Не отображаются кнопки внутри QVBoxLayout

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include<QObject>
#include<QMainWindow>
#include<QPushButton>
#include<QVBoxLayout>
 
class MainButton : public QMainWindow
 
{
    Q_OBJECT
 
public:
    explicit MainButton(QWidget *parent=0);
    QVBoxLayout *layout;
    QPushButton *button1;
    QPushButton *button2;
    QPushButton *button3;
public slots:
    void settext();
 
};
 
#endif // MAINWINDOW_H

#include "mainWindow.h"
 
MainButton::MainButton(QWidget *parent): QMainWindow(parent)
{
    QPushButton *button1 = new QPushButton();
    QPushButton *button2 = new QPushButton();
    QPushButton *button3 = new QPushButton();
 
    QVBoxLayout *layout = new QVBoxLayout(parent);
     layout->addWidget(button1);
     layout->addWidget(button2);
     layout->addWidget(button3);
    this->setLayout(layout);
 
    QObject::connect(button1, SIGNAL(clicked()), this, SLOT(settext()));
    QObject::connect(button2, SIGNAL(clicked()), this, SLOT(settext()));
    QObject::connect(button3, SIGNAL(clicked()), this, SLOT(settext()));
 
}
 
void MainButton::settext()
{
    ((QPushButton*)sender())->setText("x");
}

#include <QtGui/QApplication>
#include "mainWindow.h"
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainButton window;
 
    window.show();
    return app.exec();
 
}