Newbie - Confused about the central widget

[N3wb]

Hi,

I've been reading through the GUI programming with Qt4 and really need help understanding the central widget. Here's my code:

#include <QMainWindow>
#include <QtGui>
 
class MainWindow : public QMainWindow
{
public:
    MainWindow(); // Constructor
};
 
 
MainWindow::MainWindow()
{
    QSpinBox *spinBox = new QSpinBox;
    QVBoxLayout *mainLayout = new QVBoxLayout();
    mainLayout->addWidget(spinBox);
    this->centralWidget()->setLayout(mainLayout);
}
 
int main(int argc, char *argv[])
{
QApplication app(argc, argv); // Create application. Arguments as parameters
MainWindow *mainWindow = new MainWindow; // Create the application window
mainWindow->show(); // Show the application window
return app.exec();
}

#include <QMainWindow>
#include <QtGui>

class MainWindow : public QMainWindow
{
public:
    MainWindow(); // Constructor
};


MainWindow::MainWindow()
{
    QSpinBox *spinBox = new QSpinBox;
    QVBoxLayout *mainLayout = new QVBoxLayout();
    mainLayout->addWidget(spinBox);
    this->centralWidget()->setLayout(mainLayout);
}

int main(int argc, char *argv[])
{
QApplication app(argc, argv); // Create application. Arguments as parameters
MainWindow *mainWindow = new MainWindow; // Create the application window
mainWindow->show(); // Show the application window
return app.exec();
}

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This is my program simplified, all in one file. Although it compiles just fine, the program will crash upon being run.

I really have been trying to figure out how to do this correctly and would really appreciate if someone could point in the right direction!

Thanks

[SudaNix]

you should first set the central widget because this call
this->centralWidget()

will return 0.

here the fix:

MainWindow::MainWindow()
{
    QWidget* widget = new QWidget; // parent widget 
    QSpinBox *spinBox = new QSpinBox;
    //... use any widget you want.
 
    // set the layout
    QVBoxLayout *mainLayout = new QVBoxLayout(widget);
    mainLayout->addWidget(spinBox);
 
    setCentralWidget(widget);
}

MainWindow::MainWindow()
{
    QWidget* widget = new QWidget; // parent widget 
    QSpinBox *spinBox = new QSpinBox;
    //... use any widget you want.

    // set the layout
    QVBoxLayout *mainLayout = new QVBoxLayout(widget);
    mainLayout->addWidget(spinBox);

    setCentralWidget(widget);
}

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