Qt: les Signaux et les slots d'Erreur: undefined reference to `vtable for

Exemple suivant à partir de ce lien: http://developer.kde.org/documentation/books/kde-2.0-development/ch03lev1sec3.html

#include <QObject>
#include <QPushButton>
#include <iostream>
using namespace std;
class MyWindow : public QWidget
{
Q_OBJECT  //Enable slots and signals
public:
MyWindow();
private slots:
void slotButton1();
void slotButton2();
void slotButtons();
private:
QPushButton *button1;
QPushButton *button2;
};
MyWindow :: MyWindow() : QWidget()
{
//Create button1 and connect button1->clicked() to this->slotButton1()
button1 = new QPushButton("Button1", this);
button1->setGeometry(10,10,100,40);
button1->show();
connect(button1, SIGNAL(clicked()), this, SLOT(slotButton1()));
//Create button2 and connect button2->clicked() to this->slotButton2()
button2 = new QPushButton("Button2", this);
button2->setGeometry(110,10,100,40);
button2->show();
connect(button2, SIGNAL(clicked()), this, SLOT(slotButton2()));
//When any button is clicked, call this->slotButtons()
connect(button1, SIGNAL(clicked()), this, SLOT(slotButtons()));
connect(button2, SIGNAL(clicked()), this, SLOT(slotButtons()));
}
//This slot is called when button1 is clicked.
void MyWindow::slotButton1()
{
cout << "Button1 was clicked" << endl;
}
//This slot is called when button2 is clicked
void MyWindow::slotButton2()
{
cout << "Button2 was clicked" << endl;
}
//This slot is called when any of the buttons were clicked
void MyWindow::slotButtons()
{
cout << "A button was clicked" << endl;
}
int main ()
{
MyWindow a;
}

résultats dans:

    [13:14:34 Mon May 02] ~/junkPrograms/src/nonsense  $make
g++ -c -m64 -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/opt/qtsdk-2010.05/qt/mkspecs/linux-g++-64 -I. -I/opt/qtsdk-2010.05/qt/include/QtCore -I/opt/qtsdk-2010.05/qt/include/QtGui -I/opt/qtsdk-2010.05/qt/include -I. -I. -o signalsSlots.o signalsSlots.cpp
g++ -m64 -Wl,-O1 -Wl,-rpath,/opt/qtsdk-2010.05/qt/lib -o nonsense signalsSlots.o    -L/opt/qtsdk-2010.05/qt/lib -lQtGui -L/opt/qtsdk-2010.05/qt/lib -L/usr/X11R6/lib64 -lQtCore -lpthread
signalsSlots.o: In function `MyWindow::MyWindow()':
signalsSlots.cpp:(.text+0x1a2): undefined reference to `vtable for MyWindow'
signalsSlots.cpp:(.text+0x1aa): undefined reference to `vtable for MyWindow'
signalsSlots.o: In function `MyWindow::MyWindow()':
signalsSlots.cpp:(.text+0x3e2): undefined reference to `vtable for MyWindow'
signalsSlots.cpp:(.text+0x3ea): undefined reference to `vtable for MyWindow'
signalsSlots.o: In function `main':
signalsSlots.cpp:(.text+0x614): undefined reference to `vtable for MyWindow'
signalsSlots.o:signalsSlots.cpp:(.text+0x61d): more undefined references to `vtable for MyWindow' follow
collect2: ld returned 1 exit status
make: *** [nonsense] Error 1

vtable est pour des fonctions virtuelles, autant que je sache, quelle est la raison de l'erreur ici?