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Multiple inheritance from QObject and QThread

  • Dear All,
    I make multiple inheritance from QObject and QThread.
    @class cameraManagement : public QObject, public QThread

    I got the error of
    "QObject is an ambiguous base of cameraManagement"
    What could be the error?

    [Edit: Added @ tags around code -- mlong]

  • Since QThread already inherits QObject there is no need for your class to be derived from QThread and QObject. Just remove the public QObject from your classes declaration.

  • Is it really necessary to subclass QThread? To run your object in a different thread, subclassing is not the recommended way, anymore. See "QThread Class Reference (notes)":http://qt-project.org/doc/qt-4.8/QThread.html#notes

    Maybe you want to try it like this:
    #include <QThread>

    class CameraManagement : public QObject


    QThread * camThread = new QThread;
    CameraManagement * camMgr = new CameraManagment;

    camMgr->moveToThread( camThread );

    btw: note the captial 'C' of your class. Unless it is a typo, usually it is good style to write class names with a starting capital letter to avoid confusion.


  • Thanks, this is a good approach. No need to subclass.
    Thanks for highlighting the good practice as well.

  • Hi JSchmidt,
    My thread does not work properly.
    I have three classes, MainWindow class, CameraManagement class and ImageQueue class. CameraManagement class handles all camera related things and put images into ImageQueue. I made ImageQueue to be thread safe using mutux. Then MainWindow class retrieve images and display at QGraphicsView.
    I made a separate thread inside the CameraManagement class as

    @class CameraManagement : public QObject




    public slots:
    void captureThreadRun();


    Then in the MainWindow class, I managed to call the thread as

    CameraManagement *cam = new CameraManagement(this);
    camCaptureThread = new QThread;

    Then I have a While loop to retrieve the images from ImageQueue.
    The problem is when the While loop runs, the thread does not run (I put a break point and check). The thread runs only when the While loop is commented out.
    So what could be the problem?

  • I don't know if I got you right. You have a thread inside a thread?
    So your topology is something like:
    MainWindow -> has Thread with ( CameraManagement -> has Thread with ImageQueue )

    I don't know if this is your setup and if so, if it produces your error.

    My personal approach would look like this:
    MainWindow has two Threads:

    for CameraManagement

    for ImageQueue

    CameraManagement has a signal, to send data to ImageQueue
    ImageQueue has a slot to receive new data from CameraManagement

    In MainWindow, you connect CameraManagement-signal and ImageQueue-slot and start both threads.

    Maybe you get the idea? If I got you right, this might be a cleaner design and could prevent some hidden errors. Or you have to split up CameraManagement internally and create inside of CameraManagement two threads, if you want to have some sort of facade pattern design.

    If this is no option or won't help, maybe you want to provide a bit more code, to see where you create which objects, where you set breakpoints and/or comment out something and so on.

  • No JSchmidt,
    I have only one thread inside MainWindow.
    That thread runs
    void captureThreadRun(); method inside the CameraManagement class.
    MainWindow and CameraManagement both share the ImageQueue class for image storing and retrieving.
    I manage thread safe for ImageQueue using mutax.

    Is that make sense?

  • I assume the while loop you mentioned blocks the Eventloop so that no signals from the CameraManagmentThread are received. Where is this wile loop running and when is it started and when does it return?

    Using loops for polling is not very usefull in an event driven framework like Qt. You better use signal-slot connections to notify your classes about changes in data and so forth.

  • Yeah JSchmidt,
    I am new to QT and not familiar with signal and slot.
    I read
    and got some ideas for signal and slot.
    I still don't have right signal and slot setting.

  • Btw here is "another good article":http://developer.qt.nokia.com/wiki/QThreads_general_usage about how to use QThreads. Notice the signal slot connections at the end. They are a good basic example on how to start and stop the object in the thread gracefully.

  • AH ok. I think, now I got it.

    And what for is your mentioned while loop? Where is it running? Is it a infinte loop?

    I could think of two possible critical points:

    as far as I see, your MainWindow and your ImageQueue are using the same thread. If your mentioned while loop is a (kind of) infinite loop, you are blocking the complete thread. Your MainWindow class has no chance to get cpu time. Even more, internally, your QApplication has a EventLoop to manage Signals/Slots and running MainWindow (and in your case ImageQueue). So if you have an infinte loop, you are blocking this eventloop too. You won't be able to receive any signals (maybe you even cannot send signals, if we are talking about queued connections)

    if you use mutexes to handle thread saftey, there are always possible deadlock situations. might it be that your mutex won't release your lock? are you awaiting any signal to unlock, which are not connected or cannot be sent?

    Again, my advice would be to move ImageQueue to a separate (second) thread. Should take not more than 10 lines of code, if your classes your properly design, up to now, and might solve your problem right away.. since you managed thread saftey as you said it should be no big change in code.
    And, use signals/slots (queued connection) to communicate between threads! Design your communication in a way, that transfer of information may take some time (= aquire some kind of response, if you need to know if an information has been processed/arrived)

    Your problem seems to be a classic "Producer-Consumer-Problem":http://en.wikipedia.org/wiki/Producer-consumer_problem

    Edit: As I saw, I was to slow ;-) I agree with KA51O. Probably you don't need yor while loop

  • Thanks I'll change my program with proper signal slot connections.

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