Ok, edited the question. Bolded the edited part.

@wysota : Mmmm, I'm not sure I understand what you are asking, but here is the complete code. I hope this helps.

mainwindow.cpp
Qt Code: 
  1. mainwindow::mainwindow()
  2. {   
  3.  
  4.     #include <QtGui>
  5.     #include <iostream>
  6.     #include "mainwindow.h"
  7.     #include "game.h"
  8.     #include "menu1.h"
  9.  
  10.     men = new menu1(this);
  11.     qur = new game(this);
  12.  
  13.     qs = new QStackedWidget(this);
  14.     qs->resize(880,558);
  15.     qs->addWidget(men);
  16.     qs->addWidget(qur);
  17.  
  18.     qs->setCurrentIndex(0);
  19.  
  20.     show();
  21. }
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menu1.cpp

Qt Code: 
  1. #include "menu1.h"
  2. #include <QtGui>
  3.  
  4. menu1::menu1(QWidget *parent) :
  5.     QWidget(parent)
  6. {
  7.  
  8.    resize(880,558);
  9.  
  10. }
  11.  
  12. void menu1::paintEvent(QPaintEvent *)
  13. {
  14.     QPainter painter(this);
  15.     painter.setRenderHint(QPainter::Antialiasing, true);
  17.     path.moveTo(qrand() % 80, qrand() % 320);
  18.     path.cubicTo(200, 200, 320, 80, 480, 320);
  19.     painter.setPen(QPen(Qt::green, 8));
  20.     painter.drawPath(path);
  21. }
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