Why is there a segmentation fault error if I include an array of widgets in Qt?

Question

I'm a little new to object oriented programming, and very new to Qt and GUIs in general. I am now playing with this example in Nokia's Qt tutorial: http://doc.qt.nokia.com/4.1/tutorial-t5.html

I tried to extend the code; this is what I have now:

#include <QApplication>
#include <QFont>
#include <QLCDNumber>
#include <QPushButton>
#include <QSlider>
#include <QVBoxLayout>
#include <QWidget>

class MyWidget : public QWidget
{
public:
    MyWidget(QWidget *parent = 0);
};

MyWidget::MyWidget(QWidget *parent)
    : QWidget(parent)
{
    QPushButton *quit = new QPushButton(tr("Quit"));
    move(1600,0);
    quit->setFont(QFont("Times", 18, QFont::Bold));

    QPushButton* numbase;
    numbase = new QPushButton[4];
    numbase[0].setText("Dec");
    (numbase+1)->setText("Bin"); // Hihihi
    numbase[2].setText("Hex");
    numbase[3].setText("Oct");

    // a[i] == *(a+i)

    QLCDNumber *lcd = new QLCDNumber(8);
    lcd->setSegmentStyle(QLCDNumber::Filled);
    lcd->setMode(QLCDNumber::Hex);

    QSlider *slider = new QSlider(Qt::Horizontal);
    slider->setRange(0, 99);
    slider->setValue(0);

    connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));
    connect(slider, SIGNAL(valueChanged(int)),
            lcd, SLOT(display(int)));

    connect(numbase+0, SIGNAL(clicked()), lcd, SLOT(setDecMode()));
    connect(numbase+1, SIGNAL(clicked()), lcd, SLOT(setBinMode()));
    connect(numbase+2, SIGNAL(clicked()), lcd, SLOT(setHexMode()));
    connect(numbase+3, SIGNAL(clicked()), lcd, SLOT(setOctMode()));

    QVBoxLayout *layout = new QVBoxLayout;
    layout->addWidget(quit);
    layout->addWidget(lcd);
    layout->addWidget(slider);

    // Segmentation fault if I include those two lines:
    for(int i=0;i<4;i++)
        layout->addWidget(numbase+i);
    // -------------

    setLayout(layout);
}

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MyWidget widget;
    widget.show();
    return app.exec();
}

When I include the marked two lines, the program does its job correctly, but if I quit it (and only then) the console shows a "Segmentation fault". I'd like to know why that happens.

And also, is there a better way to reference the 4 widgets? (numbase+2) looks weird, is this really how I am supposed to do this?

Answer 1

A better way is to use a array of pointers, you should try it:

QPushButton *buttons[4];

buttons[0] = new QPushButton(this);
buttons[1] = new QPushButton(this);
buttons[2] = new QPushButton(this);
buttons[3] = new QPushButton(this);

buttons[0]->setText("foo")

connect(buttons[0], SIGNAL(clicked()), this, SLOT(bar()));

Answer 2

1. Try replacing 4 by 3 in the loop.

2. If you want Qt to do garbage collection make sure you set the parent child relationship of widget correctly i.e. QPushButton resides in QWidget so set it in constructor.

3. For simple UI like these try creating it using Qt Designer and see the generated code.

Answer 3

The possible reason of crash is that Qt, at deletion of the form, calls delete function for each widget on the form. So, at program exit, when destroying your form, Qt internals would execute the following sequence:

delete numbase[0];
delete numbase[1];
delete numbase[2];
delete numbase[3];

However, the correct way to free the memory would be

delete [] numbase;

because you allocated numbase with a special array allocation new[] operator, which is different from the usual new. And it's "undefined behavior" if you use delete instead of delete[] in C++. So, segmentation fault could happen if it's the case.

You can check if it's the reason, by allocating each emenent in your array separately, with new QPushButton;, as specified in the other answer.