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Thread: QComboBox crash

  1. #1
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    Default QComboBox crash

    Hi!

    When I try to call QComboBox::addItem from another function in my class than the constructor, my application crashes.
    I've made a short application to reproduce the crash, here's the code (.zip included for those who want).

    ComboBoxCrash.pro
    Qt Code:
    1. HEADERS += GUI.h
    2. SOURCES += GUI.cpp \
    3. main.cpp
    To copy to clipboard, switch view to plain text mode 

    main.cpp
    Qt Code:
    1. #include <QtCore/QCoreApplication>
    2. #include <QtGui>
    3. #include "GUI.h"
    4.  
    5.  
    6. int main(int argc, char *argv[]){
    7. QApplication a(argc, argv);
    8.  
    9. ComboBoxCrash *MainWindow = new ComboBoxCrash;
    10. MainWindow->show();
    11. return a.exec();
    12. }
    To copy to clipboard, switch view to plain text mode 

    GUI.h
    Qt Code:
    1. #ifndef GUI_H
    2. #define GUI_H
    3.  
    4. #include <QWidget>
    5.  
    6. class QComboBox;
    7.  
    8. class ComboBoxCrash : public QWidget{
    9. Q_OBJECT
    10.  
    11. public:
    12. ComboBoxCrash(QWidget *parent=0);
    13. public slots:
    14. void function();
    15. private:
    16. QComboBox *comboBox;
    17. QPushButton *crashButton;
    18. };
    19.  
    20. #endif // GUI_H
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    GUI.cpp
    Qt Code:
    1. #include <QtGui>
    2. #include "GUI.h"
    3.  
    4. ComboBoxCrash::ComboBoxCrash(QWidget *parent) : QWidget(parent){
    5.  
    6. QComboBox *comboBox = new QComboBox;
    7. QPushButton *crashButton = new QPushButton("Crash");
    8.  
    9. QGridLayout *mainLayout = new QGridLayout;
    10. mainLayout->addWidget(comboBox,0,0);
    11. mainLayout->addWidget(crashButton,0,1);
    12. setLayout(mainLayout);
    13. setWindowTitle("ComboBox Crash");
    14.  
    15. comboBox->addItem("Item1");
    16. comboBox->addItem("Item2");
    17. comboBox->addItem("Item3");
    18.  
    19. connect(crashButton, SIGNAL(clicked()), this, SLOT(function()));
    20. }
    21.  
    22. void ComboBoxCrash::function(){
    23. comboBox->addItem("Item4");
    24. }
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    When the function which calls addItem is called, the application crashes.
    I'm using Qt 4.5.2 and Qt Creator 1.2.0 under Windows XP SP2.

    If anyone know why this happens, or even better, have a solution, I would be very happy.

    Thanks in advance

  2. #2
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    Default Re: QComboBox crash

    Oh, that's easy. You have two "comboBox" variables - one is a member variable and the other is a local variable created (and destroyed) in the constructor. You are assigning an object to the local one and try to dereference the member which obviously results in a crash.
    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.


  3. The following user says thank you to wysota for this useful post:

    sfilez (8th July 2009)

  4. #3
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    Default Re: QComboBox crash

    Thanks for the fast reply wysota!

    What a silly mistake, got it fixed now.

  5. #4
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    Default Re: QComboBox crash

    Replace

    Qt Code:
    1. QComboBox *comboBox = new QComboBox;
    2. QPushButton *crashButton = new QPushButton("Crash");
    To copy to clipboard, switch view to plain text mode 
    replace that with
    Qt Code:
    1. comboBox = new QComboBox;
    2. crashButton = new QPushButton("Crash");
    To copy to clipboard, switch view to plain text mode 
    :-)
    We can't solve problems by using the same kind of thinking we used when we created them

  6. The following user says thank you to sunil.thaha for this useful post:

    sfilez (8th July 2009)

  7. #5
    Join Date
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    Default Re: QComboBox crash

    Already fixed it, but thanks for replying anyways sunil.thaha

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