I am pretty new with Qt, and I am sure for an experienced programmer this problem is more than ridiculous. But I am eager to learn it. Hence, I am most interested to understand why my code is not working.
Thanks to everybody, who brings me into the right direction to solve my problem!

Okay, here is my problem:
In an QApplication I have QTabWidget (NOT QTableWidget!) as a central Widget.
The QTabWidget has three tabs.
The content for each Tab should come from a "selfmade" class derived from QWidget.

For the first tab "ra", it work as desired. But now I am stuck with the second tab "rf". This second tab should show a QTableView. I have marked the relevant parts in the code below with "//PROBLEM".

mainwindow.cpp
Qt Code: 
  1. #include <QtGui/QMainWindow>
  2. #include <QAction>
  3. #include <QMenuBar>
  4. #include "mainwindow.h"
  5. #include "Reiseangaben/reiseangaben.h"
  6. #include "Reiseabfolge/reiseabfolge.h"
  7. #include "Reisespesen/reisespesen.h"
  8.  
  9. MainWindow::MainWindow() {
  10.  
  11.     setWindowTitle(tr("Alexander's Reisekostenabrechner"));
  12.     resize(400, 200);
  13.     createActions();
  14.     createMenus();
  15.  
  16.     widget_TabCentral = new QTabWidget(this);
  17.     setCentralWidget(widget_TabCentral);
  18.  
  19.     widget_TabCentral->setObjectName(QString::fromUtf8("widget_TabCentral"));
  20.  
  21.     Reiseangaben *ra = new(Reiseangaben);                                       // WORKS WELL
  22.     widget_TabCentral->addTab(ra, "Allgemeine Reiseangaben");
  23.  
  24.     Reiseabfolge *rf = new(Reiseabfolge);                                           // PROBLEM
  25.     widget_TabCentral->addTab(rf, "Reiseabfolge / Tagespauschale");
  26.  
  27.     Reisespesen *sp = new(Reisespesen);
  28.     widget_TabCentral->addTab(sp, "Spesen");
  29. }
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To keep it simple for the beginning, I decide just to display in the second (Problem-)Tab rf a QTableView with content about the current directory (QDirModel). I have putted the QTableView into a QGridLayout. I am not sure if this is necessary. I did this to make sure my QTableView is within a widget, since addTab requires a widget as an argument.

reiseabfolge.cpp
Qt Code: 
  1. #include <QtGui>
  2. #include "reiseabfolge.h"
  3.  
  4. Reiseabfolge::Reiseabfolge() {
  5.  
  6.   QGridLayout *layoutGrid = new QGridLayout(this);
  7.  
  8.   QDirModel dirModel;
  9.  
  10.   QTableView *tav = new QTableView;
  11.   tav->setModel(&dirModel);
  12.   layoutGrid->addWidget(tav, 0, 0);
  13.  
  14.   QModelIndex cwdIndex = dirModel.index(QDir::currentPath());
  15.   tav->setRootIndex(cwdIndex);
  16. }
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What is my problem:
The second Tab shows only a white box with noting in it. The white box must be the QGridLayout, but this QGridLayout is empty. It does not show the QTableView .

For completeness, here some further files:
reiseabfolge.h
Qt Code: 
  1. #ifndef REISEABFOLGE_H
  2. #define REISEABFOLGE_H
  3.  
  4. #include <QtGui/QWidget>
  5.  
  6. class Reiseabfolge : public QWidget {
  7.  
  8. public:
  9.         Reiseabfolge();
  10. };
  11.  
  12. #endif // REISEABFOLGE_H
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mainwindow.h:
Qt Code: 
  1. #ifndef MAINWINDOW_H
  2. #define MAINWINDOW_H
  3.  
  4. #include <QMainWindow>
  5.  
  6. class MainWindow : public QMainWindow {
  7.  
  8.     private:
  9.         Q_OBJECT
  10.         QTabWidget *widget_TabCentral;
  11.         void createActions();
  12.         void createMenus();
  13.  
  14.         QMenu *dateiMenu;
  15.         QAction *neueDatei_Action;
  16.         QAction *oeffneDatei_Action;
  17.         QAction *speicherDatei_Action;
  18.         QAction *speicherDateiUnter_Action;
  19.         QAction *beendeProgramm_Action;
  20.  
  21.     public:
  22.         MainWindow();
  23.         ~MainWindow();
  24.  
  25.     private slots:
  26.         void neueDatei();
  27.         void oeffneDatei();
  28.         bool speicherDatei();
  29.         bool speicherDateiUnter();
  30.         bool beendeProgramm();
  31.  
  32. };
  33.  
  34. #endif // MAINWINDOW_H
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Thanks to everybody, who can tell me what I have misunderstood and made wrong.