title: what is the Macro "QT_BEGIN_NAMESPACE" mean in Qt 5?
body: <p>In Qt 5.6 Example "basicLayout",there is a Macro "QT_BEGIN_NAMESPACE",I try to search for it in Qt Documentation,but I still can't get the answer.So what is the meaning in Qt?</p>
  
  <p>The Code related is here:</p>
  
  <pre><code>#ifndef DIALOG_H
  #define DIALOG_H
  
  #include &lt;QDialog&gt;
  
  QT_BEGIN_NAMESPACE
  class QAction;
  class QDialogButtonBox;
  class QGroupBox;
  class QLabel;
  class QLineEdit;
  class QMenu;
  class QMenuBar;
  class QPushButton;
  class QTextEdit;
  QT_END_NAMESPACE
  
  //! [0]
  class Dialog : public QDialog
  {
      Q_OBJECT
  
  public:
      Dialog();
  
  private:
      void createMenu();
      void createHorizontalGroupBox();
      void createGridGroupBox();
      void createFormGroupBox();
  
      enum { NumGridRows = 3, NumButtons = 4 };
  
      QMenuBar *menuBar;
      QGroupBox *horizontalGroupBox;
      QGroupBox *gridGroupBox;
      QGroupBox *formGroupBox;
      QTextEdit *smallEditor;
      QTextEdit *bigEditor;
      QLabel *labels[NumGridRows];
      QLineEdit *lineEdits[NumGridRows];
      QPushButton *buttons[NumButtons];
      QDialogButtonBox *buttonBox;
  
      QMenu *fileMenu;
      QAction *exitAction;
  };
  //! [0]
  
  #endif // DIALOG_H
  </code></pre>