About QBoxLayout Problem

[pipfeng]

In my code, I create two buttons, and use QHBoxLayout to set these layout.
if my application is inherited from QDialog, run the application , it's ok.
if my application is herited from QMainWindow, run the application, it is wrong.

//example 1
class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0);
    ~Dialog();
private:
    QPushButton *button1;
    QPushButton *button2;
};


Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    button1 = new QPushButton(tr("Button 1"));
    button2 = new QPushButton(tr("Button 2"));
    QHBoxLayout *hLayout = new QHBoxLayout;
    hLayout->addWidget(button1);
    hLayout->addWidget(button2);

    QHBoxLayout *vLayout = new QHBoxLayout;
    vLayout->addLayout(hLayout);
    setLayout(vLayout);
}


//example 2
class MainWindow : public QMainWindow {
    Q_OBJECT
public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();

protected:
    void changeEvent(QEvent *e);

private:
    Ui::MainWindow *ui;
    QPushButton *button1;
    QPushButton *button2;    
};

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
   // ui->setupUi(this);
    button1 = new QPushButton(tr("Button 1"), this); //if it does not have "this", the application will not have Button
    button2 = new QPushButton(tr("Button 2"), this);

    QHBoxLayout *hLayout = new QHBoxLayout;

    hLayout->addWidget(button1);
    hLayout->addWidget(button2);

    QHBoxLayout *vLayout = new QHBoxLayout;
    vLayout->addLayout(hLayout);
    setLayout(vLayout);
}

[xizzhu]

I think it's because you're supposed to use the central widget of QMainWindow to place your widgets.