main.cpp:

 #include <QtGui>

 #include "mainwindow.h"
 #include "zweitwindow.h"
 
 int main(int argc, char *argv[]) {
   
     QApplication app(argc, argv);
     MyWidget widget;
     widget.show();
     return app.exec();
 }	


mainwindow.h:

 #ifndef MAINWINDOW_H
 #define MAINWINDOW_H

 #include <QApplication>
 #include <QFont>
 #include <QLCDNumber>
 #include <QPushButton>
 #include <QSlider>
 #include <QVBoxLayout>
 #include <QWidget>

 cloass My2_Widget;

 class MyWidget : public QWidget
 {
     Q_OBJECT

 public:
     MyWidget(QWidget *parent = 0);
     ~My2_Widget();
 private:
     My2_Widget *w2;

 public slots:
     void TuWas();
 };


 #endif	

mainwindow.cpp:

 #include <QtGui>

 #include "mainwindow.h"
 #include "zweitwindow.h"

 MyWidget::MyWidget(QWidget *parent)
     : QWidget(parent) {
       
     QPushButton *quit = new QPushButton(tr("Quit"));
     quit->setFont(QFont("Times", 18, QFont::Bold));

     connect(quit, SIGNAL(clicked()), this, SLOT(TuWas()));

     QVBoxLayout *layout = new QVBoxLayout;
     layout->addWidget(quit);
     setLayout(layout);
 }

 MyWidget::~MyWidget() { delete w2; }

 void MyWidget::TuWas() {
     qWarning("ENDLICH!");
     w2 = new My2_Widget;
     w2->show();
}

zweitwindow.h:

 #ifndef ZWEITWINDOW_H
 #define ZWEITWINDOW_H

 #include <QApplication>
 #include <QFont>
 #include <QLCDNumber>
 #include <QPushButton>
 #include <QSlider>
 #include <QVBoxLayout>
 #include <QWidget>

 class My2_Widget : public QWidget
 {
     Q_OBJECT

 public:
     My2_Widget(QWidget *parent = 0);
 
 };


 #endif	

zweitwindow.cpp:

 #include <QtGui>

 #include "mainwindow.h"
 #include "zweitwindow.h"

 My2_Widget::My2_Widget(QWidget *parent)
     : QWidget(parent) {
       
     QLabel *label=new QLabel(QApplication::translate("windowlayout", "Name:"));
     
    
     QVBoxLayout *layout = new QVBoxLayout;
     layout->addWidget(label);
     setLayout(layout);
 }