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Thread: QDialog - get parent name

  1. #1
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    Default QDialog - get parent name

    My program has to QListwidgets. When I right click on the list I have a QDialog popup. When the QDialog pops up I would like to know which QListWidget has opened it. I tried using this line of code
    Qt Code:
    1. qDebug()<<QDialog->parentWidget()->objectName();
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    but it crashed my program.

    Here is some code from my .cpp file.

    (this is in the main function/constructor)
    Qt Code:
    1. lists = new QTabWidget;
    2. lists->setFixedWidth(200);
    3. etfList = new QListWidget;
    4. ipoList = new QListWidget;
    5. etfList->setObjectName("ETFList");
    6. etfList->setContextMenuPolicy(Qt::CustomContextMenu);
    7. ipoList->setObjectName("IPOList");
    8. ipoList->setContextMenuPolicy(Qt::CustomContextMenu);
    9. lists->addTab(etfList, "ETF List");
    10. lists->addTab(ipoList, "IPO List");
    11.  
    12.  
    13. connect(etfList, SIGNAL(customContextMenuRequested(const QPoint &)), this,SLOT(menuPopup(const QPoint &)));
    14. connect(ipoList, SIGNAL(customContextMenuRequested(const QPoint &)), this,SLOT(menuPopup(const QPoint &)));
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    (this is the slot)

    Qt Code:
    1. void MainWindow::menuPopup(const QPoint &point){
    2.  
    3. QDialog *dia = new QDialog();
    4. qDebug()<<dia->parentWidget()->objectName(); <---crashes on this line
    5. dia->show();
    6. }
    7. }
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    And in my header file:


    Qt Code:
    1. class MainWindow : public QMainWindow
    2. {
    3. Q_OBJECT
    4. QTabWidget *lists;
    5. QListWidget *etfList;
    6. QListWidget *ipoList;
    7.  
    8. public:
    9. explicit MainWindow(QWidget *parent = 0);
    10.  
    11. ~MainWindow();
    12.  
    13. signals:
    14.  
    15. private slots:
    16. void menuPopup(const QPoint &point);
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  2. #2
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    Default Re: QDialog - get parent name

    qDebug()<<dia->parentWidget()->objectName(); <---crashes on this line
    Of course it does, because on the line before that you created your QDialog instance with no parent, so parentWidget() returns NULL. And by the way, your method of creating the QDialog on the heap (using "new") causes a memory leak. The more common method is to create temporary QDialog instances on the stack, which will automatically be destroyed when the slot exits and the QDialog instance goes out of scope.

    Anyway, it isn't the QDialog's parent you are interested in - that has nothing to do with which QListWidget issued the signal that the slot is handling. Look at the QObject::sender() method. When you are in the slot, it tells you which QObject issued the signal that your slot is handling. If the only signals connected to that slot are those from the QListWidget instances, then you can qobject_cast() the pointer returned by the sender() method to QListWidget * and see which one of your two QListWidget pointers it matches.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

  3. #3
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    Default Re: QDialog - get parent name

    Quote Originally Posted by d_stranz View Post
    The more common method is to create temporary QDialog instances on the stack, which will automatically be destroyed when the slot exits and the QDialog instance goes out of scope.
    Not for a non-modal dialog though, like in this case (QDialog::show() being called).
    For those it is usually either explicit delete or using the DeleteOnClose widget attribute.

    Cheers,
    _
    Last edited by anda_skoa; 24th August 2016 at 15:25.

  4. #4
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    Default Re: QDialog - get parent name

    Not for a non-modal dialog though, like in this case (QDialog::show() being called).
    For those it is usually either explicit delete or using the DeleteOnClose widget attribute.
    True, but I don't think that was the intent in this case. The code pattern implies that the QDialog instance was intended to be temporary, since neither the DeleteOnClose attribute was set nor the destructor called.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

  5. #5
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    Default Re: QDialog - get parent name

    Thank you d_stranz and anda_skoa. The QObject::sender() works perfect. As for the temporary instances, anda_skoa is correct. Creating a temporary instance
    Qt Code:
    1. QDialog *dia;
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    ends up crashed the program but using
    Qt Code:
    1. dia->setAttribute(Qt::WA_DeleteOnClose, true);
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    runs with our a flaw.

  6. #6
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    Default Re: QDialog - get parent name

    QDialog *dia;
    This is not "creating a temporary instance". This is a declaration of a (uninitialized) local variable named "dia" of type "QDialog *". And of course, it crashes too, because it is uninitialized and points to garbage.

    This is what I meant:

    Qt Code:
    1. void MyClass::on_some_slot()
    2. {
    3. QDialog dlg;
    4. if ( dlg.exec() == QDialog::Accepted )
    5. {
    6. // do something with the result
    7. }
    8.  
    9. // dlg goes out of scope when method exits, and is automatically destroyed
    10. }
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    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

  7. #7
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    Default Re: QDialog - get parent name

    Quote Originally Posted by d_stranz View Post

    This is what I meant:

    Qt Code:
    1. void MyClass::on_some_slot()
    2. {
    3. QDialog dlg;
    4. if ( dlg.exec() == QDialog::Accepted )
    5. {
    6. // do something with the result
    7. }
    8.  
    9. // dlg goes out of scope when method exits, and is automatically destroyed
    10. }
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    Sorry d_stranz for the confusion.

    I tried your code but it does not enter the if condition. It does however run fine outside the if condition (Or appears to anyway).

    Maybe there us something else that I am missing?

  8. #8
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    Default Re: QDialog - get parent name

    I tried your code but it does not enter the if condition.
    Well, no, it won't as written. QDialog is like QWidget - it is a base class that contains nothing; you generally have to derive a class from it that you fill up with other GUI child widgets in a layout. In particular, classes derived from QDialog generally have an OK button or something like that which is connected to the QDialog::accept() slot. This causes the exec() function to exit with the QDialog::Accepted result code.

    If you execute the code I wrote above, the only way to close the dialog is to click the "X" button on the dialog's frame. This is connected to the QDialog::reject() slot. That causes the exec() method to exit with the QDialog::Rejected result code, so the if() clause is not entered. Change my code to replace "Accepted" with "Rejected" and see what happens.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

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