Object? But then, does edge keep the funtion of graphic object?

    thanks


  • Moderators

    @tmoranduzzo
    You need to derive the class from QObject or from a class which has already inherited from QObject. Otherwise it cannot send a signal.

    QGraphicsItem is not derived from QObject. It does not send signals either.

    Try:

    class Edge : public QGraphicsItem, public QObject  {
         Q_OBJECT
     public:
    

    Note: I am not sure, if one can do it this way. You need to check.



  • Subclass it from QGraphicsObject instead.


  • Moderators

    @mchinand said:

    Subclass it from QGraphicsObject instead.

    That looks better, thanks.



  • @mchinand @koahnig

    If I subclass Edge in this way:

        class Edge : public QGraphicsItem , public QObject  {
           Q_OBJECT
    

    I obtain the following errors:
    C2039 'StaticMetaObject' is not a member of 'QGraphicItem'
    C2039 'qt_metacast' is not a member of 'QGraphicItem'
    C2039 'qt_metacall' is not a member of 'QGraphicItem'

    while if I do in this way:

        class Edge : public QObject, public QGraphicsItem {
          Q_OBJECT
    

    I have no error when I compile but when I run the program I obtain:
    QObject::connect: No such signal MainWindow::length_COMPUTED( qreal ) in ..\MainWindow\MainWindow.cpp:31
    QObject::connect: (sender name: 'MainWindow')
    QObject::connect: (receiver name: 'MainWindow')

    The program works but it seems that the SIGNAL/SLOT functions do not happen



  • Your connect statement is wrong. Your signal should be emitted from an Edge object but you're trying to connect a signal from MainWindow ('this' below) but there is no such signal in your MainWindow.

     this->connect(this, SIGNAL(length_COMPUTED( qreal ), this,SLOT(official_length_computation(qreal )));


  • @mchinand

    Sorry if I bother you, but I undestood that but if I use:

    this->connect(this, SIGNAL(length_COMPUTED( qreal ), this,SLOT(official_length_computation(qreal )));
    

    in the my edge object I'm not able to see the SLOT function of MainWindow!



  • Your connect statement should be called in from your MainWindow as you have it, but the first parameter is the sender object which is not your MainWindow object but an Edge object. You should create an Edge object and use a pointer to that object as the first parameter of your connect statement.


  • Moderators

    @tmoranduzzo
    I assume that you include Edge declaration in MainWindow.cpp.

    You can write something similar to:

    Edge *myEdge = new Edge;
    
    connect ( Edge, SIGNAL(length_COMPUTED( qreal ), this, SLOT(official_length_computation(qreal ) ) );
    

    The signal is in Edge class and the connect statement above allows connecting from myEdge (using the pointer) to the slot function pf MainWindow (using this pointer).



  • @koahnig @mchinand

    Thank you guys! I'm close to the final solution! Now the program compile and runs without error. The only problem is that it seem that there is no connection between the emission of length_COMPUTED and the slot official_length_computation because the program never enters in that function.

    I declared myEdge and the connect function in this way:

        test1 = new MyItem();
        test2 = new MyItem();
        Edge *myEdge = new Edge(test1,test2);
        this->connect(myEdge, SIGNAL(length_COMPUTED( qreal )), this, SLOT(official_length_computation( qreal)));
    

    the returning value of connect is true so I guess that the connection is ok

    After several attempts and after several Debug I saw that in the function adjust() the SIGNAL is never emitted even though the connection is ok.


  • Moderators

    @tmoranduzzo

    Well, the question how adjust is called in your original design?

    Since you have adjusted to inherit from QGraphicsObject it is emitting signals for different changes. Probably you have to connect an appropriate signal to trigger your adjust method (which has to be declared under slots then). Alternatively, you may need an intermediate slot method to be triggered which calls adjust.



  • @koahnig

    The program calls correctly the function adjust() and in it it does all the required operation except for the emission of the signal. the function adjust is called two times in the code:
    1: in edge.cpp

    Edge::Edge(MyItem *sourceNode, MyItem *destNode) : arrowSize(10)   {
    setAcceptedMouseButtons(0);
    source = sourceNode;
    dest = destNode;
    source->addEdge(this);
    dest->addEdge(this);
    adjust();
    }
    

    and in Myitem.ccp (the is the function to create a rect used as extreme of the edge.):

     QVariant MyItem::itemChange(QGraphicsItem::GraphicsItemChange change, const QVariant &value){
        qDebug()<<"myIteme change";
        switch (change) {
        case ItemPositionHasChanged:
              foreach (Edge *edge, edgeList){
                    edge->adjust();}
               break;
        default:
               break;
        };
          return QGraphicsItem::itemChange(change, value);
       }
    

    I do not get exactly why even if the connection is ok the signal is not emitted. Anyway, I'll try as you suggested by invoking the adjust() with a SLOT


  • Moderators

    @tmoranduzzo
    Depending on complexity of your application you may set a breakpoint in the debugger on the emit as well on the first statement in the slot method for tracking purposes. Not sure if this is possible in your application.

    Also you can use dumpObjectInfo() and dumpObjectTree() in a debug compilation for tracking if there should be still a connection when the emit is happening.



  • @koahnig

    Please correct me if I'm wrong. The SIGNAL/SLOT concept is associated to a specific object right? In my case where during the execution of the process multiple object Edge are created I should create and connection for each object right? and consequently the declaration:

      test1 = new MyItem();
      test2 = new MyItem();
      Edge *myEdge = new Edge(test1,test2);
      this->connect(myEdge, SIGNAL(length_COMPUTED( qreal )), this, SLOT(official_length_computation( qreal)));
    

    that I did in the MainWindow.cpp is not enough because it connect the object myEdge that in the process is not use.

    Am I completely wrong?



  • My reasoning was correct!!! I solved my problem! I added the connection in the code where the edge that I effectively use is created!

    By Thinking right now it was a really stupid and obvious error.

    Thank you very much for your help!


  • Moderators

    @tmoranduzzo
    You are right. Connections are always between specific objects. Therefore, you need to repeat the connections each time you are defining a new instance.

    It looks like you have mastered the signal slot concept. After you got used to there are lots of possibilities.

    Please mark your thread with [solved] in title line. This indicates to others that there might be a solution for them. Also upvoting of responses are appreciated.


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