C ++ / Qt macro Q_OBJECT throws error

I have just started programming in the Qt environment. Below is a very simple program:

#include <QtCore/QCoreApplication>
#include <QDebug>

class MyClass : public QObject
 {
     Q_OBJECT

 public:
     MyClass() {}
 };

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    MyClass *c = new MyClass();
    return a.exec();
}

      

        
        
        
      

    

But I get the following error when I try to compile and run it:
In function MyClass

:
undefined reference to vtable for MyClass


But when I remove the QObject macro everything works fine. Note that the class is defined in the same file as the main function.
I am using Qt version 4.7, running Win 7.
What is causing this problem?

Update: I am getting the same error when I define my class in a separate header file. mytimer.h:

#ifndef MYTIMER_H
#define MYTIMER_H
#include <QtCore>

class MyTimer : public QObject
{
    Q_OBJECT

public:
    QTimer *timer;
    MyTimer();
public slots:
    void DisplayMessage();
};


#endif // MYTIMER_H

      

        
        
        
      

    

mytimer.cpp:

#include "mytimer.h"
#include <QtCore>

MyTimer::MyTimer()
{
    timer = new QTimer();
    connect(timer,SIGNAL(timeout()),this,SLOT(DisplayMessage()));

    timer->start(1000);
}

void MyTimer::DisplayMessage()
{
    qDebug() << "timed out";
}

      

        
        
        
      

    

And this is main.cpp:

#include <QtCore/QCoreApplication>
#include <QDebug>
#include "mytimer.h"


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    MyTimer *mt = new MyTimer();
    return a.exec();
}