#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
public slots:
    void changeCentralWidget();
};

#endif // MAINWINDOW_H

#include "mainwindow.h"
#include <QtGui>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    show();
    setCentralWidget(new QPushButton("hello, this is a test................"));
    layout()->setSizeConstraint(QLayout::SetFixedSize);

    connect(centralWidget(), SIGNAL(clicked()), SLOT(changeCentralWidget()));
}

void MainWindow::changeCentralWidget()
{
    layout()->setSizeConstraint(QLayout::SetDefaultConstraint);
    setCentralWidget(new QTextEdit(QString::number(windowFlags(), 16)));
    layout()->setSizeConstraint(QLayout::SetDefaultConstraint);

    /* Adds the possibility to resize the window */
    setWindowFlags(windowFlags() ^ Qt::WindowMinMaxButtonsHint);
    setWindowFlags(windowFlags() | Qt::WindowMinMaxButtonsHint);
    show();
}