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Thread: QMainWindow: problem changing centralWidget

  1. #1
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    Default QMainWindow: problem changing centralWidget

    Hello,
    I get a segmentation fault on the setCentralWidget() call in the SetBackground() slot, that is when the QMainWindow attempts to change the central widget. I do know that it tries to delete the widget but do not understand how to cope with that. Please help!

    class myClass : public QMainWindow
    {
    Q_OBJECT

    QImage fond;
    QLabel *wfond;
    };

    myClass::myClass()
    : QMainWindow()
    {
    this->fond.load("somepic.jpg");
    this->wfond = new QLabel;
    this->wfond->setPixmap(QPixmap::fromImage(this->fond));
    this->setCentralWidget(this->wfond);
    }

    void
    myClass::SetBackground(QImage im)
    {
    this->fond = im;
    this->wfond = new QLabel;
    this->wfond->setPixmap(QPixmap::fromImage(this->fond));
    this->setCentralWidget(this->wfond);
    }

  2. #2
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    Default Re: QMainWindow: problem changing centralWidget

    Did you try debugging? What does the backtrace say?

  3. #3
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    Default Re: QMainWindow: problem changing centralWidget

    try to use QImage pointer.
    class myClass : public QMainWindow
    {
    Q_OBJECT

    QImage *fond; //use pointer
    QLabel *wfond;
    };

    myClass::myClass()
    : QMainWindow()
    {
    fond = new QImage; //allocate here.
    this->fond->load("somepic.jpg");
    ...
    }

    why you using this pointer everywhere?

  4. #4
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    Default Re: QMainWindow: problem changing centralWidget

    Quote Originally Posted by rajesh View Post
    try to use QImage pointer.
    class myClass : public QMainWindow
    {
    Q_OBJECT

    QImage *fond; //use pointer
    QLabel *wfond;
    };

    myClass::myClass()
    : QMainWindow()
    {
    fond = new QImage; //allocate here.
    this->fond->load("somepic.jpg");
    ...
    }

    why you using this pointer everywhere?
    Here is my new code, modified according to your advice:

    class myClass : public QMainWindow
    {
    Q_OBJECT

    QImage *fond;
    QLabel *wfond;
    };

    myClass::myClass()
    : QMainWindow()
    {
    this->fond = new QImage("somepic.jpg");
    this->wfond = new QLabel;
    this->wfond->setPixmap(QPixmap::fromImage(*this->fond));
    this->setCentralWidget(this->wfond);
    }

    void
    myClass::SetBackground(QImage im)
    {
    this->fond = new QImage(im);
    this->wfond = new QLabel;
    this->wfond->setPixmap(QPixmap::fromImage(*this->fond));
    this->setCentralWidget(this->wfond);
    }

    but stil the same segmentation fault on setCentralWidget() !

  5. #5
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    Default Re: QMainWindow: problem changing centralWidget

    Quote Originally Posted by rajesh View Post
    try to use QImage pointer.
    QImage is an implicitly shared class. There is no advantage in allocating it on the heap. It only makes things unnecessarily more complicated.

    Caius Aérobus: Why not just change the pixmap? There is no need to create a new QLabel every time.
    J-P Nurmi

  6. #6
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    Default Re: QMainWindow: problem changing centralWidget

    just for testing , try following code, is it working or not?
    void myClass::SetBackground(QImage im)
    {
    QLabel *center =
    new QLabel("test");
    setCentralWidget(center);
    }

  7. #7
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    Default Re: QMainWindow: problem changing centralWidget

    Hmmm, I suppose I have fixed the problem. I use Qt 4.1 and I tried it on another system on which Qt 4.3 is installed and... everything works fine! So probably a bug in Qt 4.1. I will upgrade ASAP.
    Thanks to all of you for trying to help me!
    Best regards.

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