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Thread: focusNextChild Segmentatiion Fault

  1. #1
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    Default focusNextChild Segmentatiion Fault

    Hi, everyone!

    I'm getting a SegFault while implement an eventFilter in a QDialog subclass.

    The code bellow works fine in Qt 4.8.6, in Qt 5.3.1 it Segfault.

    Qt Code:
    1. ISCBase::ISCBase(QWidget *parent) : QDialog(parent)
    2. {
    3. setGeometry(0, 0, 768, 450);
    4.  
    5. this->installEventFilter(this);
    6. }
    7.  
    8. bool ISCBase::eventFilter(QObject *object, QEvent *event)
    9. {
    10. if (event->type() == QEvent::KeyPress) {
    11. QKeyEvent *keyEvent = (QKeyEvent*) event;
    12.  
    13. if ((keyEvent->key() == Qt::Key_Return) || (keyEvent->key() == Qt::Key_Enter)) {
    14. ISCBase *base = qobject_cast<ISCBase*>(object);
    15.  
    16. if (
    17. (base) && (
    18. (qobject_cast<QRadioButton*>(base->focusWidget())) ||
    19. (qobject_cast<QCheckBox*>(base->focusWidget())) ||
    20. (qobject_cast<QComboBox*>(base->focusWidget())) ||
    21. (qobject_cast<QDateEdit*>(base->focusWidget())) ||
    22. (qobject_cast<QDoubleSpinBox*>(base->focusWidget())) ||
    23. (qobject_cast<QFontComboBox*>(base->focusWidget())) ||
    24. (qobject_cast<QSlider*>(base->focusWidget())) ||
    25. #if QT_VERSION >= 0x050000
    26. (qobject_cast<QKeySequenceEdit*>(base->focusWidget())) ||
    27. #endif
    28. (qobject_cast<QLineEdit*>(base->focusWidget())) ||
    29. (qobject_cast<QRadioButton*>(base->focusWidget())) ||
    30. (qobject_cast<QSpinBox*>(base->focusWidget())) ||
    31. (qobject_cast<QTimeEdit*>(base->focusWidget())))
    32. ) {
    33. focusNextChild();
    34.  
    35. return true;
    36. }
    37. }
    38. }
    39.  
    40. return QObject::eventFilter(object, event);
    41. }
    To copy to clipboard, switch view to plain text mode 

    Any help will be welcome.

  2. #2
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    Default Re: focusNextChild Segmentatiion Fault

    Why are you installing an event filter on your own class? Event filters are intended so that some other class can monitor events in a different class.

    If you want to handle key press events, simply reimplement the QWidget::keyPressEvent() handler in your dialog class. Have you determined that using an event filter is the only way you can trap the return key?

    You are calling QObject::eventFilter() at the end, instead of QDialog::eventFilter(), but again, you shouldn't be using an event filter in this way in the first place. You are also not setting the accept() flag on the event when you do handle it.

  3. #3
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    Default Re: focusNextChild Segmentatiion Fault

    Installing the eventFilter in my own class give me the possibily to handle all kinds of events in one central location. And the Qt documentation doesn't point this as bad habit, nor recomend against it.

    Qt Creator use this kind of eventFilter control (src/plugins/coreplugin/locator/locatorwidget.cpp), see line 262.
    Qt Code:
    1. // We set click focus since otherwise you will always get two popups
    2. m_fileLineEdit->setButtonFocusPolicy(Utils::FancyLineEdit::Left, Qt::ClickFocus);
    3. m_fileLineEdit->setAttribute(Qt::WA_MacShowFocusRect, false);
    4. m_fileLineEdit->installEventFilter(this);
    5. this->installEventFilter(this);
    To copy to clipboard, switch view to plain text mode 

    The error persists calling QObject::eventFilter or QDialog::eventFilter.

    Oddly enough this same code runs correctly under Qt 5.3.1 in windows. It's seems that it could be related to the linux version of Qt.

    I track the error to a QPaintEvent using gdb.

    Anyway thanks for the help.

  4. #4
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    Default Re: focusNextChild Segmentatiion Fault

    Quote Originally Posted by croscato View Post
    Installing the eventFilter in my own class give me the possibily to handle all kinds of events in one central location.
    In such case you should reimplement event() for the object. All events pass through there before reaching their respective event handlers. Installing an event filter on yourself is not a good solution. And in your particular case you are handling one type of event so there is no reason not to put it in keyPressEvent().

    Maybe you should state what exactly you are trying to do?
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


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