C++/Qt - Compile problem: syntax error : missing ';' before identifier - No idea

#include "ui_library_browser.h"
#include <QtGui/QApplication>
#include "StartWindow.h"
int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  StartWindow w;
  w.show();
  return a.exec();
}

#ifndef STARTWINDOW_H_
#define STARTWINDOW_H_

#include <qwidget>
#include "MainWindow.h"

class StartWindow : public QWidget
{
  Q_OBJECT

public:
  StartWindow();
  ~StartWindow();
  MainWindow main_window;  //<-- Problem
};
#endif

#ifndef MAINWINDOW_H_
#define MAINWINDOW_H_

#include <qdialog.h>
#include "StartWindow.h"

class MainWindow : public QDialog
{
  Q_OBJECT

public:
  MainWindow();
  ~MainWindow();
};
#endif