connect() problem

[harmodrew]

Hello you all!
I'm a newbie in Qt programming so my question will probably seem a stupid question but all guides I read didn't help me...
I'm developing a C/C++ program in Win Vista & Qt Creator IDE with the Qt framework. It is a Qt GUI Application (the one you can create by clicking New -> project)
I'd like to connect the event of clicking a PushButton with a function I wrote earlier. Here is the code of my mainwindow.cpp file:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include<QMessageBox>
class act: public QObject
{
 public slots:
     void buttonClicked()
     {
         QMessageBox("Title","Msg",QMessageBox::NoIcon,QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
     }
 };
  MainWindow::MainWindow(QWidget *parent) :
     QMainWindow(parent),
     ui(new Ui::MainWindow)
 {
QPushButton *PB= new QPushButton(this);
     PB->setText("Button text");
      act a;
     connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));
     PB->show();
     }
  MainWindow::~MainWindow()
 {
     delete ui;
 }

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include<QMessageBox>
class act: public QObject
{
 public slots:
     void buttonClicked()
     {
         QMessageBox("Title","Msg",QMessageBox::NoIcon,QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
     }
 };
  MainWindow::MainWindow(QWidget *parent) :
     QMainWindow(parent),
     ui(new Ui::MainWindow)
 {
QPushButton *PB= new QPushButton(this);
     PB->setText("Button text");
      act a;
     connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));
     PB->show();
     }
  MainWindow::~MainWindow()
 {
     delete ui;
 }

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It is executed correctly but when I push the button the message doesn't show!

What could be the problem?

Thanks a lot, harmodrew