I want to implement a vertical scroll bar. I have implemented layouts for this(see the attached layouts.jpg), and also scroll area(see snippet in below code). Yet, when I run, the window does not have menubar. What am I missing here? I tried adding the menubar and submenus through Qt designer, as well as through code, but the window does not show the menubar.

If i use scroll area snippet, i can use the scrollbar, but don't see menubar. However If i comment out the scroll area snippet, i can see the menubar, but lose the scroll bar. I need both of them. How to solve this?
layouts.JPG

Here is the code:- mainwindow.h
Qt Code: 
  1. #ifndef MAINWINDOW_H
  2. #define MAINWINDOW_H
  3.  
  4. #include <QMainWindow>
  5. #include <QScrollArea>
  6. namespace Ui {
  7. class MainWindow;
  8. }
  9.  
  10. class MainWindow : public QMainWindow
  11. {
  12.     Q_OBJECT
  13.  
  14. public:
  15.     explicit MainWindow(QWidget *parent = 0);
  16.     ~MainWindow();
  17.  
  18. private:
  19.     Ui::MainWindow *ui;
  20.  
  21.  
  22. };
  23.  
  24. #endif // MAINWINDOW_H
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mainwindow.cpp
Qt Code: 
  1. #include "mainwindow.h"
  2. #include "ui_mainwindow.h"
  3.  
  4. MainWindow::MainWindow(QWidget *parent) :
  5.     QMainWindow(parent),
  6.     ui(new Ui::MainWindow)
  7. {
  8.     ui->setupUi(this);
  9.     layout()->setMenuBar(menuBar());
  10.     QMenu *viewMenu;
  11.     viewMenu = menuBar()->addMenu("VIEW");
  12.     QMenu *resolutionSubMenu = viewMenu->addMenu("Resolution");
  13.     QAction *QVGA = resolutionSubMenu->addAction("QVGA");
  14.  
  15.  
  16.     QImage image(":\\image.jpg");
  17.     ui->referenceImageLabel->setPixmap(QPixmap::fromImage(image));
  18.     ui->deltaImageLabel->setPixmap(QPixmap::fromImage(image));
  19.     ui->reconstructedImageLabel->setPixmap(QPixmap::fromImage(image));
  20.     ui->referenceImageLabel->setMinimumSize(480, 640);ui->referenceImageLabel->setMinimumSize(480, 640);ui->referenceImageLabel->setMinimumSize(480, 640);
  21.  
  22.     /*Scroll area snippet*/
  23.     QScrollArea *p_ScrollArea = new QScrollArea(this);
  24.     p_ScrollArea->setAutoFillBackground(true);
  25.     p_ScrollArea->setMinimumSize(width(), height());
  26.     p_ScrollArea->setWidget(ui->centralWidget);
  27.  
  28. }
  29.  
  30. MainWindow::~MainWindow()
  31. {
  32.     delete ui;
  33. }
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main.cpp

Qt Code: 
  1. #include "mainwindow.h"
  2. #include <QApplication>
  3.  
  4. int main(int argc, char *argv[])
  5. {
  6.     QApplication a(argc, argv);
  7.     MainWindow w;
  8.     w.show();
  9.  
  10.     return a.exec();
  11. }
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