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QDialog question

  • Hi!, I'm developing a ground station in Qt. The groundstation is meant to have a mainwindow and multiple dialogs. The question is if it's possible to create the QDialog in a QObject or is mandatory to initiate the QDialog in a QMainWindow.

    Thanks in advance for your help.

  • @DavidPL
    What do you mean by "create" and "initiate"? You can display a QDialog any time. You can give it a parent (must be QWidget) if you want, or leave that out, as you please. You do not need a QMainWindow. QObject does not seem relevant here.

  • I'll try to explain better. I have some objects moved into dedicated threads in order to avoid the MainWindow from freezing. One of those objects takes data from a gamepad. That data is meant to be displayed in a QDialog. The problem I'm having is that the QDialog breaks the app when I implement it in the mentioned object code (I include the .h and .cpp files of the QDialog, create a pointer, i.e Dialog *mydialog, and state: mydialog->setmodal(false); mydialog->exec.

  • Lifetime Qt Champion


    Ok, then the updated answer is: all GUI elements have to live in the same (main/GUI) thread.

    you should use signals &slots to exchange data with the objects in the other threads.


  • @aha_1980
    If use signal slot which you recommended @aha_1980 do not use mydialog->exec(); use mydialog->show();

  • @aha_1980 Ah ok, I didn't know that. Thanks :). Only to know, imagine you need a lot of dialogs in your app or they are updating large amounts of data at a fast rate. In that case is likely to get frozen the mainwindow many times so is there any way to move some dialogs into other threads to avoid the mentioned issue?

  • Lifetime Qt Champion

    Nothing GUI related can live in other threads.
    The normal design is to use Threads for the data processing. It will then emit
    signals to GUI thread to show something on screen.

  • @mrjj Ok I understand. But there is another problem I'm getting while emitting the signal. I want to pass an array from one object living in one of the other threads to the dialog. I connect the signal and slots but It says that the size of the array is too large to be put in qeue. So, how could I send the data then? (the array is only [1][21] size).

  • @DavidPL
    I have absolutely no idea, but have you tried passing the array via QVariant instead of as-is?

  • @JonB No, I haven't tried. I attach the code for the signal, slot and the connect line and I'll try the QVariant.

    void update_commandedcontrols(double commanded_controls[1][21]);


    void update_commandedcontrols(double commanded_controls[1][21]);


    connect(gamepad_obj, SIGNAL(update_commandedcontrols(double[1][21])), this, SLOT(update_commandedcontrols(double[1][21])));

  • @DavidPL
    I don't know, but other people don't seem to be complain an array can't be passed. Or, they use things like std::vector.

    You might like to give the exact text of whatever runtime error you say you're getting?

  • @JonB This is the error I'm getting:

    QObject::connect: Cannot queue arguments of type 'double[1][21]'
    (Make sure 'double[1][21]' is registered using qRegisterMetaType().)

  • @DavidPL
    But David, earlier you wrote:

    I connect the signal and slots but It says that the size of the array is too large to be put in qeue

    Where does it say that it is "too large"? What you report now is completely different. And it tells you what you can do about it....

  • @JonB Yeah you're right, I don't know why but I thought it was related to the size, sorry. Nevertheless I'm newbie and I'b be so grateful if you could explain to me how can I solve that error and what does it mean.

  • @DavidPL
    OK, that's why the exact error message is so important! :)
    So you have to search for qRegisterMetaType, and read like:

    Basically, Qt needs to know about the types of variables you want to pass through signals/slots. It naturally supports simple types like int or QString, but once you want to pass something like your 'double[1][21] you have to tell it about that before you can pass it.

    enum QMetaType::Type (in shows you what types it automatically supports. Either you convert/pass your type to one of these, if that's possible, or you have to do the work to declare your meta-type to Qt first.

    I can't give you an exact example 'coz I don't use it (and I'm not C++), but there are plenty around if you search for those words.

  • Why not use a QList<double> ?


    class Receiver : public QObject
        public slots:
            void update(QList<double> data)
                qDebug()<<"update with data: "<<data;
    class Sender : public QObject
        void sendData()
            QList<double> d;
            d<<1.2<<2.5<<12.78;  // list of double to send
            emit update(d);
            void update(QList<double>);
    int main(int argc, char *argv[])
        Receiver r;
          Sender s;
          QObject::connect(&s,&Sender::update, &r, &Receiver::update);
          s.sendData(); // send data to r
          return 0;
    #include "main.moc"  // only needed because QObjects create in main.cpp, not in header .h

    On the other hand,
    double[1][21] is a single dim array !
    and in C when you pass an array as a argument you 're passing an address to the first element of the array.
    How do you create this array, cause it seems odd ...

  • @DavidPL
    Like @mpergand just wrote, because yours is double[1][...] it's only really holding a single list of doubles.

    Then tells you:

    Some types are registered automatically and do not need this macro:
    QList<T>, QVector<T>, QQueue<T>, QStack<T>, QSet<T> or QLinkedList<T> where T is a registered meta type

    So if you change your code to use QList<double> for your array you won't have to register your own type.

  • @mpergand Yeah I know it's a single dimension array currently but in the near future it may be multidimensional, and that's the reason I decided to use that.

    I create the array in this line: double commanded_controls [1][21];

    But if it's too difficult to use the array I'll try the QList for now. Thank you so much for the example :).

  • Lifetime Qt Champion

    just as a note:
    There is (often) very little reason to use a traditional
    c array unless you must call older c code.
    I recommend you new friend be
    for a fixed size, very close to c array but with benefits.
    Along with std::vector etc. ( QList, QMap )

    Anyway, here QList seems better as no need to register anything. (as the others mentions)

  • @DavidPL said in QDialog question:

    @mpergand Yeah I know it's a single dimension array currently but in the near future it may be multidimensional, and that's the reason I decided to use that.

    Not a big deal ;)

    Replace QList<double> with QList<QList<Double> that's all !

    In my example above, to initialize the list of list :

    QList<QList<double>> d;
     d<<QList<double>{1.2,2.5,12.78};  // first row
     d<<QList<double>{11.2,2.15,1002.78};  // second row
    // etc
     emit update(d);

    In the receiver:

    void update(QList<QList<double>> data)
                qDebug()<<"update with data: "<<data;

    The debug ouput :

    update with data:  ((1.2, 2.5, 12.78), (11.2, 2.15, 1002.78))

    You can retrieve each element like that:

    double val=data[1][2]; // = 1002.78

    easy isn't it ?

  • @mrjj Thanks a lot for the note haha, I appreciate it so much :).

  • @mpergand

    Replace QList<double> with QList<QList<Double> that's all !

    The way I read the docs meta types description, I did not think QList<QList<>> was covered as a "built-in type" acceptable without adding your own Meta. You saying that does work?

  • @mpergand since you know how to do it yes, easy haha. Thank you again for your help and time, that's really nice of you.

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