I am getting the error:

mainwindow.cpp:44: undefined reference to `StartWindow::StartWindow(QWidget*)

What is the reason? The code is below:

mainwindow.h
Qt Code: 
  1. #ifndef MAINWINDOW_H
  2. #define MAINWINDOW_H
  3.  
  4. class MainArea;
  5. class StartWindow;
  6.  
  7. class MainWindow : public QMainWindow
  8. {
  9.     Q_OBJECT
  10.  
  11.     public:
  12.         MainWindow();
  13.         ~MainWindow();
  14.     private slots:
  15.         void newGame();
  16.  
  17.     private:
  18.  
  19.         void setupActions();
  20.  
  21.         MainArea *m_graphicsViewArea;
  22.         StartWindow *m_startWindow;
  23.         KTextEdit *m_textArea;
  24.  
  25. };
  26.  
  27. #endif // MAINWINDOW_H
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mainwindow.cpp
Qt Code: 
  1. #include "mainwindow.h"
  2. #include "mainarea.h"
  3. #include "startwindow.h"
  4.  
  5.  
  6. //Constructor
  7. MainWindow::MainWindow()
  9. {
  10.  
  11.  
  12.     m_graphicsViewArea = new MainArea(this);
  13.     m_startWindow = new StartWindow(this); //Here is the error
  14.     setCentralWidget(m_startWindow);
  15.     setWindowTitle("Mancala");
  16.  
  17.     setupActions();
  18.  
  19. }
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startwindow.h
Qt Code: 
  1. #ifndef STARTWINDOW_H
  2. #define STARTWINDOW_H
  3.  
  4. class StartWindow : public QGraphicsView{
  5.  
  6.     Q_OBJECT
  7.  
  8.     public:
  9.         StartWindow(QWidget *parent);
  10.  
  11.     private:
  12.  
  13.         QGraphicsSimpleTextItem *m_start_text;
  14. };
  15.  
  16. #endif // STARTWINDOW_H
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startwindow.cpp
Qt Code: 
  1. #include "startwindow.h"
  2.  
  3. StartWindow::StartWindow(QWidget *parent)
  4.     : QGraphicsView(parent)
  5. {
  6.  
  7.     m_start_text = new QGraphicsSimpleTextItem("This is a collection of Mancala Games");
  8.  
  9.     m_start_text->setFont( QFont("Comic Sans MS", 24, QFont::Bold) );
  10.  
  11.    scene()->addItem(m_start_text);
  12.  
  13. }
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Mahfuz